having pectinated gills and two nuchal cirri like _Eunice_. In making its
tube it employs small pebbles, bits of shell, and even echinid spines,
which it glues together with mucus, so that it bears a general resemblance
{320}to its surroundings. _O. conchylega_ Sars, has a flattened,
scabbard-like tube, which can be carried about by its owner. Atlantic.
_Lumbriconereis_ has a more or less conical prostomium, without any
tentacles, but with large palps: segments without gills. _L. fragilis_
Müll. is reddish or brownish, with a beautiful iridescence; it is
cylindrical, very narrow, and some 5 or 6 inches long; _L. tricolor_ Jnstn.
is much larger.
_Ophryotrocha_ (Fig. 170) is a small form often occurring in aquaria; it is
chiefly remarkable for the possession of segmentally-arranged girdles of
cilia—a permanent larval feature. _Lysidice ninetta_ Aud. and Edw. belongs
here.
FAM. 10. _Glyceridae._—Elongated worms with numerous segments. The
prostomium, though narrow, is long, conical, annulated, and carries at its
apex four very small tentacles; at its base a pair of palps. Special
retractile gills are present. The armed pharynx is very long, and when
protruded appears wider than the animal. The members of this family are
without any system of blood-vessels, but the coelomic corpuscles are
coloured red. _Glycera_ has four jaws, the parapodia are all alike (Fig.
136, C). _G. capitata_ Oerst. is 2 or 3 inches in length, is yellowish in
colour, with a dark-red median line. It may be found burrowing in sand. The
setigerous lobes of each foot are coalesced to form one large lobe with
pointed apex. The dorsal cirrus is a small wart above the base of the foot.
Atlantic and Mediterranean. A second species, which is much larger and
flesh-coloured, also occurs.
[Illustration: FIG. 171.—_Glycera meckelii_ Aud. and Edw. with pharynx
everted, × 1. (_Règne Animal._)]
_Goniada_ is distinguished from the preceding by the fact that the
parapodia suddenly change in size and character at about one-third the
length of the body. The pharynx has numerous paragnaths. _G. maculata_
Oerst. occurs off our coasts.
FAM. 11. _Sphaerodoridae._—The dorsal and ventral cirri of each segment are
spherical. The chaetae are usually jointed, and there is an aciculum to
each {321}parapodium. _Ephesia_ Rthke. (_E. gracilis_ R. = _Sphaerodorum
peripatus_ Jnstn.) is exceptional in having unjointed chaetae. North Sea,
Arctic Ocean, and the Channel. The family, which is much modified, is
allied in some respects to the Syllidae.
FAM. 12. _Ariciidae._—These worms burrow in sand between tide-marks. The
body consists of many short segments, and is nearly cylindrical. The
prostomium is more or less pointed; the chaetae are all capillary; in the
first few segments they project laterally but soon come to lie dorsally,
and are carried by slight conical papillae (supported by acicula), which
are longer in the middle of the body. Most of the segments carry filiform
"gills," representing the dorsal cirri (Fig. 137, B).
_Scoloplos armiger_ Müll. is extremely common on our coast. It is about an
inch long, yellowish, with red gills, commencing about the twelfth segment.
Each of the lobes of the parapodium possesses an aciculum, and the chaetae
are bent in a peculiar way. The everted buccal region has the form of a
six- or eight-rayed star. The spawn of this species may be found on the
shore in spring as brown, pear-shaped, jelly-like masses, each with a long
stalk, by which the mass is fixed to the sand. In the jelly are the eggs,
which may be watched passing through the earlier stages of development.
Atlantic on both shores, even off Spitzbergen, and Nova Zembla. Another
representative is _Theodisca mamillata_ Clap., which occurs amongst the
roots of _Laminaria_.
FAM. 13. _Typhloscolecidae._[382]—Pelagic, greatly modified forms,
apparently related to the Phyllodocidae, but with very uncertain
affinities. The prostomium is pointed and carries a pair of foliaceous
tentacles; each of the first two segments bears a pair of foliaceous cirri;
the remaining segments possess a dorsal and a ventral pair of foliaceous
cirri, with a small bunch of chaetae and a single aciculum. All the cirri
have peculiar rod-cells. _Typhloscolex_ Busch, _Sagitella_ Wagner, and
_Travisiopsis_ Uljanin: all small worms. North Sea Atlantic.
SUB-ORDER 2. SPIONIFORMIA.
FAM. 1. _Spionidae._—Mostly small worms, with small ridge-like prostomium
carrying a pair of eyes, but no tentacles or palps. The peristomium, which
extends forwards on each side {322}of the prostomium, bears a pair of very
long cirri (usually termed "tentacles") normally directed backwards, very
mobile, and more or less coiled. They are readily thrown off by the animal.
The notopodial cirri are long, finger-shaped, and curved over the back;
they are vascular and ciliated, and function as "gills." The neuropodia
project laterally. Both are usually provided with a "podal membrane" along
their outer margin. There are no ventral cirri; the dorsal chaetae are
fringed capillaries; the ventral are "crotchets." The buccal region is
eversible. The worms burrow in mud and sand.
_Spio seticornis_ Fabr. is a small worm less than an inch in length,
colourless except for the red blood in its vessels. It builds long and
flexible tubes of sand in the clefts of rocks and under stones in the upper
part of the littoral zone. The prostomium is notched at the anterior
margin. The gills commence on the twelfth segment, and do not extend to the
end of the body. A membrane-like cirrus exists also on the second
chaetigerous segment. The podal membrane is adnate to the gill throughout
its extent. Four short anal cirri occur. Greenland and Scandinavia.
[Illustration: FIG. 172.—_Nerine vulgaris_ Jnstn., enlarged. (From
Cunningham.) _a_, Prostomium; _c^1_, cirrus of peristomium; _c^2_, "gill";
_l_, lobes; _m_, podal membrane; I, peristomium; II, III, IV, following
segments.]
_Nerine_ is represented by two species, sometimes called "Ragworms." The
genus is very similar to _Spio_, but the worms are of larger size. The
prostomium is compressed by the forward growth of the peristomium, and
appears as a ridge on the latter segment, extending downwards in front
towards the mouth. The "gills" commence on the second segment, and are
continued in every segment except the hindmost. _Nerine_ (_Scolecolepis_)
_vulgaris_ Jnstn. is readily distinguished from other species by its
somewhat T-shaped prostomium. It is an extremely common worm under stones
and amongst seaweed at low water. It is some 3 or 4 inches in length and
more slender than the following species. Its colour is yellowish-brown, and
the red gills directed upwards and backwards give the appearance of oblique
red lines. The podal membrane does not reach the tip of the gill. North
Atlantic. It is {323}said to ascend rivers and live in brackish water. _N.
coniocephala_ Jnstn. is much the same colour, but reaches a length of 8
inches, and a diameter of ¼ inch. The prostomium is conical. The podal
membrane reaches to the tip of the gill in the anterior segments. The worm
burrows rather more deeply and nearer low-water mark than the preceding
species.
FAM. 2. _Polydoridae._—_Polydora_ Bosc (= _Leucodore_ Jnstn.) is readily
distinguished from the other Spionids, and, indeed, from any other
Polychaet (except _Chaetopterus_), by possessing specially strong chaetae
in the enlarged fifth chaetigerous segment. The anterior segments differ
from the rest in the absence of gills and in the character of the chaetae
(Fig. 133, A, p. 261).
_P. ciliata_ Jnstn. inhabits soft mud tubes near low water; it also makes
U-shaped galleries in stones and shells, and the tube projects from each
mouth. The worm is about ½ inch long, consists of some forty segments, and
is yellowish or flesh-coloured. The prostomium resembles that of _Spio_;
the peristomium is raised into a slight collar at each side. The anus is
surrounded by an incomplete funnel. The species has almost a world-wide
distribution, having been recorded from Iceland, Australia, the Philippine
Islands, as well as from the European seas. _P. coeca_ Oerst. often lives
commensally with a sponge, having a protective odour.
FAM. 3. _Chaetopteridae._—The family is represented on our coasts by
_Chaetopterus variopedatus_ Ren.,[383] which is found at the Channel
Islands, the Scilly Isles, the Isle of Man, and the west Scottish coast,
and probably at various other places, at low water and down to a depth of
some 15 fathoms. It occurs in all European seas. The animal builds a long
tube, the basis of which is a tough, parchment-like substance; this is
coated externally with sand, small pebbles, and other débris: it is of
considerable length and about ¾ inch in diameter, is U-shaped and open at
both ends, the greater part of it being embedded in sand or in crevices of
rocks. The animal, whose body-wall is thin and delicate, never leaves its
tube. The body has a bizarre appearance; three regions are readily
{324}distinguishable, which may be denoted by the letters A, B, and C. The
most anterior region, A, is flattened, and carries nine pairs of conical
lobes with delicate chaetae, though the fourth lobe possesses special
stouter chaetae (as in _Polydora_). The anterior end of the body terminates
in a wide funnel, the boundary of which is formed chiefly by the
peristomium; on its dorsal surface is a pair of tentacle-like processes
(peristomial cirri); the region between which represents the prostomium.
[Illustration: FIG. 173.—_Chaetopterus._ (From Panceri.) Natural size of a
young specimen. A is the anterior region of the body; B, the middle region;
C, the hinder region. _c_, Peristomial cirri; _d_, "sucker"; _e_, the great
"wings"; _f_, the first of the three "fans"; _m_, mouth.]
The second region, B, is very curiously modified; it is formed of five
segments. The most anterior is produced on either side into a great
wing-like process, which in life is directed forwards above the region A.
Each is grooved on its inner side, the ciliated grooves being continuous
with a median groove running forwards along the back of A; this apparatus
serves to bring food to the great funnel-like mouth. The next segment
(twelfth) carries a dorsal and ventral "sucker," representing the
parapodia. Each of the segments 13, 14, 15 carries a membranous fold
encircling the body. By the constant movement of these "fans," which have
nearly the same diameter as the tube, a current of water is constantly
washed over the animal. The fans represent the notopodia; the neuropodia
are bilobed rounded knobs. The region C consists in the adult of about
thirty segments, all alike, and less modified than the preceding. The
animal is the most truly tubicolous of the Polychaetes, and is much
modified on this account. No locomotor chaetae are present, though the
great wings and notopodial processes of region C contains chitinous
bristles, which, however, do not project;[384] the anterior region with its
stiff chaetae, and the neuropodial uncinal plates of the rest of the body
serve in its movements up and down the tube, while the "suckers" fix the
worm temporarily to the wall of its house.
{325}_Chaetopterus_ is highly phosphorescent (see p. 295). It is further
interesting on account of the green colouring-matter, which is extracted by
alcohol. Two commensal Polynoids occur in the tube, viz. _Polynoë glabra_
and _P. cirrosa_. The larva is "mesotrochal" (with a ciliated ring round
its middle), that region of the body lying in front of the cilia giving
rise to the region A, whilst the rest of the body gives rise to regions B
and C.
FAM. 4. _Magelonidae._—This family includes only the very peculiar worm,
_Magelona papillicornis_ Fr. Müll., which lives buried in sand, between
tide-marks, in various parts of our coast and that of the United States.
Its chief features are the large, flat, spoon-shaped prostomium; the long
peristomial cirri, slightly expanded terminally, carrying papillae along
one side; the enormous, eversible buccal region, which is an important
respiratory organ. The blood is of a madder-pink colour, and the
blood-vessels in the thorax are greatly dilated. The body of the worm is
divisible into two well-marked regions, owing to differences in the
chaetae.[385]
FAM. 5. _Ammocharidae._—This family contains only one species, _Owenia
filiformis_ D. Ch. Some of the anterior segments are longer than the hinder
ones, though the arrangement of chaetae is alike throughout. The mouth is
wide, like that of _Chaetopterus_, and is surrounded, except ventrally, by
a membrane, so deeply notched as to give rise to flattened filaments
containing blood-vessels. These "gills" appear to belong to the
peristomium. The small worm in its sandy tube is plentiful on our coasts in
about 20 fathoms. Off Greenland and the Mediterranean.
SUB-ORDER 3. TEREBELLIFORMIA.
FAM. 1. _Cirratulidae._—These worms have a cylindrical body, more or less
attenuated at each end; the segments are distinct, and similar throughout,
with capillary chaetae on each side in two bundles, carried by small
papillae. The prostomium is conical, the peristomium usually without cirri.
On more or fewer segments the dorsal cirri are long and filamentous, and
function as gills. There is a single pair of anterior nephridia: the septa
and genital ducts are repeated throughout the hinder part of the body. The
worms usually live in burrows.
{326}_Cirratulus._—The prostomium is long, sometimes annulated. In addition
to the segmental filamentous "gills" there is a transverse row of long
"tentacular filaments" across one of the anterior segments, and it has been
suggested that they are really prostomial tentacles which have shifted
backwards. They and the gills twist about in a very active fashion during
life, and look like small independent worms, especially when they are
detached. _C. cirratus_ Müll, is a brown or dirty yellowish worm about 4 to
6 inches in length, usually to be found under stones, partially embedded in
the mud or sand. The prostomium carries a pair of linear groups of
"eye-spots"; the first chaetigerous segment carries a transverse row of
tentacular filaments, and the red gill-filaments commence on the same
segment. Common. _C. tentaculatus_ Mont. is a larger worm, dark red in
colour, and is distinguished from the preceding by the absence of eyes and
by the fact that the tentacular filaments are on the seventh chaetigerous
segment, while the gills commence more anteriorly. Atlantic and the
Mediterranean.
[Illustration: FIG. 174.—_Cirratulus tentaculatus_ Mont. (½ nat. size).
(From _Règne Animal_.)]
_Chaetozone setosa_ Mgrn. occurs in the North Atlantic. _Dodecaceria
concharum_ Oerst. is about an inch in length, olive-green or brownish in
colour, and is not uncommon amongst roots of _Laminaria_. It is stated to
live also in tortuous tubes bored in shells and stones, but whether it
makes these tubes is uncertain. The worm has two thick tentacular
filaments, and the thinner gills are only on four segments.
_Hekaterobranchus shrubsolii_ Buch.[386] is a small worm some ½ inch long,
found at Sheerness, where it occurs at low tide in soft mud; here it forms
a loosely coherent tube, though it also moves freely in the mud. Its chief
features are (1) a pair of long, ciliated {327}"cephalic tentacles,"
probably peristomial, and similar to the "tentacular filaments" of
_Dodecaceria_; (2) a pair of filamentous gills (dorsal cirri) on the first
chaetigerous segment; (3) a pair of large green nephridia in the anterior
segments. The describer placed it amongst the Spionids, but the above and
other features point to Cirratulid affinities.
FAM. 2. _Terebellidae._—The body is cylindrical, and generally larger in
front than behind. The prostomium is generally flattened, and forms a
mobile upper lip, which always carries a transverse series of many
tentacles; it may bear "eye-spots," but never palps; the lower lip is
formed by the peristomium. There are one to three pairs of gills, which are
usually more or less branched, on as many segments.[387]
The chaetigerous lobes are small; the dorsal ones contain capillary
chaetae, which are frequently confined to the anterior segments, whilst the
ventral chaetae are uncini. The ventral surface of the anterior segments is
thickened by glands which secrete the mucus employed in tube-building; the
number of these "shields" and of the dorsal bundles of chaetae have to be
noted in identifying the worms. There are one to three pairs of large
anterior nephridia. A very strong "diaphragm"—usually more or less
pouched—cuts off this anterior region of the body-cavity from the rest, and
is the only complete septum in the body; from three to twelve pairs of
small generative ducts occur behind it. The family is tubicolous, foreign
materials being generally used in the formation of the tube.
[Illustration: FIG. 175.—_Amphitrite johnstoni_ (½ nat. size). _g_, Gills;
_t_, prostomial tentacles. (From Cunningham and Ramage.)]
There are six genera which are fairly common round our coast, and their
identification may be facilitated by means of the following table[388]:—
{328}
{ equal in _Amphitrite_.
{ 3 pairs, { size. 24 notopodia.
A. Capillary chaetae { which are {
confined to the { { unequal in _Terebella_.
anterior part of Gills { { size. 17 notopodia.
body; commencing ramose. {
on the fourth { { Gills equal.
segment. { { Eye-spots. _Nicolea._
{ 2 pairs. {
{ 17 notopodia. { Gills unequal
{ and of _Pista._
{ peculiar shape.
B. Capillary chaetae { arborescent; 3 pairs. _Leprea._
throughout the {
body; commencing Gills { filiform; in transverse
on the third { series in two _Thelepus._
segment. { segments.
_Amphitrite johnstoni_ Mgrn. (Fig. 175) is brown in colour, about 4 to 6
inches in length, and nearly ½ inch in breadth anteriorly. Each of the
gills consists of a curved stem; from the convex side of which arise a
number of branches, themselves dichotomously divided, the final branches
being long (Fig. 176, A). There are twelve ventral "gland shields." The
worm is fairly common between tide-marks, below stones in muddy places: the
end of its tube of mud projects above the surface. Atlantic.
_Terebella_ (_Polymnia_) _nebulosa_ Mont. is distinguished by its bright
red colour, spotted with white; it is 6 or 7 inches in length, and ½ inch
across. Large specimens of this beautiful worm may be obtained at Weymouth
and elsewhere on the south coast, where it lives in about 14 fathoms. Each
gill appears much more arborescent than in the preceding (Fig. 176, B); it
consists of a main stem, from which comparatively few branches arise; these
subdivide frequently, and the terminal branchlets are quite short. The
"gland shields" are fourteen to sixteen in number. The tube is of mud.
North Sea and Mediterranean. _T._ (_Leprea_) _lapidaria_ L. is 1 inch in
length, orange-red in colour; and has 12 ventral shields. The tube, of fine
mud, lies horizontally on the under surface of stones.
_T._ (_Lanice_) _conchilega_ Pall. (the "sand mason") forms a very
characteristic tube of sandy particles, small pebbles, and pieces of shell.
It is buried in the sand, but a short portion protrudes, and bears, set
round its edge, a fringe of branching sandy threads (Fig. 153) commonly
seen on sandy shores between tide-marks. The worm may be distinguished from
the preceding species by the fact that the series of fourteen to
{329}seventeen gland shields are red, and continuous from segment to
segment. The gill is shown in Fig. 176, C. North Sea, Atlantic, and
Mediterranean.
[Illustration: FIG. 176.—Gills of various Terebellids. × 4. A, _Amphitrite
johnstoni_ Mgrn.; B, _Terebella nebulosa_ Mont.; C, _T. conchilega_ Pall.;
D, _Nicolea_ Mgrn. (the finer branches are not indicated); E, _Pista_
Mgrn.; F, _Terebellides_ Sars (after Malmgren). _g_, Gill; _m_, mouth; _t_,
tentacles.]
_Nicolea venustula_ Mont. has only two pairs of equal, arborescent gills
(Fig. 176, D); the tentacles are comparatively few. The animal, which is
about an inch in length, is cinnamon-yellow with white spots, and has
seventeen gland shields. 20 fathoms, North Sea and Mediterranean. _Pista
cristata_ Müll. is readily recognised by the shape of the gills (Fig. 176,
E), of which there are only two pairs. Each consists of a long peduncle,
bearing a number of dichotomously dividing, rather spirally-arranged
branches, the whole having the appearance of a "bottle-brush." The worm is
2 to 4 inches long, of greyish-red to yellow colour. Atlantic east and west
(even at the mouth of the Congo), and Mediterranean. _Thelepus cincinnatus_
Fabr. is about the same length, pale red in colour, marked on its back with
clear areas, giving the appearance of lacework. The gills are represented
by numerous unbranched filaments arising separately in two transverse rows.
The tube, which is adherent to shells, etc., along its whole length, is of
thin, transparent, and flexible material like mica, covered with foreign
bodies, and even with Polyzoa and Hydrozoa. 30 fathoms, Atlantic and
Mediterranean.
{330}_Polycirrus aurantiacus_ Gr. is sometimes placed in a special
sub-family, as it has no gills. The numerous tentacles are very long, and
arise from a great hood over the mouth; the capillary chaetae commence in
the first segment and extend for about half the length of the body; the
uncini commence in the ninth segment. The ventral "shields" are paired. The
animal is highly coloured; its phosphorescence and its distastefulness have
already been mentioned on p. 294. In _Terebellides stroemi_ Sars, four
comb-like gills arise from a single common thick peduncle on the back of
the second segment (Fig. 176, F). The ventral surface of the body bends
upwards anteriorly so as to bring the mouth to the dorsal surface. 13 to 16
fathoms, muddy bottoms, North Sea and Mediterranean.
FAM. 3. _Ampharetidae._—This family differs from the Terebellids chiefly in
the shape of the head and in the presence of a bundle of strong chaetae (or
paleae) on each side of the head in front of the gills, of which there are
four on each side. Each gill is a simple filiform process, considerably
longer than the tentacles, which are very few in number. _Amphicteis
gunneri_ Sars, _Ampharete gracilis_ Mgrn., and _Melinna cristata_ Sars,
occur on our coasts.
[Illustration: FIG. 177.—_Pectinaria belgica_ Pall. Slightly enlarged. _a_,
Neuropodial chaetae; _b_, notopodium; _ch_, paleae; _g_, gills; _sc_,
scapha; _t_, prostomial tentacles; _I_, peristomium. (From Malmgren.)]
FAM. 4. _Amphictenidae._—This contains the interesting genus _Pectinaria_,
in which the head is protected by great golden chaetae on the second
segment; they are flattened, curved, and pointed, and are arranged in a
single transverse row on each side, serving as an operculum to the tube.
The posterior end of the worm has undergone great degeneration, and is
represented by a small leaf-like "scapha" which serves to close the tube
posteriorly. The worm is 1½ inches in length and consists of only twenty
segments, of which seventeen are chaetigerous. The tube is nearly
cylindrical, but wider anteriorly than posteriorly (Fig. 152, p. 288); the
sand grains are uniform in size, and are embedded in the secreted mucus in
a very regular way, the surface being smooth both {331}inside and out.
These tubes can be carried about by the worm, but may be found projecting
from the sand at very low tides. _P. belgica_ Pall. forms a straight tube,
whilst in _P. auricoma_ Müll. the tube is larger and slightly curved. The
former species appears to be confined to the North Sea; the latter occurs
in deeper water, and is also present in the Mediterranean.
SUB-ORDER 4. CAPITELLIFORMIA.
FAM. _Capitellidae._[389]—_Capitella capitata_ v. Ben. occurs pretty
frequently in the sand under stones near low tide-mark. It is a red worm,
about 1½ to 2 inches long, greatly resembling a Tubificid Oligochaete. It
may readily be distinguished from other Polychaeta by the strong genital
chaetae in the male, which replace the notopodial chaetae of segments 8 and
9; those in the former segment are pointed backwards, and in the latter
forwards. There is but a single pair of generative ducts in either sex in
the eighth segment. North Sea, Mediterranean.
_Notomastus latericeus_ Sars is a longer worm, living in shallow water, off
our coast and in the Mediterranean. The anterior twelve segments are wider
than the rest of the body. The notopodial chaetae of the anterior segments
of the hind body form a ring. _Dasybranchus caducus_ Gr., which occurs in
the Mediterranean, but not on our coast, reaches a length of 2 or 3 feet.
It has gills on the hinder segments above the neuropodia.
_Mastobranchus_ Eis. is found in the Mediterranean.
SUB-ORDER 5. SCOLECIFORMIA.
FAM. 1. _Opheliidae._—Comparatively short, rather ugly worms of a pearly
colour, no prostomial processes: parapodia obscure. The family is
represented in British waters by four species, occurring in shallow water.
_Ammotrypane aulogaster_ Rathke, is about 2 inches long; the nearly
cylindrical body has a ridge running along each side below the chaetae. The
conical prostomium is tipped with a small knob, and carries at each side a
ciliated pit (Fig. 144, p. 273). Every segment, except the first
chaetigerous, is provided with a filamentous gill (dorsal cirrus). The
segmentation {332}is very obscurely marked, for internally there are only
three complete septa, placed far forwards. The intestine is bent upon
itself. In _Ophelia limacina_ Rthk. the gills commence in the eighth
segment, and the longitudinal ridge does not extend in front of this
segment. The worm is about 1½ inches long, and occurs between tide-marks.
_Travisia forbesi_ Jnstn., North Sea. _Polyophthalmus pictus_[390] Duj. is
very abundant at some parts of the coast. There are two bundles of chaetae
on each side of every segment; each bundle contains three chaetae, of which
only one projects to any distance. Paired eye-like spots exist on the sides
of twelve segments. The worm is about an inch in length.
FAM. 2. _Maldanidae_ (= _Clymenidae_).—Represented on our coasts by four
fairly common species. They form sandy tubes, which are embedded in the
sand with a short portion projecting. In some places they are so abundant
that at low water the sand has quite a rough appearance. The prostomium is
frequently truncated and depressed, and is always fused with the
peristomium. A horny plate may be developed on the upper surface of the
head, and the skin at the side of the prostomium is frequently raised into
a more or less prominent fold. The hinder end of the body carries a funnel
surrounding the anus. There are no gills or sensory processes on the body.
Some of the segments towards the middle of the body may be longer than the
rest. Peculiar serrated hooks of characteristic shape constitute the
neuropodial "torus." The buccal region is eversible.
_Nicomache lumbricalis_ Fabr. is a rosy-pink worm with white spots
anteriorly; the chaetigerous ridges are red. The worm consists of
twenty-six segments, and measures 2 or 3 inches. It is very narrow and
readily breaks in pieces. The prostomium is laterally compressed; the anal
funnel is fringed with a number of short equal processes. Under stones in
the Laminarian zone.
[Illustration: FIG. 178.—Anal funnel of _Nicomache lumbricalis_. (From
Malmgren.)]
_Axiothea catenata_ Mgrn., which may reach a length of 3 or 4 inches,
resembles the above in general colour, though of a deeper tint. There are
only eighteen chaetigerous segments. The head has a membranous fold of skin
on each side, and the anal funnel is produced into longer and shorter
processes. Both {333}these species are also found on the west side of the
Atlantic. _Clymene lumbricoides_ Qfg. is about 8 inches long; pink, with a
light ring round each segment; the seventh segment is larger and
reddish-brown. The prostomium is laterally compressed. Anus on a cone,
which rises from the bottom of a funnel, the margin of which is entire.
Atlantic.
[Illustration: FIG. 179.—_Axiothea catenata._ × ½. _a_, Anterior end; _b_,
notopodial and _c_, neuropodial chaetae; _p_, perianal funnel. (From
Malmgren.)]
[Illustration: FIG. 180.—_Arenicola marina._ × 1. Dorsal view. The anterior
end is seen partly from the side. _III_, The first chaetigerous segment;
_IX_, the ninth chaetigerous, and first branchial segment; _XXI_, the last
branchial segment; _b_, notopodial and _c_, neuropodial chaetae; _g'_, _g_,
the first and last gills; _t_, the non-chaetigerous tail.]
FAM. 3. _Arenicolidae._—Here belongs the common "lug-worm" _Arenicola
marina_ L., which occurs all round our coasts between tide-marks, and is so
generally used as bait in fishing. The worm, which measures 5 to 8 inches,
is of a dark tint, usually brownish-green. It burrows to a depth of some 18
inches or 2 feet, and throws up a considerable quantity of "castings" or
"sand-ropes," which are noticeable on every shore consisting of mud or
muddy sand. The body of the worm is cylindrical, thicker anteriorly; the
segments are indistinct, owing to the secondary groovings and furrows on
the skin. The prostomium is in the adult fused with the peristomium; this
and the second segment are achaetous. Then follow twenty chaetigerous
segments with dorsal bundles of capillary chaetae and ventral rows of short
crotchets. The hinder region of the body is achaetous and narrower than the
rest, forming a "tail." There are twelve (sometimes thirteen) pairs of
arborescent red gills on segments 9 to 20 (21). Internally there are
{334}only four complete septa, and six pairs of nephridia, which are of
large size: the fore-gut is eversible. Atlantic and Mediterranean. A second
and smaller species, _A. ecaudata_ Jnstn., occurs on our southern coasts;
it is readily distinguished by the absence of a "tail," the chaetae and
gills being continued to the end of the body.
FAM. 4. _Scalibregmidae._—Prostomium ill-marked, fringed with small
processes. Parapodia represented by slight papillae; two bundles of
chaetae; usually cirriform lobes above and below them. _Lipobranchius
jeffreysii_ M‘I. has a grub-like body pointed at each end; forms tubes of
mud. Firth of Clyde and elsewhere in North Sea. _Sclerocheilus_ Gr. in old
oyster shells. Channel, Mediterranean. _Eumenia crassa_ Oerst. has gills on
first six segments. North Sea. _Scalibregma inflata_ Rthke. has arborescent
gills on segments 4 to 7. The anterior part of the body is dilated. North
Sea.
FAM. 5. _Chlorhaemidae._—The family derives its name from the green colour
of the blood, due to chlorocruorin. The representatives are comparatively
short worms, with capillary chaetae on all the segments, the limits of
which are not evident. The prostomium carries a pair of long grooved
yellowish processes, which are perhaps palps, and several green tentacles,
acting as "gills," arranged in a transverse series above the mouth (Fig.
134, F, p. 262). The peristomium is achaetous; the whole "head" can be
withdrawn into the body. The chaetae of the anterior segments are
especially long, and directed forwards so as to form a "cage" for the head.
The body-wall is covered with longer or shorter papillae. Internally, the
chief points of interest are the presence of only two septa (_Trophonia_)
or only one septum (_Siphonostoma_), situated somewhere in front of the
middle of the body, and forming a great backwardly-directed pouch, which
contains a part of the looped intestine, and the nephridia, of which there
are only two or four.
_Trophonia plumosa_ Müll. is about 2 to 4 inches long, yellowish-brown in
colour, with a rough skin; the head is usually retracted. It lives in the
mud amongst Laminarian roots down to 50 fathoms. North Atlantic.
_Siphonostoma_ (_Flabelligera_) _diplochaitos_ Otto, has a transparent
body-wall, so that the coloured viscera are visible. The skin carries long
papillae, which traverse a thick jelly-like envelope secreted by it, in
{335}which numerous diatoms live (symbiotically?); the surface is covered
by particles of mud, etc. This species, which may be found under stones at
low tide, occurs also in the Mediterranean.
[Illustration: FIG. 181.—_Sternaspis scutata_ Ranz. × 2. (From Vejdovsky.)
The left figure shows the ventral surface; the right represents the
internal organs as seen when the body-wall is pinned aside, having been
slit up along its dorsal surface. _a_, Anus; _c_, gills; _ch_, anterior
strong chaetae; _ch^1_, bundles of chaetae along the lateral margin of the
shield; _ch^2_, the posterior marginal chaetae; _d_, oviduct; _g_, the
external tube carrying genital pore; _i_, coiled intestine; _n_, nephridium
("brown tube"); _o_, ovary, amongst the coils of the alimentary canal; _p_,
pharynx; _pr_, prostomium, with mouth just behind it; _s_, shield (on left
figure); _s_, stomach (on right); _sh_, outline of shield seen through the
ventral body-wall (in right figure); _x_, chaetae embedded in the
body-wall, with nerves passing by them.]
FAM. 6. _Sternaspidae._—The single genus, _Sternaspis_, has not been
recorded on our coasts, but is of so peculiar a structure as to deserve a
description.[391] _S. scutata_ Ranzani, occurring in the Mediterranean, is
rather less than an inch in length, and derives its name from the
possession of a pair of peculiar "horny" plates or shields on the ventral
surface posteriorly. Around their margins are set about thirty bundles of
long capillary chaetae. There are three half rings of stronger chaetae on
each side near the anterior end of the body. The mouth is overhung by a
very {336}small rounded knob (prostomium), which in _S. spinosa_ Sluiter,
is prolonged outwards on each side to form a grooved palp-like organ,
recalling that of _Bonellia_. The anus is placed posteriorly; and in front
of it, on the dorsal surface, are two bundles of many long thread-like
gills. On the ventro-lateral surface, in front of the middle of the body,
is a pair of finger-shaped processes containing the genital ducts. The
anterior segments of the body can be withdrawn into the body, as in the
_Chlorhaemidae_. Further examination leads to the conclusion that the body
of _Sternaspis_ consists of about thirty segments, most of them provided
with paired bundles of capillary (neuropodial?) chaetae, distributed as
follows:—Each of the segments 2, 3, 4 has a half ring of strong chaetae on
each side; segments 5 to 7 are without chaetae; segments 8 to 14 have
chaetae embedded in the body-wall, but not projecting. The shields cover
the remaining segments; and along the outer edge of each are some ten
bundles of chaetae, and along the hinder edge some five or six bundles,
representing as many segments. Thus the worm consists of about thirty
segments whose outlines are nearly obliterated (as in the _Chlorhaemidae_),
and whose chaetae, except those which are specially developed, are
disappearing: while posteriorly a great shortening of the body brings the
bundles close together. A continuation of this process, involving a further
disappearance of chaetae, leads readily to the condition met with in
_Echiurus_, one of the chaetigerous Gephyrea. Internally, further evidence
of the relation between _Sternaspis_ and the Chlorhaemids with the Gephyrea
is afforded by the absence of septa, by the coiled alimentary canal, and by
the presence of a single pair of nephridia, which in the latter group act
both as excretory organs and as genital ducts.
BRANCH B. CRYPTOCEPHALA.
SUB-ORDER 1. SABELLIFORMIA.[392]
FAM. 1. _Sabellidae._—The branchial crown consists of a usually
considerable number of filaments arising from a semicircular base. The
peristomium may be reflexed to form a collar, {337}which is frequently
notched, so that a lateral and a ventral lobe on each side may be
distinguished (Fig. 133, B, p. 261). The thorax consists of nine segments,
and is provided with ventral "gland shields," which are continued along the
abdomen, where they are subdivided into two by a ciliated "faecal groove,"
which sometimes bends to one side on reaching the thorax, and may extend
forwards along the dorsal surface to the head: this groove serves to carry
the faeces out of the tube. The gill filaments are always provided with
secondary processes, and may be provided with compound eyes.[393] The tubes
of the Sabellidae are always of fine mud or of sand.
_Sabella pavonia_ Sav. is about 10 to 12 inches long and about ¼ inch
across; the tube of fine mud is considerably longer and embedded deeply in
the mud, with its free end projecting to some 2 or more inches, where it
serves for the attachment of seaweed, Polyzoa, Hydrozoa, etc. The colour of
the animal is orange-brown; the gills, which are about 1½ inches long, are
green (due to contained blood) marked with more or less extensive brown or
purple-brown spots, which may even hide the green tint. There is a pair of
dark filaments arising between the dorsalmost gill filaments, which have
been erroneously regarded as "prostomial tentacles"; they are, in fact,
prolongations of a peculiar membrane or lip round the base of the gills,
which bounds a groove leading to the mouth. These lip-processes (Fig. 133,
B, _l_) occur in other Sabellids. Atlantic, North Sea, and Mediterranean.
[Illustration: FIG. 182.—A, A gill filament of _Branchiomma_; B, of
_Dasychone_. _a_, Axis; _f_, secondary filaments; _o_, eye; _x_, dorsal
appendices.]
_Branchiomma vesiculosum_ Mont. forms a sandy tube near low tide-mark. The
animal, measuring 6-7 inches, is rich brown, darker anteriorly, abundantly
speckled with white; the ventral surface is pink; the gills are green or
{338}olive-brown, marked with white bands in a fairly regular fashion. Each
gill filament has, just below its tip, a compound eye, consisting of
several lenses and retinae. North Sea and Mediterranean.
_Dasychone bombyx_ Dalyl. is a short, comparatively stout worm usually 1 to
1½ inches long; reddish-brown in colour, with a darker spot on each side of
every segment. The gills are lighter with greenish marks. This worm may
readily be recognised, for each of the gill filaments carries some six to
ten pairs of dark compound eyes at intervals along its length, and near to
each pair there arise two short processes from the outer side of the
filament, which are known as "dorsal appendices." The worm forms a tube of
mud, more or less mixed with sand. It occurs at low water and to some depth
round the coasts of the Atlantic, North Sea, and Mediterranean.
_Chone infundibuliformis_ Kröyer may be recognised by the absence of lobes
on the collar, the presence of a membrane connecting the gill filaments,
and the passage of the faecal groove along the dorsal surface of the
thorax. The worm is 6 inches long, with purple gills, spotted with
yellowish-white. The tube is formed of yellowish membrane covered with
sand, and is fixed to stones and other objects. _Potamilla reniformis_
Müll. is about 3 inches long, with about twelve brown gill filaments, some
of which have eyes near the base. The tube is transparent and horny, with
sometimes a slight covering of sand. Found in old oyster shells. North Sea,
Atlantic, Mediterranean.
The genus _Spirographis_ contains one of the largest European Sabellids,
_S. spallanzanii_ Viv., which occurs off the Channel Islands and in the
Mediterranean. The two gill plumes are unequal; the large rone forms an
upright, spirally-coiled column.
FAM. 2. _Eriographidae._—_Myxicola infundibulum_ Mont. has its gill
filaments connected by a membrane reaching nearly to their tips. Each gill
plume forms a semicircle; there are no eyes; the peristomium does not form
a collar; no gland shields. The worm requires neither of these structures,
since it is practically a free-swimmer, envelopes itself in mucus, and
moves tail first. The faecal groove is not well marked, though continued
dorsally. In the abdomen the _tori uncinigeri_ extend dorsally and
ventrally {339}(beyond the neuropodial chaetae) and nearly encircle the
body. The animal is 4 or 5 inches long, dull green, with purplish gills.
Between tide-marks. North Sea and Mediterranean.
_Amphiglena mediterranea_ Leyd. is only about ¼ inch long, hermaphrodite,
and has eyes on the peristomium and on the anal segment. It is a very
elegant little worm, and as a living object under the microscope, with the
cilia on the gills, is very beautiful. The gills consist of six filaments
on each side, provided with the usual double row of ciliated processes.
FAM. 3. _Amphicorinidae._—Small hermaphrodite Sabellids in which each gill
tuft contains only a few branching filaments. The simplest form is
_Haplobranchus aestuarinus_ Bourne,[394] which occurs in the rather foul
mud at low tide in the estuaries of the Thames, the Liffey, and other
rivers. The animal is about ¼ inch long, with four finger-shaped processes
on each side, and a pair of larger, vascular processes on the ventral
surface. These five branches are gills (palps), although, owing to the
small size of the worm and simple vascular system, the four lateral
filaments have no blood-vessels. The animal consists of only eleven
chaetigerous segments, and lives in a tube made of mud particles.[395]
_Fabricia sabella_ Ehrenb. (_Amphicora fabricia_ Müll.) has three gills on
each side, each with a number of secondary branches of different sizes, but
so arranged as all to reach the same level. It has eyes in its tail and
swims backwards.
FAM. 4. _Serpulidae._—The thorax is provided with an undulated membrane on
each side, chiefly employed in smoothing the inside of the tube; it
represents the dorsal and ventral cirri of these segments. The gland
shields are confined to the thoracic segments. In many genera the
dorsalmost gill filament on one or both sides is terminally dilated and
serves as an operculum. The tube is calcareous, and attached to rocks,
shells, etc., for a greater or smaller part of its extent.
_Serpula vermicularis_ L. forms a pinkish tapering tube about 3 inches
long; the narrower fixed end is coiled. It is marked at irregular intervals
with encircling ridges, indicating cessation {340}in formation, and has a
circular aperture. The worm itself is about 1½ inches long. The horny
operculum is conical, with its base upwards, fringed with short processes.
20 fathoms in the North Sea and Mediterranean; also from 275 fathoms off
the west coast of Ireland.
_Pomatoceros triqueter_ L.—The white shell is adherent, with a distinct
keel along its upper surface; the aperture is overhung by a spine. The
tubes are abundant everywhere, attached to rocks, stones, shells, etc.,
between tide-marks and down to 18 fathoms. The animal is very handsome, the
thorax being deep blue, the abdomen red in the female, whitish in the ripe
male. The branchiae are barred and spotted with blue, orange, and white;
the operculum is calcareous, and furnished with a couple of horn-like
processes.
[Illustration: FIG. 183.—A group of tubes of _Serpula vermicularis_ L.,
from the mouths of two of which the animals are protruding, that on the
right being seen from above. _br_, The gill plume; _m_, thoracic membrane;
_op_, operculum; _op^1_, corresponding gill filament of the opposite side;
_t_, tube. (From Cuvier's _Règne Animal_.) Nat. size.]
_Filigrana implexa_ Berkeley is a small worm, but the slender white tubes
intertwine and adhere together in masses 3 or 4 inches high, occurring at
low tide and down to 18 fathoms in the North Sea and Mediterranean. The
animal has only eight gill filaments on each side, one of which on each
side is slightly expanded to serve as an operculum. The worm multiplies by
transverse division.
_Spirorbis borealis_ Daudin is a still smaller worm, the tube of which is
coiled in a flat spiral about 1/16 to ⅛ inch across; it is {341}common,
adhering to _Fucus_, shells, and other objects. It is represented by
fossils in the Palaeozoic rocks. Cosmopolitan.
_Protula_ (_Psygmobranchus_) _tubularia_ Mont. is a Serpulid without an
operculum; it forms a straight or slightly and irregularly curved tube.
Atlantic and Mediterranean. _Salmacina dysteri_ Huxley has no operculum; it
is a small worm incrusting seaweeds, or forming masses like _Filigrana_.
SUB-ORDER 2. HERMELLIFORMIA.
The single family _Hermellidae_ is represented by two species—_Sabellaria
alveolata_ L., which is littoral, and _S. spinulosa_ Leuck., occurring in
10 to 30 fathoms.
_S. alveolata_[396] is about an inch long; the thorax is purple, the
abdomen yellow to red. The narrow caudal region is bent sharply forwards,
so that the anus, situated at its tip, comes to lie at the orifice of the
tube, which is irregular and sandy. Great numbers of the animals live
together, so that the masses of their tubes may be 2 or 3 feet thick and
several feet long. They are well seen on the shore, at Paignton, near
Torquay, and on Hilbre Island, off the Cheshire coast. North Sea, Atlantic,
Mediterranean.
[Illustration: FIG. 184.—_Spirorbis._ A, the shell, enlarged. B, the
animal, × 50. _c_, Peristomial collar; _e_, eggs in the brood pouch at the
end of the operculum; _g_, gills; _m_, thoracic membrane (characteristic of
Serpulidae); _n_, the single pair of thoracic nephridia opening by a median
dorsal pore beneath the collar (common to all Sabelliformia); _o_, ova in
the anterior abdominal segments; _op_, operculum; _s_, spermatozoa in the
hinder abdominal segments; _st_, stomach. (From Claparède.)]
ORDER III. MYZOSTOMARIA.[397]
These animals are parasitic on Crinoids or Asteroids.[398] The {342}single
family, Myzostomatidae, contains but two genera, _Myzostoma_ F. S. Lkt. and
_Stelechopus_ v. Gr.
Some of them move, more or less actively, on the surface of their hosts,
others live in a sessile condition between the joints of the arms or
pinnules, causing a greater or less malformation thereof, sometimes leading
to the formation of a more or less globular cyst like a plant-gall, due to
overgrowths of the joint, as in _M. deformator_ v. Gr. and _M. cysticola_
v. Gr.: while _M. pulvinar_ v. Gr. is endoparasitic in the intestine. Two
species occur on our common _Antedon_ (_Comatula_) _rosacea_; one, _M.
cirriferum_ Lkt., creeps about the oral surface, especially along the food
grooves of the disc and arms; the other, _M. glabrum_ Lkt., lives close to
the mouth of the Crinoid, so that its pharynx can be inserted into the
oesophagus of the host; this species rarely moves from this position, and
carries a young one on its back.
[Illustration: FIG. 185.—_Myzostoma glabrum_ Lkt., on the disc of _Antedon
rosacea_. The hermaphrodite individual (⚥) lies over the mouth of the
_Antedon_, and carries on its back at the anterior end a young one (♂) with
only male organs fully developed. _ap_, The anal papilla of _Antedon_. ×
4.]
The Myzostomaria are circular or oval, more or less markedly convex
dorsally, flat ventrally; _Stelechopus_, however, which lives on
_Hyocrinus_, is elongated. The margin of the body is provided with ten or
more pairs of cirri, short (_M. glabrum_) or long (_M. cirriferum_), and
the general appearance of the animal is greatly changed in some species by
the great elongation of the hinder cirri, into which the viscera may extend
(_M. filicauda_ v. Gr.). On the ventral surface are five pairs of small
conical "parapodia," arranged, like the internal organs, in a radiate
manner. Each parapodium carries a couple of chaetae; one a hook, the other
serving as a "guide" for this hook. The four "suckers" on each side are
either glandular or sensory organs; and Wheeler considers them homologous
with the lateral organs of Capitellids; they are usually little developed
in those species which live inside cysts.
{343}[Illustration: FIG. 186.—_Myzostoma cirriferum._ (After Lang and
v. Graff.) The organs are supposed to be seen by transparency. On the right
side the more dorsal organs are shown, and on the left, those lying more
ventrally. _a_, Anus; _c_, ten pairs of marginal cirri; _f^1_ to _f^5_, the
five parapodia of the left side, each with two chaetae; _i_, the branches
of the intestine on the right side; _m_, mouth; _o_, the opening of the
oviduct (_od_) into the rectum; _ov_, the uterus or coelom, filled with
eggs, occupying the spaces between the lobes of the intestine; _p_, the
pharynx (acrecbolic introvert) lying in the pharyngeal sac; _r_, rectum;
_s_, the four "suckers" of the left side; these, like the parapodia, really
lie on the ventral surface; _st_, stomach; _t_, the branching testis; ♂,
the pore of the sperm-duct.]
The mouth and anus are usually ventral; but in _M. glabrum_ the anus is
dorsal, and in a few species both apertures are carried on to the back by
the great development of the ventral surface. The alimentary canal is
provided with a protrusible pharynx; the intestine is branched; amongst its
branches is the coelom, packed with eggs, and functioning as a uterus
(usually called "ovary"). The true ovary is a small mass of cells on each
side, a proliferation of the coelomic epithelium covering the intestinal
wall. A median continuation of the {344}uterus passes backwards above the
rectum, and opens either into it or by an independent pore dorsal to the
anus. The "lateral oviducts" of Nansen are nephridia with ciliated funnels
opening into the coelom (uterus), and with pores leading into the cloaca on
its ventral surface; or, in _M. belli_ Wheeler, opening to the exterior.
The two testes are branched, and each sperm-duct opens laterally on a
papilla, just outside the third parapodium of each side. Wheeler[399] has
recently shown that in the young _Myzostoma_ the spermatozoa ripen before
the ova, so that it is functionally a male; before the spermatozoa are all
discharged the ova mature, and the animal is for a time hermaphrodite;
later on, however, when all the spermatozoa are used up, the worm is a
female. Beard's "dwarf males" are therefore merely the young of
hermaphrodite forms. In cysticolous species each cyst usually contains a
large female individual and a small male. In these cases the young one
(male) discharges all its spermatozoa before the ova ripen, so that a
period of immaturity intervenes and a true hermaphrodite condition is
omitted; the animal is at first male, and later female. The Myzostomaria
are thus "protandric hermaphrodites."
The affinity of these animals has been much discussed; they superficially
resemble the Tardigrada in many anatomical features, and differ greatly
from Chaetopoda, but as they possess the characteristic chaetae or
parapodia, and pass through a larval stage[400] similar to that of the
Polychaetes, there is no doubt that they are closely allied to the group,
and indeed may be regarded as degenerate Chaetopods. It has been suggested
that they form a passage group between them and the Tardigrada; and von
Graff forms a group Stelechopoda, to include the Myzostomaria, the
Tardigrada, and the Linguatulida.
OLIGOCHAETA (EARTHWORMS, ETC.) AND HIRUDINEA (LEECHES)
BY
F. E. BEDDARD, M.A. (Oxon.), F.R.S.
Prosector to the Zoological Society
{347}CHAPTER XIII
OLIGOCHAETA (EARTHWORMS AND THEIR ALLIES)
INTRODUCTION—ANATOMY—REPRODUCTION—BIONOMICS—DISTRIBUTION—CLASSIFICATION—
MICRODRILI AND MEGADRILI
The Oligochaeta form a well-marked branch of that exceedingly large
assortment of animals vaguely spoken of as worms, and embracing a number of
types many of which have no near relationship to each other. From this
great and unnatural group, which has survived as "Vermes" even in some
quite modern text-books, we can separate off those forms which show a plain
segmentation or division of the body internally as well as externally into
a series of more or less similar rings, as a Class Chaetopoda. This Class,
consisting of the Orders mentioned on p. 241, includes the worms which form
the subject of the present chapter—the Oligochaeta, as they were originally
called by Grube, on account of the fewness of their chaetae as compared
with the number possessed by the majority of the Polychaeta.
Our knowledge of this group, as of so many others, dates from Aristotle,
who called the earthworms the "intestines of the earth." But it is only
very recently that the numerous and remarkable genera of exotic earthworms
have been anatomically investigated; indeed the common British species was
not really well known before the publication of the memoirs of
Lankester[401] and Claparède[402] in 1864 and 1868, in spite of the
elaborate quarto devoted to it by Morren,[403] the botanist, in 1826.
{348}Some of the aquatic species afforded material to Bonnet and
Spallanzani for their experiments upon the powers of regeneration of the
animals when cut into fragments, while the work of O. F. Müller[404] upon
various Naids is a monument of careful anatomical description. Our
knowledge of the aquatic Oligochaeta does not appear to have advanced so
rapidly as has that of the earthworms.
EXTERNAL CHARACTERS.—The most salient external characteristic of this group
of worms, which vary from 1 mm. to 2 metres in length, is of course the
segmentation. The entire body is divided into a number of rings, which are
for the most part similar to each other; a fragment of an earthworm's body
could not be accurately replaced unless it had been cut from the anterior
region. There is precisely the same regular segmentation in the aquatic
representatives of the Order. At the anterior end of the body in the common
earthworm (and in nearly all Oligochaeta) is a small unpaired lobe, which
overhangs the mouth, and is usually termed the prostomium; the mouth itself
is surrounded by the first segment of the body, which never bears any
chaetae in any Oligochaete. The prostomium is occasionally greatly
developed, and in such cases doubtless forms a tactile organ of importance.
This is especially the case with the South American genus _Rhinodrilus_,
where the lengthy prostomium can be retracted at will. The aquatic _Nais
lacustris_ (= _Stylaria proboscidea_) has also an exceedingly long
prostomium, which cannot, however, be retracted, though it is contractile.
At a certain distance from the anterior end of the body, fixed for the
species, but varying greatly from genus to genus and from species to
species, is the clitellum. This region of the body (popularly believed to
mark the spot where a worm divided by the gardener's spade has come
together again) is associated with the reproductive function, and serves to
secrete the cocoon in which the creature's eggs are deposited. It has in
the earthworm a thick glandular appearance. A more minute examination of
the worm's body will show the orifices of the reproductive ducts and of the
excretory organs which will be found described below. In addition to these,
all British earthworms and a large percentage of the tropical forms have a
row of pores along the back, which are between the successive segments in
the median line. These "dorsal pores" {349}open directly into the
body-cavity, and are mere perforations of the body-wall, not tubes lined by
a special layer of cells. Professor Spencer of Melbourne[405] has observed
a giant earthworm (_Megascolides australis_) of Gippsland which, when held
in the hand, spurts out, to a height of several inches, the fluid of the
body-cavity through its dorsal pores. The burrows, he remarks, are coated
over with the same fluid, which is regarded by him as a lubricant. This,
however, considering that the glandular cells of the epidermis can secrete
a mucous fluid, seems to be an expensive use to which to put the important
fluids of the interior of the body. It is more probable that the dorsal
pores are a means of getting rid of waste products. Lim Boon Keng[406]
suggests that the coelomic fluid possesses a bactericidal function. The
dorsal pores are missing in many earthworms, and without exception in those
Oligochaeta which live in water; but these latter worms have a pore upon
the head, which appears to be wanting in the earthworms. Dr. Michaelsen has
thought that the head-pore serves to relieve the brain from undue watery
pressure—to act, in fact, as a kind of safety-valve for the liberation of
superfluous fluid.
In some foreign worms the pores of the reproductive ducts are conspicuous
external features (Fig. 197); even in our British species the turgescent
male apertures upon the fifteenth segment are sometimes quite obvious.
STRUCTURE OF THE BODY-WALL.—The body-wall consists in all Oligochaeta of
three recognisable sheets of tissue. Outside is the epidermis, which always
consists of a single layer of cells, except in the clitellar region of
earthworms. It is a point of difference between the aquatic genera and the
terrestrial forms that in the former the clitellum is only one cell thick,
while in the higher Oligochaeta it is made up of more than one layer of
cells. The epidermis is ciliated only in the genus _Aeolosoma_, and there
only on the prostomium. It secretes a thin layer of chitin, which is
defective opposite to the glandular cells, and becomes therefore perforated
by numerous pores. The structure of the epidermis of _Lumbricus_ has been
studied by Cerfontaine, whose recent account[407] of the same is the
fullest and most accurate that exists.
Underneath the epidermis comes a layer of circular muscle-fibres, {350}and
underneath this again a layer of longitudinal muscles. In both layers the
fibres have a softer core, outside which lies the radially striated
muscular substance. The fibres are embedded in a granular matrix. It used
to be considered at one time that such medullated fibres were distinctive
of leeches as opposed to Oligochaeta. Their existence has been really known
in the Oligochaeta since the researches of Ratzel; but Cerfontaine has
fully described them, and emphasised the fact that the fibres of both
circular and longitudinal coats are alike in this respect.
[Illustration: FIG. 187.—Chaetae of Oligochaeta. × 10. (After Michaelsen,
Stolc, and Vejdovsky). 1, 2, Penial chaetae of _Acanthodrilus georgianus_;
3, _Spirosperma_; 4, _Ilyodrilus_; 5, _Lophochaeta_; 6, _Tubifex_; 7, 8,
_Nais_; 9, _Bohemilla_. Figs. 3-9 are ordinary chaetae.]
CHAETAE.—The passive organs of locomotion in these animals are the chaetae,
which are absent in only one family, Discodrilidae, and in one other genus,
_Anachaeta_. In this latter worm the chaetae are represented by large
glandular cells, which seem to correspond to the cells from which the
chaetae arise in other forms. They are in this case, as in the others,
cells of the epidermis. The chaetae of the Oligochaeta are not quite so
variable in form as in the marine Polychaeta (see Fig. 138, p. 267). Figs.
187 and 188 illustrate some of the principal shapes which these bristles
assume. The most prevalent form is an elongated S, which has been aptly
compared to the mathematical sign ∫. This kind of chaeta is found in all
earthworms, and in not a few aquatic genera such as the Lumbriculidae. In
some of the latter and in the Tubificidae and Naids there is the same form
of chaeta, which is cleft at the free end, and possibly enables the worm to
grasp the leaves of aquatic plants, and otherwise facilitates progression
in a laxer medium than the stiff soil frequented by the earthworms. Even
earthworms, at any rate the genus _Pontoscolex_, have chaetae of this kind;
some of the aquatic Oligochaeta have elongated and hair-like bristles, such
as that of _Tubifex_.
{351}[Illustration: FIG. 188.—Chaetae. × 10. 1, _Onychochaeta_; 2,
_Pontoscolex_; 3, _Trichochaeta_; 3_b_, the same, more highly magnified.]
In the Tubificid _Lophochaeta_ (Fig. 187, 5) the chaetae are ornamented on
both sides with delicate processes, which give them the appearance of
Crustacean hairs. Among earthworms the simple S-like form is sometimes
complicated by the development of sinuous ridges upon the distal end. No
doubt these bristles enable their possessor to get a firmer grip of
adjacent objects; they are very commonly found, in the family
Geoscolicidae, upon the segments of the clitellum, and permit of a firmer
union during sexual congress. In no Oligochaeta are the chaetae borne upon
parapodia, as is the case with the Polychaeta; but in many of the aquatic
forms there are a considerable number to each bundle. In earthworms the
number of chaetae varies greatly. The common earthworms of this country,
belonging to the genera _Lumbricus_, _Allolobophora_, and _Allurus_, have
only eight chaetae upon each segment of the body, and these are then, as a
rule, arranged in pairs or rather couples, two of each on each side of the
body. The genus _Perichaeta_ and some of its allies have a much larger
number of chaetae to each segment, disposed in a continuous row round the
middle of the segment. The intermediate condition is to be seen in the
genus _Deinodrilus_, where there are twelve in each segment, and in certain
members of the genus _Megascolex_, where there are eight in each segment in
the anterior region of the body, the number increasing in the posterior
segments. The four bundles of chaetae in the Naids and Tubificids have been
likened to the {352}notopodia and neuropodia of the Polychaetes; but it
does not seem certain that this comparison is justifiable. It was at one
time thought that the continuous circle of chaetae of the Perichaetidae was
the primitive condition; but Professor Bourne has lately found that in
_Perichaeta_ the young embryos have not got this continuous circle; it is
only acquired later.
BRANCHIAE.—The Oligochaeta were called by Cuvier the "Annélides abranches
sétigères." But the epithet "abranches" is now known to be inaccurate. In
fact it really was so when Cuvier wrote; for naturalists were at that time
well acquainted, chiefly through the elaborate work of O. F. Müller, with
the little fresh-water Naid _Dero_, the posterior extremity of which is
provided with a varying number of branchial processes. These are furnished
with looped blood-vessels and are covered externally by cilia, so that the
water containing oxygen is constantly renovated. The second instance of a
gilled Oligochaete was discovered in the very same family. Professor
Bourne[408] of Madras found in "tanks" a Naid which he named
_Chaetobranchus_, in which the head segments, to the number of fifty or so,
are provided with long ciliated processes, which as a rule enclose the
dorsal chaetae of their segments, and in addition a capillary loop.
Curiously enough, this very same worm made its appearance in the _Victoria
regia_ tank at the Botanical Gardens in the Regent's Park, whither it had
in all probability been accidentally imported. Two members of the family
Tubificidae were the next examples of gilled Oligochaeta made known to
science; one of these, _Branchiura sowerbyi_,[409] appeared also in the
Botanical Gardens, so that its native home is unknown. It differs from
_Chaetobranchus_ in that the gills are at the posterior end of the body,
and are contractile; during the life of the worm they are in continual
motion. A species of the South American genus _Hesperodrilus_,[410] _H.
branchiatus_, is also gilled, and, so far as can be made out from a
spirit-preserved specimen, the gills are precisely of the same pattern and
contractility as those of its ally _Branchiura_. Possibly _Branchiura_
ought to be included in the same genus with _Hesperodrilus_. A worm which
was originally described by Grube as _Alma nilotica_, should really have
been placed before the three last-mentioned {353}instances; but as this
worm was only known from a fragment, and as the description was not by any
means full, it was not thoroughly believed in; it was surmised that it
might be a member of some marine genus, perhaps of the Capitellidae. Oddly
enough, the same worm was independently described by a different name,
_Digitibranchus niloticus_, a few years later by Levinsen. Quite recently
Michaelsen has found by a reference to the original types that this worm is
really gilled, and that it is specifically identical with a worm which had
been given a totally different name, viz. _Siphonogaster_. The fact that
the gills of the latter had been overlooked was readily explained by the
circumstance that they are retractile, and not merely contractile. But all
the species of the genus _Siphonogaster_, or _Alma_, as it ought really,
following the rules of priority, to be called, have not got gills, as is
the case too with the genus _Hesperodrilus_. The gills of _Alma_ are
branched, and there is therefore no longer any justification whatever for
defining the Oligochaeta as a group of Annelids without gills. The simple
gill-like processes of _Chaetobranchus_ might have been held to be not
accurately comparable to the more complex structures which we find in the
marine worms.
[Illustration: FIG. 189.—Transverse section through _Branchiura sowerbyi_.
× 20. _d.br_, Dorsal branchia; _i_, intestine; _n_, nerve-cord; _v.br_,
ventral branchia.]
NERVOUS SYSTEM.—The central nervous system of the Oligochaeta is very
uniform in its structure in the entire group. The only family which is at
all anomalous is that of the Aphaneura. In _Aeolosoma_ there appears to be
only a pair of cerebral ganglia, which retain the primitive position of
these organs in being still in direct connexion with the epidermis. In all
other Oligochaeta there are a pair of cerebral ganglia, {354}connected by a
circumoesophageal commissure with a ventral ganglionated cord. From the
cerebral ganglia arises a system of nerve-fibres and nerve-cells, which
represents the stomatogastric nerves of other Invertebrates.
SENSES AND SENSE-ORGANS.—The only organs that can be regarded with anything
like probability as sense-organs are the pigmented eyes of certain Naids
and the tactile cells of many worms. The latter are usually elongated cells
provided at their free extremity with a stiff process; they occur
associated in groups, and often these bundles of cells have a segmental
arrangement. The head end of many of the lower Oligochaeta, for instance
the genus _Aeolosoma_, has delicate processes projecting here and there;
these appear to be also of a tactile nature, and are of course connected
with cells of the epidermis. The eyes of certain Naids are little more than
lenticular bodies embedded in a mass of pigment. In the genus _Eudrilus_
and in many Eudrilidae are peculiar integumental bodies, which were
independently discovered by Dr. Horst[411] and myself, and compared by us
to the Pacinian bodies of Mammals. Whether these structures are connected
with nerves or not is doubtful. In spite of the poor development and the
simplicity of their sense organs, the higher Oligochaeta at any rate can
feel, and can distinguish light from darkness. Darwin[412] came to the
conclusion that "light affects worms by its intensity and its duration."
And furthermore, it is only the anterior end of the body which is thus
affected. Of the sense of hearing these animals appear to be utterly
devoid. Some kept by Darwin "took not the least notice of the shrill notes
from a metal whistle, which was repeatedly sounded near them; nor did they
of the deepest and loudest tones of a bassoon." But it is always necessary
to discriminate between sound and vibrations passing through any solid
body, which would appeal rather to a sense of touch. Here worms are most
sensitive. It is quite easy, by digging with some vigour, to arouse the
worms in the neighbourhood, who will crawl to the surface and away from the
scene of action; a proceeding on their part which is sometimes put down to
a desire to escape from their enemy the mole.
Smell appears to be another sense which is somewhat deficient. {355}But
worms are epicures, and exhibit a decided taste and preference for certain
articles of diet. Like their fellow tiller of the soil, the agricultural
labourer, worms have a keen relish for onions, which, however, they must
recognise by the smell. They prefer green cabbage to red, celery to both,
and raw meat appears to be the greatest delicacy that can be offered to
them. It is only substances they are not likely to meet with, such as
perfumes, tobacco, and paraffin, that produce no impression upon the worm's
sense of smell.
COELOM AND VASCULAR SYSTEM.—When an earthworm is dissected the various
organs are seen to lie in a fairly spacious cavity, which is interrupted
and divided into a series of chambers by the mesenteries or septa which
stretch across from wall to wall of the body, and correspond roughly in
their position to the grooves which separate the body externally. This
cavity, common to all the higher animals, is known as the coelom; it is
lined by cells, which cover the intestines as well as the inside of the
body-wall; and upon the intestine assume the form so characteristic of the
group, namely, that of large yellow cells loaded with secreted matters, and
called "chloragogen-cells" by Claparède. The coelom communicates with the
exterior by means of the dorsal pores, the nephridia, and the ducts of the
reproductive organs. As in all animals which possess a coelom, the
reproductive tissues, ova and sperm, are developed on its walls.
[Illustration: FIG. 190.—_Sparganophilus tamesis_; general anatomy, × 3.
(After Benham.) I-XVIII, segments. 1, 4, 6, Perivisceral vessels (6 is one
of the hearts); 2, 3, 7, dorsal vessel; 5, spermatheca; 8, sperm sacs; 9,
intestino-tegumentary vessels; 10, ovary; 11, 12, integumentary vessels.]
The vascular system of the Oligochaeta forms a system of perfectly closed
vessels, which ramify into fine capillary networks in the body-wall, in the
coats of the alimentary canal, and upon {356}the other organs of the body.
The main trunks are a dorsal and a ventral longitudinal, which communicate
directly in the anterior end of the body by large transverse contractile
trunks, the so-called hearts (see Fig. 190, 6). The dorsal vessel is also
contractile, but not the ventral, or, when it occurs, the subnervian. The
vascular system has many degrees of complexity in different families; it is
simpler in the smaller aquatic forms. The blood is usually red, and the
pigment which is suspended in the plasma is haemoglobin. The blood is
corpusculated.
EXCRETORY ORGANS.—There appears to be a great deal more variation in the
structure of the excretory system than there is in many other groups. For a
long time only _Lumbricus_ and a few of the aquatic genera were known as
regards their excretory systems. In these there is a pair of excretory
organs or nephridia in nearly all the segments. These are much coiled
tubes, in which it is always possible to recognise three divisions. The
nephridium commences with an orifice of a funnel-like character, fringed
with long cilia, and opening into the body-cavity; from this springs a
tube, which immediately perforates the septum lying between the segment
which contains the funnel and the following one; this tube has the
peculiarity first pointed out by Claparède of being excavated in the
substance of cells; the glandular part of the nephridium is a row of cells
which are bored through by a continuous canal, the walls of which are here
and there furnished with cilia. It often happens that the main canal gives
off minute lateral ramifications, which may even form a kind of network
round the principal canal. The terminal section of the nephridium is a
muscular sac which opens on to the exterior by a pore, and from which the
products of excretion are from time to time evacuated by contractions of
its walls. This is a brief statement of the main facts in the structure of
those Oligochaeta in which there is a single pair of nephridia to each
segment of the body; small differences of more or less importance occur. In
_Chaetogaster_, for example, there is no trace of a funnel; in some genera
the terminal sac is much reduced or unusually extended, being even
sometimes provided with a caecum of moderate dimensions. In _Acanthodrilus
novae-zelandiae_ and a few other species the point of opening of the
nephridia varies from segment to segment, though it always bears some
relation to the chaetae. In these {357}species the nephridia which open
more dorsally are a little different in structure from those which open
more ventrally. One set have a caecum, and the other have not.
The nephridia of the terrestrial forms are enveloped by a richly developed
network of blood capillaries, which is absent in the smaller aquatic
genera.
A very remarkable genus, _Brachydrilus_, has lately been described by Dr.
Benham,[413] in which each segment has two pairs of nephridia instead of a
single pair. More recently, certain Australian forms, which I propose to
unite on this account into a genus _Trinephrus_, have been discovered which
have no less than three distinct and separate pairs in each segment.[414]
[Illustration: FIG. 191.—Section through body-wall of _Megascolides
australis_, highly magnified. (After Spencer.) 1, 4, 5, 6, Coils of
nephridia; 2, funnel; 3, septum; 7, external apertures.]
In many Megascolicidae there is a nephridial system of a different
character. In _Perichaeta_ when dissected the nephridia appear, on account
of their minute size, to be altogether absent. There is, however, in most
Perichaetidae, in many Acanthodrilidae, and in many Cryptodrilidae a mass
of minute tubules which cover the inside of the body-wall, and open on to
the exterior by innumerable openings; there may be in a single segment one
hundred or more of these external orifices, which are scattered about
irregularly. It is at present uncertain whether these minute tubes are
connected among {358}themselves, thus forming a network passing through the
septum and from segment to segment, or whether each tube is isolated from
its fellows, and forms a distinct nephridium, of which there are many in
each segment and entirely separate. This is, however, certain, that the
complex nephridial systems of at any rate _Octochaetus_ and _Megascolides_
are derived from the multiplication of a single pair of tubes which are
alone present in the embryo. In _Perichaeta_ the minute nephridia are
furnished with coelomic funnels; in _Octochaetus_ they are not, except in
the case of certain nephridia which open into the terminal section of the
intestine.
Both at the anterior and at the posterior end the nephridia occasionally
open into the alimentary canal. In various genera the first pair of
nephridia are larger than the others, and open into the buccal cavity; it
seems likely that they serve as salivary glands. A somewhat similar
condition of things exists in _Peripatus_ (vol. v, p. 17). In _Octochaetus
multiporus_, for example, there is a large tuft of nephridial tubes in the
anterior region of the body, which opens by a long muscular duct into the
buccal cavity. In the same species a good many of the nephridial tubes open
into the posterior section of the intestine, reminding one of the anal
vesicles of the Gephyrea (p. 436) and of the Malpighian tubes of the
Arthropods.
In many Eudrilidae the ducts of the paired nephridia form a network in the
body-wall, which opens on to the exterior by many pores.
ALIMENTARY CANAL.—The digestive tube is perfectly straight in nearly all
Oligochaeta. Only in _Plagiochaeta_ and a species of _Digaster_ is it
twisted in the intestinal region in a corkscrew-like fashion. The mouth is
under the buccal lobe (where, as in the majority of cases, this is
present); the anus is mostly terminal, or rarely, _e.g._ _Criodrilus_, a
little in advance of the end of the body on the ventral side. In the
simpler forms three regions can be distinguished, which are themselves
simple in structure. The mouth leads into a buccal cavity, which in its
turn opens into the pharynx; the latter is muscular, with thick walls. The
narrower oesophagus opens into the wider intestine, which opens
posteriorly, as already stated. In the earthworms there is as a rule some
complication. The oesophagus bears certain glandular appendages, the
calciferous glands; and a part of {359}it is modified into a gizzard. The
gizzard is merely a portion of the oesophagus with very much thickened
muscular walls and with a stout lining of chitin. It is not universally
present among earthworms, and when present varies much in position. The
rule is that one gizzard only is present. In _Digaster_, as is implied by
the name, and in some other forms there are two in successive segments; in
_Trigaster_, as the name also indicates, there are three gizzards; in
_Moniligaster_ and the Eudrilids _Hyperiodrilus_ and _Heliodrilus_ there
are four to six; and a few other forms also have a considerable number of
gizzards. The calciferous glands are diverticula of the oesophagus with
folded and sometimes ciliated walls; their epithelium secretes calcareous
particles, which are frequently of crystalline form. Darwin supposed that
this secretion was provided in order to negative the humus-acids of the
soil which is the food of earthworms. These organs are usually paired, but
in the Eudrilidae there are unpaired as well as paired glands; the unpaired
calciferous glands lie ventrally. These glands are totally wanting among
the aquatic families, with the sole exception of the Enchytraeidae. In a
few of these there are either paired or single glands of a very similar
nature; Dr. Michaelsen has suggested that the function of these is rather
absorptive than secretory. From the median unpaired gland of _Buchholzia_
arises the dorsal vessel, which at first forms a sinus round the glandular
epithelium; the epithelium, like that of the nephridia, is perforated by
the ducts. In certain Oligochaeta there are some curious modifications of
the calciferous glands. In _Stuhlmannia_ and a few other Eudrilidae the
oesophagus is beset with a larger number of paired structures than in any
other genera of the family, where the calciferous glands are more limited
in number. These glands consist of a short tube lined with epithelium
opening into the oesophagus. Round this is a mass of cellular tissue, but
the outlines of the constituent cells are lost; the whole is permeated with
abundant blood-vessels. This layer seems to be peritoneal, and the entire
gland seems to have lost its function as a secretory organ, and to have
taken on some function in connexion with the vascular system. An analogous
modification is to be found among the Enchytraeidae. In certain forms there
is a structure known as the cardiac body; this is a chord of cells lying in
the dorsal blood-vessel at the point where it springs from the intestine.
It is tempting to regard this cellular rod as being {360}the altered dorsal
glandular pouch already spoken of, which is surrounded by a blood sinus.
REPRODUCTIVE ORGANS.—All the Oligochaeta are hermaphrodite animals. But, as
is the case with other hermaphrodites, the male and female organs are in
many cases mature at different times, thus leading to a practical
unisexuality. Many of the aquatic forms appear to have fixed times for
breeding, which may be in the winter or in the summer; but the earthworms
are as a rule sexually mature the whole year round. Various accessory
organs are developed in the majority of cases. In all, the reproductive
glands lie in successive segments and are attached to the septa, from the
peritoneal covering of which they originate. Their actual position differs
greatly in different genera; the position is constant only in the
earthworms, where the testes are in the tenth and eleventh segments and the
ovaries in the thirteenth, in exactly corresponding situations. A few
earthworms have only one pair of testes. The only exception, among
terrestrial forms, to the position of the generative organs is in the
family Moniligastridae, which show so many other affinities to the lower
forms of Oligochaeta. In this family the ovaries have moved one or two
segments forward. Among the fresh-water families the position of the testes
and of the ovaries is not so uniform. They are generally more anterior than
in the terrestrial genera, particularly the ovaries.
One of the chief differences between the Oligochaeta and the Polychaeta is
that the reproductive organs of the former have special ducts to convey
their products to the exterior. In _Aeolosoma_, the only exception to this
rule, Dr. Stolc[415] has shown some reasons for believing that certain
nephridia, but slightly altered in form, serve as the conduits of the
spermatozoa, whilst the ova are extruded through a pore upon the ventral
surface of the body. In the Enchytraeidae the same pore for the extrusion
of the ova appears to exist; but a nearer examination shows that it is
really not a mere perforation of the integument, like the dorsal pores, for
example, but that its internal orifice is fringed with cells which seem to
represent a rudimentary oviduct; perhaps _Aeolosoma_ typifies a last stage
in the reduction. Even so high in the scale as in the genus _Nemertodrilus_
(Eudrilidae), there is an oviduct which can only be compared with that of
the {361}Enchytraeidae. Elsewhere the oviducts are a pair of tubes with a
wide, funnel-shaped, and ciliated mouth, which leads to the exterior by way
of a ciliated tube of varying length.
The sperm-ducts are of an essentially similar structure; but they are
commonly much longer, passing through a variable number of segments on
their way to the exterior. In most earthworms there are, moreover, two of
them on each side instead of only a single pair, as is the case with the
oviducts. Among the Tubificidae, Naids, and other aquatic families there
are only two sperm-ducts, one on each side of the body. But this is not a
character of the aquatic families, for the Lumbriculidae have generally two
pairs, as in the earthworms. It is, however, a rule with hardly an
exception, that among the aquatic Oligochaets the sperm-ducts open, as do
the oviducts in all Oligochaets, upon the segments following that which
bears internally the ciliated funnel. It is only in the Moniligastridae
among earthworms that the sperm-duct only traverses two segments in its
course. But where it is short as regards the actual distance traversed
between the two extremities, the tube itself is commonly long and coiled.
Sometimes, as in our common earthworms, the sperm-duct opens directly on to
the exterior of the body, the lips of the external orifice being swollen by
the development of cutaneous gland-cells. In the majority of cases the
sperm-duct or ducts open near or into a glandular structure which in
earthworms has been called "prostate"; in the aquatic forms, on the other
hand, "atrium." As these terms are objectionable from the different way in
which they have been used for structures of Vertebrates, I have suggested
for both the term "spermiducal glands," indicating the identity of the
structure in all Oligochaeta. The number of pairs of these glands varies,
as does also their shape and size. The typical form is perhaps illustrated
in the lower Oligochaeta, where there is but a single pair into which the
sperm-duct or ducts of the same side open. The Naids, Tubificidae,
Lumbriculidae, and Moniligastridae have a simple gland of this description
on each side of the body. These glands may consist of a tuft of pear-shaped
glandular cells attached to the organ at one side, as in most Tubificidae,
or of a complete investment of gland-cells, as in _Branchiura_. Among
earthworms it is only the Moniligastridae and the Eudrilidae in which the
sperm-duct opens directly into {362}the end of the spermiducal gland; in
the Perichaetidae the gland is differentiated into two sections; there is a
muscular duct leading to the exterior, and a lobate glandular part, which
is formed by a complicated branching of a single sac such as exists in the
Tubificidae; in the Acanthodrilidae and in many Cryptodrilidae the
spermiducal glands are of a tubular form and are not branched, though there
is the same differentiation into a duct and a secreting portion. There are
in the Acanthodrilidae two pairs of these, and as many as three pairs in
_Dichogaster_; in the latter case in three successive segments. In the
Acanthodrilidae the glands are upon the seventeenth and nineteenth
segments. In most Cryptodrilidae the sperm-ducts do not open into the duct
of the spermiducal gland, but on to the body-wall near to its orifice, the
distance varying in different genera. In the Acanthodrilidae the male pore
is on the eighteenth segment, removed therefore by the distance of a
segment from the aperture of either of the glands. It may be that a large
series of structures which exist in _Microchaeta benhami_[416] and in other
Geoscolecids, and which have been termed copulatory glands, are the
equivalents of the spermiducal glands.
[Illustration: FIG. 192.—Diagrammatic longitudinal section of _Lumbricus_,
showing the generative segments. × 3. (After Hesse.) _sp_, Spermathecal
pore; _t_, testis; _s.s_, seminal sac; _sp.s_, sperm-sac; _o_, ovary;
_e.s_, egg-sac; ♀, female pore; ♂, male pore.]
In many earthworms there are, at the external opening of the male ducts,
bundles of specially modified chaetae, which have been called, from their
supposed function, penial chaetae; they are usually ornamented at the free
end with spinelets or ridges, and frequently offer valuable specific
characters. In the Lumbricidae and the Geoscolicidae there are modified
chaetae upon the {363}clitellum; in a few forms, such as, for example,
_Acanthodrilus schmardae_, the spermathecae have bundles of similar chaetae
in their neighbourhood, often associated with glands not unlike the
spermiducal glands.
In most, perhaps in all Oligochaeta the sperm is not matured in the testes,
or even in the body-cavity; it is received into special sacs which are
called sperm-sacs, and there ripens. These sacs, the vesiculae seminales,
have been shown to be outgrowths of the septa; their cavity is thus a
portion of the body-cavity shut off more or less completely from the
general body-cavity.
The reproductive organs of the Eudrilidae, and particularly the female
organs, are so divergent in many particulars from those of other
Oligochaeta that it is convenient to treat them separately. The testes are
normal, save that they are often adherent to the posterior wall of their
segment, as, however, is the case with some other earthworms. In many
Eudrilidae, for instance in the genus _Hyperiodrilus_, the funnels of the
sperm-ducts are dependent from the anterior wall of the segment which
contains them; the narrow tube which follows projects into the segment in
front, and is there immediately dilated into a wide chamber, which again
narrows, and bending round, re-traverses the same septum; the two ducts of
each side (if there are two, which is not invariably the case) remain
separate and open separately into the glandular part of the spermiducal
gland. There is occasionally only a single median gland; and as a general
rule the two glands open by a median unpaired orifice. Penial chaetae may
or may not be present.
[Illustration: FIG. 193.—Female reproductive organs of _Hyperiodrilus_.
XII-XV, Segments of the body; 1, spermathecal sac; 2, egg-sac; 3,
spermatheca; 4, ovary.]
The structure of the female organs differs considerably in detail in the
different genera. But _Hyperiodrilus_ may be taken as an instance of a
genus in which these organs are as complicated as they are anywhere. The
ovaries (Fig. 193, 4) are {364}perfectly normal in structure and in
position. So also are the oviducts; but both are enclosed in sacs which
communicate in rather an elaborate fashion. Each ovisac is somewhat rounded
in form, and the two communicate by a narrow tube; from the ovisac also
arises another narrow tube, which soon dilates into a chamber lying in the
thirteenth segment; this contains the mouth of the oviduct and is
continuous with the egg-sac; the latter is quite normal in position. Beyond
the egg-sacs the two tubes unite round the intestine and open into a large
median sac, which contains sperm and may be called the spermathecal sac
(1). There is, however, a true spermatheca, single and median. This opens
on to the exterior in the middle of the thirteenth segment, but lies
chiefly in the right-hand sac behind the ovarian portion of the same. I
never found this spermatheca to contain sperm. Dr. Rosa inferred on
anatomical grounds, and I have been able to prove developmentally (in
_Libyodrilus_), that these sacs which involve the ovaries and oviducts, and
which also contain sperm, are derivatives of the septa; that in fact the
spaces which they enclose are coelomic. In some Eudrilids these sacs are
the only "spermathecae"; in others, as in _Hyperiodrilus_, there are in
addition blind pouches lying within them which must be regarded as true
spermathecae; these are smaller in some than in others. In fact there are
various transitions in the entire replacement of true spermathecae
apparently homologous with those of other earthworms by pouches which are
derived from the septa, and which are therefore of an entirely different
morphological significance; here is an excellent case of the substitution
of organs, analogous to the replacement of the primitive notochord of the
Vertebrate by the vertebral column.
So far as is known, all the Oligochaeta deposit their eggs in special
chitinous cases, the cocoons. They share this peculiarity with the
Hirudinea. The cocoons have long been known, but were originally mistaken
for the eggs themselves. The cocoons contain several eggs and a variable
quantity of albumen for the nutrition of the growing embryos. In the
majority of earthworms they are more or less oval with projections at the
two ends, and are of a brownish colour. In others the tint is rather to be
described as green. The genera _Criodrilus_ and _Sparganophilus_ have a
cocoon which is greatly elongated. These structures seem to be undoubtedly
formed by the clitellum, the earlier {365}opinion of D'Udekem being that
they were the product of certain glands developed in _Lumbricus_ at the
breeding season, which he thence called the capsulogenous glands. It is
more probable that these glands, which have been up to the present but
little investigated, are the seat of the formation of the albumen which is
found within the cocoons. The cocoons are deposited at varying depths in
the ground, or on the surface. Among the aquatic genera they are often
attached to aquatic plants. The process of formation has been carefully
watched by Vejdovsky[417] in the genus _Rhynchelmis_. The worm throws off
the cocoon over its head, crawling backwards to free itself therefrom. The
eggs, spermatozoa and albumen, reach the interior of the cocoon as it
passes over the orifices of the respective ducts. Out of the numerous eggs
which a single cocoon originally contains, only a few, sometimes only one,
reaches to maturity. Among the Enchytraeidae, however, quite a large number
of young emerge from a single cocoon. The development of all the
Oligochaeta is direct, there being no free larval stage. It seems to be the
rule for a process of fission to take place in the embryos of
_Allolobophora trapezoides_[418] at least, according to the observations of
Vejdovsky, in warm weather. In cold weather he found in each cocoon as a
rule single embryos, and only 10 per cent of double embryos.
[Illustration: FIG. 194.—Cocoons of Lumbricidae. (After Vejdovsky.) A,
_Lumbricus rubellus_, nat. size and × 3; B, _Allurus_, nat. size and × 6;
C, _Allolobophora foetida_, nat. size and × 3.]
HABITAT.—Earthworms are found in almost every part of the world where they
have been looked for. They occur far to the north, in Siberia and Nova
Zembla,[419] while South Georgia and Kerguelen mark their southern limits.
From arid tracts of country they are naturally absent, and also, which is
more {366}curious, from certain districts of North America. In the tropics
these animals seem to be on the whole less abundant than in more temperate
climates. But this deficiency of individuals is counterbalanced by the
greater variety of generic and specific types. From tropical Africa, little
explored as it has been from this point of view, no less than thirty
genera, including about ninety species, have been recorded; whereas in
Great Britain only four genera and seventeen species occur, and in all
probability but few remain to be discovered. The vertical range of these
Annelids is also considerable. Several species have been met with in Europe
and elsewhere at an altitude of 10,000 feet.
For the bulk of the species the term earthworm is an accurate description
of their habitat. But there are not a few which occasionally or habitually
prefer other localities. The genus _Allurus_ is equally at home in soil or
in water; I have taken it in the fast-flowing river Plym in Devonshire. The
genus _Acanthodrilus_ includes a few species which have at present only
been met with in water; _A. schmardae_ comes from fresh water in
Queensland, _A. stagnalis_ from ponds in South America; _A. dalei_ is like
_Allurus_ in that it is to be found both on land and in streams and ponds.
The Enchytraeidae are just as amphibious; _Criodrilus_ and _Sparganophilus_
appear to be purely aquatic. A more curious locality for a creature that is
so characteristically terrestrial is the margin of the sea. For a long time
a species belonging to a peculiar genus _Pontodrilus_ has been known from
the shores of the Mediterranean in the neighbourhood of Nice. It lives
there among seaweed above high-water mark, but it must at least
occasionally be splashed by the waves. Another species of the same genus
occurs on the coast of Brazil and some of the West Indian islands;
_Pontoscolex corethrurus_ and _Diachaeta littoralis_ were described by
Schmarda[420] from the shores of Jamaica. The former species is one of the
most widely distributed of earthworms, and, except in this particular part
of the world, has been always taken on the land far from the sea. There are
also partly marine forms among the Tubificidae; _Clitellio arenarius_ is
common on our coasts.
While there are several kinds of earthworms that are thus met with in fresh
water, others will live for some time submerged {367}in water. Perrier
found by experiment that various species could undergo with impunity a
prolonged immersion in water, and I confirmed his experiments myself with a
common species of _Allolobophora_. A correspondent of "Nature" stated that
a certain number of species (not particularised) of earthworms in Ceylon
could suffer with impunity the effects of sea-water. The importance of this
fact will be again dealt with in considering the geographical distribution
of the group.
Among the aquatic genera of Oligochaeta we do not as a rule meet with
amphibious species. The Enchytraeidae however, as already mentioned, are an
exception; so too appears to be the genus _Phreoryctes_, which in its
structure is to some extent intermediate between the earthworms and the
aquatic families.
TERRESTRIAL AND AQUATIC FORMS.—There are many obvious structural
peculiarities which would prevent the normally aquatic worms from being
thoroughly at home on dry land. The gills of _Branchiura_ and the other
gilled species would be injured, in all probability, by friction with the
earth; the delicate and long chaetae of Naids and _Tubifex_ are also most
unsuited for progression through dry soil; and it is to be noted that those
Oligochaeta, which, belonging to aquatic groups, are yet found away from
water, have chaetae of the simple sigmoid pattern which characterises the
earthworms.
There are other peculiarities found only in the aquatic species which have
not so obvious a relation to their habitat. In no genus that is mainly
aquatic in habit are the ova small and nearly unprovided with yolk as in
_Lumbricus_; the ova of aquatic forms are invariably large and filled with
abundant yolk.
The more delicate organisation of the aquatic Oligochaeta is not so hard to
understand. The comparatively unresisting nature of the medium in which
they live, water or fine mud, does not necessitate so strong a development
of the layers of the body-wall as is essential to the earth-living forms,
which have also thick septa in the anterior region, to protect the organs
of reproduction as the strong muscular contractions of the body force the
worm's way through the dense soil. With the weak structure of the
integument are perhaps also correlated the simplicity of other organs of
the body in the aquatic Oligochaeta. With thin body-walls, through which
gases can diffuse with great ease, there would seem to be less need for the
development of a system of {368}integumental blood capillaries. These are
indeed for the most part absent in the aquatic forms, being only faintly
developed in a few, an example possibly of degeneration.
EARTHWORMS AND THE SOIL.—Darwin has explained the enormous effects which
these soft-bodied and small creatures have had upon the superficial
structure of the earth. Their castings, brought up to the surface, are
blown about by the wind when dry, and are thus spread over the ground in a
fine layer. It has been calculated that in the space of an acre .2 of an
inch in thickness of earth is annually brought to the surface. It is clear
therefore that in a long period of years there would be a very large effect
produced. On the sides of a hill this matter brought up from below would
tend to roll down the slopes when dry, and would increase the débris
carried away to the sea by streams and rivers, so that continents formerly
deposited under the sea may owe no small proportion of their size to the
continued work of earthworms in past ages.
Darwin has also pointed out the benefits to the agriculturist which accrue
from the industry of these Annelids. The soil is thoroughly mixed and
submitted to the action of the atmosphere. The secretions of the worms
themselves cannot but have a good effect upon its fertility, while the
burrows open up the deeper-lying layers to the rain. Mr. Alvan
Millson,[421] in detailing the labours of the remarkable Yoruba worm
(_Siphonogaster millsoni_ Beddard), hints that they may serve as a check
upon the fatal malaria of the west coast of Africa. By their incessant
burrowings and ejecting of the undigested remains of their food many
poisonous germs may be brought up from below, where they flourish in the
absence of sunlight and oxygen, and submitted to the purifying influence of
sun and air.
PHOSPHORESCENCE.—Phosphorescence has been observed in several species of
Oligochaeta. The most noteworthy instance of recent times is the discovery
by Giard of the small worm which he called _Photodrilus phosphoreus_ at
Wimereux. During damp weather it was sufficient to disturb the gravel upon
the walks of a certain garden to excite the luminosity of these Annelids.
In all probability this species is identical with one whose luminosity had
been noticed some years before (in 1837) by Dugès, and named by him
_Lumbricus phosphoreus_. According {369}to Giard, the light is produced by
a series of glands in the anterior region of the body debouching upon the
exterior. This same worm has since been found in other localities, where it
has been shown to be phosphorescent, by Moniez[422] and by Matzdorf[423].
It is remarkable that in some other cases the luminosity, though it exists,
is very rarely seen. The exceedingly common Brandling (_Allolobophora
foetida_) of dunghills has been observed on occasions to emit a
phosphorescent light. This observation is due to Professor Vejdovsky,[424]
and was made "upon a warm July night of 1881." He thinks that the seat of
the light is in the secretion of the glandular cells of the epidermis, for
when this and other worms are handled the phosphorescence clings to the
fingers, as of course does the mucous secretion voided by the glands.
Phosphorescence has been observed also in some other families of
Oligochaetes. The late Professor Allen Harker noticed a small worm in
marshy ground in Northumberland which emitted a distinct light, and which
was subsequently identified as a member of the family Enchytraeidae.
GEOGRAPHICAL DISTRIBUTION.[425]—In the succeeding pages some of the details
of the geographical range of the Oligochaeta will be found. The present
section deals with a few generalities, which appear to result from an
examination of the facts.
As to the aquatic genera but little is known at present with regard to
their range; they have not been widely collected in extra-European
countries. What little is known points to the conclusion that while many
parts of the world have their peculiar genera (such as _Hesperodrilus_ in
South America, _Phreodrilus_ and _Pelodrilus_ in New Zealand), some of the
common European species are widely distributed. I have, for example,
received _Henlea ventriculosa_ from Kirghiz Tartary, and from New Zealand;
and a New Zealand _Tubifex_ appeared to me to be indistinguishable from the
common _T. rivulorum_ of our rivers and ponds. It is possible that these
and similar instances may, at least in some cases, be due to accidental
importation at the hands of man, a matter into which we shall enter later.
But the aquatic genera have, many of them, facilities for extending their
range in a natural fashion, which are greater than those possessed by
earthworms. {370}It has been pointed out that the chaetae of the aquatic
Oligochaeta are generally hooked at the extremity and bifid, which would
give them a greater chance of holding on to the feet or feathers of aquatic
birds; I am not myself disposed to lay much stress on the possibilities of
migration by these means, since the tender bodies of the small worms would
be liable to be soon dried up by wind while in the act of migration. More
likely in every way is a migration when enclosed in the cocoon. The cocoons
being small, and often deposited at the edges of ponds frequented by
aquatic birds, there would be many chances of their being carried away with
tolerable frequency; moreover, as Dr. Michaelsen has pointed out, the
cocoons of some species, particularly among the Enchytraeidae, contain a
large number of embryos; so that when such a cocoon reached a foreign shore
there would be a better chance of the species establishing itself there. I
have referred elsewhere[426] to the singular habit of forming a temporary
cyst which characterises one species of the genus _Aeolosoma_; this would
perhaps tend to facilitate its transference in the way indicated from one
spot to another.
Earthworms, on the other hand, have not such easy means of travelling from
country to country; the assistance which the cocoons in all probability
give to the smaller aquatic Oligochaeta cannot be held to be of much
importance in facilitating the migrations of the earthworms. In the first
place, the animals themselves are of greater bulk, and their cocoons are
naturally larger, and thus less easy of transportation. Secondly, they are
deposited as a rule upon dry land, where the chances of their sticking to
the feet of birds would be less; and thirdly, they are often deposited deep
in the ground, which is a further bar to their being taken up. Another
possible method by which earthworms could cross the sea is by the help of
floating tree-trunks; it is, however, the case with many species that they
are fatally injured by the contact of salt water. There are, it is true, a
few species, such as _Pontodrilus_ of the Mediterranean coast, which
habitually live within reach of the waves; but with the majority any such
passage across the sea seems to be impossible.[427] On the other hand,
rivers and lakes are not a barrier to the dispersal of the group. There are
a few species, such as _Allurus tetragonurus_, {371}which live
indifferently on land and in fresh water; and even some habitually
terrestrial species can be kept in water for many weeks with impunity. A
desert, on the other hand, is a complete barrier; the animals are
absolutely dependent upon moisture, and though in dry weather the worms of
tropical countries bury themselves deep in the soil, and even make
temporary cysts by the aid of their mucous secretions, this would be of no
avail except in countries where there were at least occasional spells of
wet weather.
The range of the existing genera and species is quite in keeping with the
suggestions and facts already put forward. But in considering them we must
first of all eliminate the direct influence of man. Every one who studies
this group of animals knows perfectly well that importations of plants
frequently contain accidentally-included earthworms; and there are other
ways in which the transference of species from one country to another could
be effected by man. There are various considerations which enable us to
form a fair opinion as to the probability of a given species being really
indigenous or imported. Oceanic islands afford one test. There are species
of earthworms known from a good many, but with a few exceptions they are
the same species as those which occur on the nearest mainland; in those
cases where it is supposed that the animal inhabitants have reached an
oceanic island by natural means of transit, it is a rule that the species
are different, and even the genera are frequently different. That the bulk
of them are the same seems to argue either frequent natural communication
with the mainland or a great stability on the part of the species
themselves. It is more probable that the identity is in this case to be
ascribed to accidental transference.
Another argument comes from the distribution of the family Lumbricidae.
This family forms the bulk of the earthworms of the European and North
American continents. But they are also found all over the world. With one
or two exceptions, such as _Allolobophora moebii_, from Madeira, the
extra-north-temperate species are identical with those found within that
region. Now, if the migration had been by natural means there would surely
in the lapse of time been some differentiation of species. Furthermore, Dr.
Michaelsen has pointed out that in South America the presumably European
forms (_i.e._ _Lumbricus_ and _Allolobophora_) are found upon the coast and
in cultivated ground; it is inland that {372}the presumably indigenous
species are met with. This again looks very like accidental transference.
A mapping of the world in regions indicative of the distribution of
earthworms produces a result which is slightly different from the accepted
division. North America, Europe, and Northern Asia so far as is known agree
in having as their distinctive earthworms the family Lumbricidae, which is
very nearly the only one represented in these parts of the world. The
majority of the species are common to the two continents; there cannot, in
fact, be a separation of Nearctic and Palaearctic; we must accept the
Holarctic region of Professor Newton. The Ethiopian region, on the other
hand, is quite as it is in other groups, being bounded to the north by the
desert of Sahara. The Neotropical region is quite distinct, and includes
Central as well as South America, and the West Indian islands, even the
Bermudas. It is, however, a question whether the more southern portions of
the continent should not be cut off from the rest and joined with New
Zealand, to form an Antarctic region. In these two countries, and also in
Kerguelen and Marion Islands, the prevailing genera are _Acanthodrilus_ and
_Microscolex_. In America _Acanthodrilus_ is found nowhere but in the more
southern regions of the southern continent, as well as in the Falklands and
South Georgia. New Zealand is characterised by other genera of
Acanthodrilids besides _Acanthodrilus_ itself; but the bulk of the species
belong to the latter genus. _Acanthodrilus_ also occurs (three species
only) in Queensland and at the Cape of Good Hope. _Microscolex_ is rather
more widely dispersed, being found in other parts of America and in Europe,
the island of Madeira (? accidentally imported); but it is undoubtedly
chiefly concentrated in South America and in New Zealand. Apart from New
Zealand, which, as already said, can only be doubtfully referred to the
Australian region, the latter appears to form one with the Oriental region
(to which, on account of its Perichaetidae, Japan should be added) of other
writers. There is, so far as earthworms are concerned, no "Wallace's line"
at all. The characteristic genera _Perichaeta_ and _Megascolex_ range from
one extremity of the Indo-Australian region to the other. It is true that
_Cryptodrilus_ and _Megascolides_ are limited to Australia itself (with the
apparent exception of a species or two in America, for I can hardly
separate _Argilophilus_ of Eisen from _Megascolides_); {373}but they are
not at all well-defined genera, and indeed the generic distinctions of the
whole family Cryptodrilidae are not in a satisfactory condition.
CLASSIFICATION.—The Oligochaeta do not shade into the Polychaeta so
imperceptibly as might be inferred from the current schemes of
classification. Apart from minor points, which are not universally
characteristic of the two groups, though never found except in one or the
other, the Oligochaeta are to be defined by the complicated reproductive
system; although in a few undoubted Polychaets there is a faint approach to
this in the specialisation of some of the nephridia as sperm-receptacles
and even as sperm-ducts. But nowhere among the Polychaeta are there the
diversified sperm-ducts and oviducts, spermathecae and sperm-sacs, that are
universal among the Oligochaeta. Moreover, no Polychaet has a clitellum,
which is so distinctive of the Oligochaeta, and of their near allies the
Leeches. Dr. Eisig has compared the glandular modification of the
integument at the mouths of the sperm-ducts in the Capitellidae to the
beginnings of a clitellum. This may be the case, but it is, in my opinion,
more comparable to the similar glandular spots at the male pores in
earthworms. The reproductive glands in the Oligochaeta (save for a few
apparently abnormal cases) are restricted to at most two pairs of each,
which occur in the same individual; the Polychaeta being dioecious. There
is, in short, no form known which cannot be definitely referred to either
the Polychaeta or the Oligochaeta, excepting perhaps _Ctenodrilus_, the
anatomy of whose reproductive organs is at present unknown.
It is a difficult task to classify the different families of the
Oligochaeta; and to enter into the historical aspect of the matter would
take too much space. I am myself disposed to divide them first of all into
two main groups, for which I use Dr. Benham's[428] names of Microdrili and
Megadrili.
The MICRODRILI are, as a rule, small and aquatic in habit; they have short
sperm-ducts which open on to the exterior in the segment which immediately
follows that which contains the internal aperture; the clitellum is only
one cell thick; the egg-sacs are large; the epoch of sexual maturity is at
a fixed period. This group, to my thinking, includes the Moniligastridae;
although Professor Bourne has denied my statement with regard {374}to the
clitellum, and in this case it is not so easy to decide their systematic
position.
The MEGADRILI are characterised by the precisely opposite characters. The
sperm-ducts are longer; the clitellum is composed of many layers of cells;
the egg-sacs are rudimentary; sexual maturity appears to be more or less
continuous.
There is, however, a substantial agreement about the families which I here
adopt, which may be fairly taken to express our present knowledge of the
Order. For fuller details the reader is referred to my Monograph of the
Order Oligochaeta.[429]
[Illustration: FIG. 195.—_Aeolosoma hemprichii_ dividing transversely, ×
30. (After Lankester.)]
I. MICRODRILI.
FAM. 1. _Aphaneura._[430]—This name was originally given to the present
family by Vejdovsky; the family contains a single genus, _Aeolosoma_, of
which there are some seven species. The name is taken from, perhaps, the
most important though not the most salient characteristic of the worms. The
central nervous system appears in all of them to be reduced to the cerebral
ganglia, which, moreover, retain the embryonic connexion with the
epidermis. The worms of the genus are fairly common in fresh waters of this
country, and they have been also met with in North and South America, and
in Egypt, India, America, and tropical Africa. They are all small,
generally minute (1 to 2 mm. long), and have a transparent body variously
ornamented by brightly-coloured oil globules secreted by the
{375}epidermis. These are reddish brown in _A. quaternarium_, bright green
in _A. variegatum_ and _A. headleyi_, in the latter even with a tinge of
blue. In the largest species of the genus, _A. tenebrarum_ they are olive
green. In _A. niveum_ the spots are colourless, and _A. variegatum_ has
colourless droplets mixed with the bright green ones. Fig. 195 shows very
well the general appearance of the species of this genus. The body has less
fixed outlines than in most worms, and the movement of the creatures is not
unsuggestive of a Planarian. As the under side of the prostomium is
ciliated, and as the movements of these cilia conduce towards the general
movement of the body, the resemblance is intelligible. One species of
_Aeolosoma_, at any rate, has a curious habit which is unique in the Order.
At certain times, for some reason at present unknown, the worm secretes a
chitinous capsule, inside which it moves about with considerable freedom;
these capsules when first observed were mistaken for the cocoons of the
worms; they are really homologous with the viscid secretion which the
common earthworm throws off when in too dry soil, and with which it lines
the chamber excavated in the earth in which it is lying. The worms of this
genus multiply by fission; sexual reproduction has been but rarely
observed.
FAM. 2. _Enchytraeidae._[431]—This family consists at present of rather
over fifty well-characterised species, which are distributed into eleven
genera. It is common in this country and in Europe generally; it has been
met with in Spitzbergen and the extreme north; it occurs in the American
continent from the north to the extreme south; it is also an inhabitant of
New Zealand. The worms of this family are nearly always of small size,
sometimes minute; they never exceed an inch or so in length, and that is a
rare occurrence. They are equally at home in water and in soil, some
species being common to the two media; a few are marine or littoral in
habit, while others are parasitic in vegetable tissues. Like most
earthworms, and unlike the majority of aquatic worms, the chaetae are
without a bifid termination; the body-wall, too, is comparatively thick.
The perivisceral fluid is often (as in certain Naids) loaded with
elliptical or rounded corpuscles. Resemblances to earthworms rather than to
the aquatic families of Oligochaeta are suggested by the long distance
which separates the {376}spermathecae from the male pores (segments 5 and
12), and by the paired or unpaired glands that have been already compared
to the calciferous glands so universally present among earthworms. On the
other hand, the male ducts are confined, as in the lower Oligochaeta, to
two segments, upon one of which the internal, upon the other the external
orifice is situated, and the oviduct is reduced to a simple pore, as in
Naids; but this may be merely a matter of convergence by degeneration.
Perhaps the most remarkable genus in the family is _Anachaeta_, which has
no chaetae, but in their place a large cell projecting into the
body-cavity, which appears to represent the formative cell of the chaeta.
The integument of this genus contains true chlorophyll, according to
Vejdovsky.
A singular character, found, however, also in _Rhynchelmis_ and _Sutroa_
among the Lumbriculidae, is the opening of the spermathecae into the
alimentary canal. This was originally discovered by Dr. Michaelsen, but has
been abundantly confirmed.
_Stercutus_ is a singular genus which was originally found in manure, and
has the peculiarity that the alimentary canal is often aborted; this
degeneration seems to bear some relation to the food and conditions of
life.
FAM. 3. _Discodrilidae._[432]—This family consists of small parasitic forms
which were at one time assigned to the Hirudinea; there seems, however, to
be no doubt that they are rightly included in the present Order.
_Branchiobdella_ is found upon the gills of the Crayfish, _Astacus
fluviatilis_; the American _Bdellodrilus_ upon _Cambarus_. The chief reason
for the former inclusion of these worms among the leeches was due to the
absence of chaetae and to the presence of chitinous jaws and of suckers;
apart from these structures there is nothing whatever leech-like about the
worms. _Bdellodrilus_ has two pairs of testes in segments 5 and 6; there
are two pairs of sperm-ducts, all opening, however, by a common "atrium" on
the sixth segment; on the fifth open a pair of spermathecae, likewise by a
common pore. The ovaries are in segment 7, and the ova escape by a pair of
pores apparently like the single pore of the Enchytraeidae. The entire worm
consists of only eleven segments.
FAM. 4. _Phreoryctidae._[433]—This family contains only two {377}genera,
_Phreoryctes_ and _Pelodrilus_. The former is widely spread, occurring in
Europe, North America, and New Zealand. _Pelodrilus_ is limited to New
Zealand. Most species of _Phreoryctes_ are distinguished by their
extraordinary length and thinness, and there is frequently a tendency to
the disappearance of the chaetae. The most important anatomical fact about
_Phreoryctes_ (at any rate _P. smithii_) is that there are two pairs of
ovaries as well as two pairs of testes, and that the ducts of all are
simple and very much alike. This seems to argue the low position of the
family in the series.
FAM. 5. _Naidomorpha._[434]—This family contains eight or nine genera,
perhaps more; they are all of them aquatic and of small size, and they
multiply by fission as well as sexually. The most noticeable peculiarity of
the family is the "cephalisation" which occurs in the head segments. In
some genera, in _Pristina_ for example, there is no such cephalisation to
be observed; but in others the dorsal bundles of chaetae commence a few
segments farther back than the ventral, the segment where they commence
being different and characteristic in the various genera. Thus in _Dero_
the first four segments are without dorsal chaetae, and in _Nais_ the first
five are in this condition. There is thus a kind of "head" formed, whence
the expression "cephalisation." _Dero_, _Nais_, and _Pristina_ are commonly
to be met with in ponds, lakes, etc., in this country. _Bohemilla_ is
rarer, and is to be distinguished by the remarkable serrated chaetae of the
dorsal bundles. Of _Dero_ and _Nais_ there are a considerable number of
different species; indeed it is usual perhaps to regard as distributable
among three genera, _Nais_, _Stylaria_, and _Slavina_, the species which I
am disposed to place in one genus, _Nais_. _Stylaria_ is defined on this
view by its extremely long prostomium, which has given rise to both its
popular and technical names. "Die gezungelte Naide" was the term applied by
one of its earliest investigators, and the name _Stylaria proboscidea_
signifies the same peculiarity. But as the same inordinately long
"proboscis" occurs in the South American _Pristina proboscidea_, belonging
to a genus of which the other member does not possess so well developed a
prostomium, it seems too variable a character upon which to differentiate a
genus. _Chaetogaster_ and {378}_Amphichaeta_ have been placed by some
systematists in a separate family. The first named contains four species
which are fairly common. It is one of those worms in which the chaetae are
not exactly related to the segmentation of other organs, which moreover
sometimes show an independence in their segmentation; thus there are more
nerve ganglia in the anterior segments of the body than there are septa.
FAM. 6. _Tubificidae._—The worms belonging to this family are of small
size, and are all inhabitants of fresh or salt water, or the margins of
pools and the sea. They differ from the last family in that asexual
reproduction never occurs, and that the reproductive organs are situated
rather farther back in the body. The male pores are upon segment 11, and
the oviduct-pores upon the following segment. This family differs from the
Lumbriculidae in the fact that there are only a single pair of sperm-ducts.
The earliest known Tubificid was the common _Tubifex rivulorum_, so widely
dispersed in this country and elsewhere; but with it was at first
confounded the somewhat similar genus _Limnodrilus_, which only differs in
that the chaetae are all of the cleft variety, and never capilliform, as in
_Tubifex_. The genera are mainly distinguished by the characters of the
chaetae and of the male ducts. At the base of the series perhaps lies
_Ilyodrilus_, which has many points in common with the Naids. The form of
the terminal chamber into which the sperm-duct opens has the same
simplicity as in that group, and the intestine is surrounded with a network
of blood-vessels as in the Naids, a structure which is otherwise wanting in
the Tubificidae. The development of the ova also is upon a plan which is
met with in the Naids. The atrium (see p. 361) becomes more complicated in
other Tubificidae. The extremity also is as a rule modified into a
retractile penis. The discrete "prostate," of which we have already spoken,
marks out a considerable number of genera, such as _Tubifex_,
_Limnodrilus_, _Spirosperma_, _Hemitubifex_. In the marine _Clitellio_
there is no such structure at all, and it is also wanting in the South
American _Hesperodrilus_. In _Branchiura_ there is a complete prostatic
investment of the atrium, and in _Telmatodrilus_ a large number of separate
aggregations forming as many distinct prostates. _Vermiculus_, a genus
consisting of but one species, found by Mr. Goodrich on the sea-shore in
the neighbourhood of Plymouth, is remarkable for the unpaired character of
the {379}generative organs, a peculiarity which is shared by Stolc's genus
_Bothrioneuron_. The gills upon the posterior segments of _Branchiura
sowerbyi_ and _Hesperodrilus branchiatus_ have been already noticed above
(p. 352). A very aberrant genus, perhaps not rightly referable to this
family, is _Phreodrilus_,[435] from New Zealand, first collected in water
from a subterranean spring. It differs from all other Tubificids except
_Hesperodrilus_ in that the spermathecae lie behind the male pores, a state
of affairs which is met with in the Lumbriculidae. Another singularity of
structure concerns the sperm-duct, which is wrapped in a thin-walled sac,
which has every appearance of being simply the outer muscular wall of the
duct. Within this are the complicated coils of the duct, and also a
quantity of free spermatozoa, whose mode of ingress is difficult to
understand. Many of the Tubificidae live in tubes fabricated by themselves,
whence the tail end protrudes. The integument in more than one species is
vascular. This integumental blood system, universal among the earthworms,
appears to be restricted to the present group among the Limicolae of
Claparède.
FAM. 7. _Lumbriculidae._[436]—This family is not a large one, and is nearly
limited in range to Europe and North America; indeed, if we omit the
doubtful _Alluroides_, entirely to the Palaearctic region. There are only
fourteen species, which are referred to eight genera. A number of dubious
forms, as is the case with other families, may possibly ultimately swell
this list. The type genus of the family, viz. _Lumbriculus_, upon which
Bonnet made his experiments in section and subsequent regeneration, has
only within the last year been thoroughly explored anatomically. But all
the other genera are well known. The Lumbriculidae are of small or moderate
size, and all of them aquatic in habitat. There are three characters which
are nearly or quite universal in the genera of the family. In all of them
the chaetae are only eight to each segment, arranged in couples, and are
either cleft at the extremity or simple. As a rule which has but two
exceptions, the genera _Alluroides_ and _Lumbriculus_, there are two pairs
of sperm-ducts, which, however, communicate with the exterior through a
single terminal chamber on each side of the body.
The dorsal blood-vessel has in the Lumbriculidae a series of
{380}contractile and blind appendages, which were at first mistaken for
caeca of the intestine itself. There are two genera of this family in North
America, which are not very different anatomically from their European
representatives. The genera described by Eisen are _Sutroa_[437] and
_Eclipidrilus_.[438] The latter lives in cold torrents at a great height in
the mountains of the Sierra Nevada of California.
FAM. 8. _Moniligastridae._—This family, terrestrial in habit, is probably
Oriental in range; but I have described a single species from the Bahamas
which may possibly be referable to the category of accidentally introduced
specimens. Our knowledge of this family is conveniently summed up in
Professor Bourne's paper[439] upon the genus _Moniligaster_. There are some
eighteen species, which range in size from an inch or so in length (_M.
bahamensis_) to about two feet; this last measurement is that of the huge
_M. grandis_, of which, together with many others, Professor Bourne gives
coloured drawings. There is a second genus, _Desmogaster_, which is mainly
characterised by the doubling of the reproductive organs. This was
described by Rosa from Burmah. The family is noteworthy on account of the
fact that every species belonging to it has at least four distinct
gizzards, sometimes more; but as this multiplication of the gizzards has
been also found in _Heliodrilus_ among the Eudrilidae, and indeed
elsewhere, it is insufficient to define the family. More characteristic is
the fact that the sperm-ducts open on to the next segment to, or even the
same segment as, that which contains their funnels; consequently the
apertures of the oviducts are behind instead of in front of them. These
pores are also situated in a very anterior position, the male pores being
upon the tenth segment or between the tenth and eleventh, and the oviducal
pores upon the following one. In these features the family presents
resemblance to the aquatic Oligochaeta, from which, however, its
stoutly-built gizzards, and vascular nephridia differentiate it.
II. MEGADRILI.
FAM. 9. _Perichaetidae._[440]—The Perichaetidae comprise a larger
{381}number of species than any other family of earthworms; but it is a
matter of considerable difficulty to divide the family satisfactorily into
genera. The family as a whole may be defined as having numerous chaetae in
most of the segments of the body.
There is no other definition which will distinguish this family from the
next two families, and even this definition is not absolutely distinctive.
There are Acanthodrilids which have a large number of chaetae in each
segment. The only difference is that in this case—in the genus
_Plagiochaeta_—the chaetae are implanted in twos; this is not the case in
the Perichaetidae. In all Perichaetidae that are known the sperm-ducts open
in common with the ducts of the spermiducal glands; they generally open
into them at some distance from the common external pore. In _Megascolex_,
_Perichaeta_, and _Pleionogaster_ the nephridia are of the diffuse type so
widely spread among these worms, and the spermiducal glands are lobate.
_Megascolex_ differs from the others in the fact that in addition to the
small scattered nephridia there are a pair of large nephridia in each
segment, and the chaetae do not form absolutely continuous circles, but are
interrupted above and below. _Pleionogaster_ has more than one gizzard but
otherwise agrees with _Perichaeta_; it is confined to the East.
_Perichaeta_ is tropical and occurs—no doubt introduced—in Europe and
America. _Megascolex_ is Old World only, and, like _Perichaeta_, Australian
as well as Oriental. But whereas _Perichaeta_ is rare in the Australian
region, _Megascolex_ is common there. _Perionyx_ and _Diporochaeta_ are the
other genera which it is possible to recognise. Both of them have paired
nephridia, and neither of them have intestinal caeca, a peculiarity which
they both share with _Megascolex_ and _Pleionogaster_. _Perionyx_
principally differs from _Diporochaeta_ in that the spermiducal glands are
lobate, whereas in the latter they are as in the Acanthodrilidae.
_Perionyx_ is Oriental; _Diporochaeta_ occurs in Australia and New Zealand.
[Illustration: FIG. 196.—_Perichaeta everetti_ F. E. B. × 1. _sp_,
Spermathecal pores; _cl_, clitellum; ♀, female pore; ♂, male pore.]
A very distinctive feature of _Perichaeta_—perhaps only of the {382}genus
_sensu stricto_—is its exceeding activity. The first specimens ever noticed
in this country, or at least of whose existence printed notice was taken,
were exhibited by the late Dr. Baird of the British Museum, at a meeting of
the Zoological Society. He remarked in that communication upon the agile
fashion in which these tropical Annelids will spring off a table when
touched or in any way interfered with. Numerous other observers have seen
the same manifestations, and the name of "eel-worm" has been given to these
_Perichaeta_ by gardeners. It is worth putting on record here that in a
species of _Acanthodrilus_ (_A. capensis_) the same irritable behaviour is
visible. When a _Perichaeta_ moves it helps itself greatly by extending, or
rather protruding, the buccal cavity, which serves as a sucker, and grips
the ground in front until the rest of the body is brought forward. It is
possibly on account of this extra facility for movement that the genus can
climb trees with such ease. A species of _Perichaeta_ has been recorded by
Mr. Willey upon an epiphyte of a palm, and Dr. Benham has found that it is
a new species, to which the name of _Perichaeta willeyi_ has been given.
The Lumbricid genus (if it be admitted as a genus), _Dendrobaena_, was so
named on account of a similar habit of climbing trees. Very singular in its
habit is the not inaptly-named _Perichaeta musica_ of Java. It is a monster
of its kind, several feet in length, and during the night makes "a sharp
interrupted sound," apparently by the friction of the chaetae against
stones. The species figured (p. 381) is, as are a few others, remarkable
for the presence of twelve or seventeen spermathecae in segments 6 and 7.
FAM. 10. _Cryptodrilidae._[441]—This family is one of the largest of the
Oligochaeta; there are rather over 120 different species, which can be
arranged in at least sixteen genera. They are found in most parts of the
world, but abound principally in the tropics. Australia may be considered
to be the headquarters of the family, which form its principal
earthworm-inhabitants. Peculiar to this continent, or at least mainly
confined to it, are the genera _Megascolides_, _Cryptodrilus_,
_Fletcherodrilus_, _Trinephrus_, and _Digaster_. _Microscolex_, though
occurring in many parts of the world, is characteristic of the more
southern regions of South America and of New Zealand. Tropical Africa has
the genera _Nannodrilus_ and _Millsonia_ limited to itself, and has besides
nearly {383}all the species of the genus _Gordiodrilus_. This family is one
which it is exceedingly difficult to define and to split up into different
genera. It shades almost imperceptibly into the Perichaetidae on the one
hand, and is very hard to differentiate from the Acanthodrilidae on the
other. A Cryptodrilid, like any member of the genus _Cryptodrilus_, with
complete circles of chaetae would be a Perichaetid; and as there are
species of _Perichaeta_ in which the anterior segments have only a few
chaetae in each segment, it is perhaps wrong to separate the two families
at all. Apart from the chaetae, there is no peculiarity in the organisation
of the family Perichaetidae that is not also met with in the
Cryptodrilidae. Even the highly characteristic intestinal caeca so
distinctive of the genus _Perichaeta_ itself, as contrasted with
_Megascolex_ and the other genera, occur, though more numerously, in the
African _Millsonia_, where there are forty or fifty pairs of them. A fairly
common feature in the family is the presence of two, or even three, pairs
of gizzards, a character which is also met with in the genus _Benhamia_
among the Acanthodrilidae, and occurs also in some other families. The
names _Digaster_, _Didymogaster_, _Perissogaster_, and _Dichogaster_ have
been founded upon this character. The excretory organs may be paired (in
_Trinephrus_ there are three pairs to each segment) or of the diffuse kind.
The male pores are usually upon the eighteenth segment, but not
unfrequently upon the seventeenth, and are often armed with long and
ornamented chaetae. Spermiducal glands are invariably present, and may be
lobate or tubular. There are two groups of small-sized genera, which in
their simplicity of organisation stand at the base of the series; but it is
very possible that the simplification is rather due to degeneration than to
primitive position. One of these groups includes the semi-marine genus
_Pontodrilus_ (with which I include the phosphorescent _Photodrilus_) and
_Microscolex_. In these forms the gizzard has disappeared, or is
represented by a rudimentary structure, and the male pores are upon the
seventeenth segment. In the other group are the genera _Ocnerodrilus_,[442]
_Gordiodrilus_,[443] and _Nannodrilus_, which are of even smaller size, and
have in the same way the male pores upon the seventeenth segment. The
species of this group are often aquatic, and there is not only no gizzard
{384}(in most of the species), but the calciferous glands have been reduced
to a single pair, which lie in the ninth segment. The latter character is
also found in the Acanthodrilid _Kerria_, which has been associated with
the above named. _Gordiodrilus_ has the peculiarity that there are, as in
Acanthodrilids, two pairs of tubular spermiducal glands.
FAM. 11. _Acanthodrilidae._[444]—This family is only with difficulty to be
distinguished from the last. The following definition applies to all the
members of the family with one exception, and does not apply to any
Cryptodrilid with, so far as is known, one exception only. There are two
pairs of spermiducal glands, opening upon the segments in front of and
behind that which bears the apertures of the sperm-ducts.
The one exception to this definition is the species _Acanthodrilus
monocystis_, which I formerly placed in a distinct genus, _Neodrilus_.
_Microscolex modestus_ is the exception among the Cryptodrilidae; in that
worm the male pores are upon the segment which follows that upon which the
spermiducal glands open. The Acanthodrilidae show a considerable range of
structural variation. This enables them to be separated into several
well-marked genera. The type genus _Acanthodrilus_ has a pair of nephridia
in each segment. It contains thirty-five species, which are all from the
southern hemisphere. These species show but little variation among
themselves. _Benhamia_ is a genus that differs from _Acanthodrilus_ in the
fact that the nephridia are of the complex type, so often met with in
earthworms with many external pores. The segment that bears the male pores
is entirely without any traces of the ventral chaetae. Here again there are
a large number of species which are nearly confined to the continent of
Africa. Dr. Michaelsen is indeed of opinion that the few species found in
the East Indies and America are accidental importations. I have proposed to
separate some of the New Zealand Acanthodrilids into a distinct genus,
_Octochaetus_, which is somewhat intermediate between _Acanthodrilus_ and
_Benhamia_. They have multiple nephridia, but only a single gizzard.
_Plagiochaeta_ of Benham, from New Zealand, is in any case clearly a
distinct form. It is mainly to be distinguished by the numerous chaetae in
each segment. _Trigaster_ Benham, is West Indian. _Deinodrilus_ (New
{385}Zealand) has twelve chaetae in each segment. _Diplocardia_, from North
America, has the male pores on segments 18, 19, 20.
FAM. 12. _Eudrilidae._[445]—This is perhaps the most remarkable family of
terrestrial Oligochaeta. Its distribution is no less curious than its
structure. Up to the present it is not known outside tropical Africa, with
the exception of the genus _Eudrilus_ itself, which is almost world-wide in
range. As, however, but one species of _Eudrilus_ is found out of Africa,
and as that species is so common in gatherings from various tropical
countries, it seems to be an instance of a species with large capacities
for accidental transference from country to country. The type genus,
_Eudrilus_, has been known since 1871, when it was originally described by
M. Perrier.[446] Since that date nineteen other genera have been described
from Africa by Dr. Michaelsen, Dr. Rosa, and myself. The most salient
external character of the group, not universal but general, is the unpaired
male and female orifices. The orifices are commonly very conspicuous (see
Fig. 197).
[Illustration: FIG. 197.—_Libyodrilus violaceus_ F. E. B. × 2. _sp_,
Spermathecal pore; _cl_, clitellum; ♂, male pore.]
The peculiarities of internal structure mainly concern the reproductive
organs, the differences in which from genus to {386}genus are often very
great. We have already referred to the remarkable branching of the
nephridial duct in the body-wall, and to the much modified calciferous
glands of _Stuhlmannia_ and some other genera. These structural variations
perhaps permit the family to be divided into two sub-families. In one there
are calciferous glands of the normal type, though peculiar in that one or
more are median and ventral in position, and are unpaired; there is no
branching of the nephridium in the body-wall; there are always, so far as
is known, the Pacinian-corpuscle-like bodies in the integument. In the
other sub-family the calciferous glands, if present (they are absent, for
instance, in _Libyodrilus_), have undergone much modification in structure;
the nephridia, where they have been investigated, have been found to branch
copiously in the body-wall; the peculiar integumental bodies hardly ever
occur.