with the exception of a few anteriorly and a few posteriorly. The
nephridium of _Nereis_ differs from that of most other Polychaetes hitherto
examined carefully, and rather resembles that of the Oligochaetous
Enchytraeids. It consists of a compact gland-like organ, containing a much
coiled tube, ciliated for the greater part of its length, but deprived of
cilia in its last coils; this latter part—or duct—leaves the "gland" and
pierces the body-wall, opening to the exterior at the base of the
parapodium. The ciliated canal passes forwards into the next segment, where
it opens by a funnel into the coelom. The lip of the funnel is extremely
curious, for the cells constituting it are drawn out into very long,
delicate processes covered with cilia.[307]
[Illustration: FIG. 128.—Nephridium of _Nereis_. (From Goodrich.) _f_,
Funnel; _n_, neck, which passes through a septum; _t_, coiled tubule; _c_,
connective tissue; _d_, duct.]
In most Polychaetes the nephridium is a wide, sac-like tube as in
_Arenicola_[308] (Fig. 129). Its walls are covered by a dense network of
blood-vessels, and it not only acts as an excretory organ, but also as a
genital duct (see p. 273).
Excretion, in the strict sense of the word, is carried out by {254}the
cells forming the wall of the tube; they remove waste materials from the
blood distributed over the surface of the organ. But, in addition, there is
a removal from the coelom, by means of the funnel, of any dead or dying
coelomic corpuscles which in their turn have eaten up or otherwise
destroyed foreign bodies (such as Bacteria, etc.) that may have entered the
animal.
In _Nereis_ there is in each segment, in addition to the pair of nephridia,
a pair of "dorsal ciliated organs" (Goodrich) (_cil.org_ in Fig. 124). Each
appears as a wide-mouthed funnel, greatly folded, and without any permanent
outlet. But it is possible that these organs function as genital ducts, and
that the external aperture will make its appearance temporarily at the
period of maturity. This "dorsal ciliated organ" has not been met with in
allied genera—such as _Eunice_, _Nephthys_, _Polynoë_, _Glycera_—where the
nephridium is a wide tube, and serves as a genital duct.
[Illustration: FIG. 129.—Nephridium of _Arenicola_. (From Benham.) × 4.
_d_, Dorsal lip of funnel; _v_, ventral lip of funnel; _b_, blood-vessel
(all the black lines are blood-vessels); _m_, dilated bladder; _x_, part
cut away from body-wall where the nephridium is passing to the exterior.]
The NERVOUS SYSTEM, as in all Chaetopods, consists of a dorsal cerebral
ganglion or "brain" (Fig. 127, _br_), connected by circum-buccal
commissures with the anterior end of a ventral chain of ganglia. The brain
occupies the prostomium,[309] and from it nerves pass away to the
prostomial tentacles and palps. The circum-buccal commissures spring from
the outer corner of the brain, and from each arises a nerve to the first
pair of peristomial cirri. The first ventral ganglion lies in the third
segment, and represents at least two ganglion-pairs fused together, for
from it arise (1) a pair of nerves to the second pair of peristomial cirri
and (2) a {255}pair to the first parapodium. In the remainder of the body
there is a ganglion in each segment, whence nerves pass outwards to the
parapodium and muscles of the segment (Fig. 124).
In _Nereis_ the apparently single ganglion in each segment really consists
of two halves, and the apparently single cord which traverses the whole
length of the body consists of two closely apposed cords. In some worms,
such as Serpulidae, the two cords are more or less widely separated, and
the two ganglia of each segment are thus distinct, and connected by a
transverse commissure. In _Nereis_, as well as in many other Polychaeta,
the nerve-cords lie within the body-wall, but in other cases they lie in
the epidermis, as they do in Archiannelida.
The visceral nervous system, supplying the muscles of the pharynx, is
frequently highly developed. In _Nereis_ it arises on each side by two
roots, one from the brain, the second from the circum-buccal commissure.
[Illustration: FIG. 130.—Eye of _Nereis_. (After Andrews.) × 150. A,
Section through the entire eye; _c_, cuticle; _e_, epidermis; _l_, lens;
_h_, rods; _r_, retina; _n_, optic nerve: B, isolated retinal element; _c_,
cell; _p_, pigment; _h_, rod: _f_, nerve-fibre.]
The ORGANS OF SENSE in _Nereis_ are eyes, tentacles, palps, and cirri. The
four eyes, which rest upon the brain, have the structure represented in
Fig. 130. The retina consists of a single layer of cells containing
pigment; each cell is drawn out peripherally into a nerve-fibre, whilst
centrally it forms a cuticular product—the "rod" (_h_). The edges of the
retina are continuous with the surrounding epidermis, and the cup thus
formed remains widely opened to the cuticle in a few Polychaetes, _e.g._
_Autolytus_, and in the young of _Nereis_, but more usually it has the
relations represented in the figure. The lens is produced by the retinal
cells (according to Andrews[310]), and is in some cases (_Eunice_,
_Amphinome_) continuous with the cuticle. It appears to be composed in
other cases (_Lepidonotus_) of continuations of the retinal rods. The
structure of the other sense organs {256}indicates their adaptation to a
tactile function; in each case a nerve traverses the axis of the organ, and
the nerve-fibrils terminate in sensory cells. Very probably the palps have
a certain power of testing the food—a combination of the senses of taste
and smell.
The GENERATIVE SYSTEM.—In all the Polychaeta, with very few exceptions, the
sexes are separate; and the reproductive cells—ova and spermatozoa—are
produced at certain seasons of the year by the rapid proliferation and
modification of coelomic epithelial cells surrounding the blood-vessels in
the parapodium and its immediate neighbourhood. The sexual cells remain in
the coelom till they are ripe.
The egg-cells become filled with yolk globules; a vitelline membrane is
present, and an outer coat of albuminous material. It is doubtful by what
means these sexual cells are discharged in _Nereis_. There is some evidence
that the "dorsal ciliated organ" may act as a genital duct. In some other
worms the nephridia serve this purpose, whilst in others a rupture of the
body-wall allows the products to escape into the sea. According to
Wistinghausen,[311] at the time of discharge the females of _Nereis
dumerilii_ become surrounded by a kind of gelatinous tube formed from a
secretion of the parapodial glands, and into this tube the ova are
discharged, and arranged in a single layer round its wall.
The common species _Nereis diversicolor_ is viviparous. In a large number
of species of _Nereis_ the sexually-mature individuals undergo very marked
changes in various parts of their body, so that they differ very greatly
from the immature individuals.
These changes resulting in the "heteronereid" condition will be dealt with
at some length in Chap. X. p. 276. The larvae of Polychaetes and other
facts connected with reproduction are described in the same chapter.
{257}CHAPTER X
CLASSIFICATION OF THE POLYCHAETA—SHAPE—HEAD—PARAPODIA—CHAETAE—
GILLS—INTERNAL ORGANS—JAWS—SENSE ORGANS—REPRODUCTION—LARVAL FORMS—
BUDDING—FISSION—BRANCHING—REGENERATION.
The POLYCHAETA are marine worms whose bodies are usually elongated and
cylindrical; they either lead a free life, swimming in the open sea, or
crawling along the bottom; or they pass their life in burrows or definite
tubes of various kinds.
Each segment is normally provided on each side with a single or a couple of
bundles of chaetae, by means of which locomotion is effected. These, in the
free-living forms, are carried at the ends of lateral muscular outgrowths
of the body, known as "parapodia," which are practically limbs.
The "head" of the worm generally carries eyes, and frequently more or less
elongated tactile organs, the "tentacles" dorsally and "palps" ventrally.
The foregut is frequently provided with a masticating apparatus in its
anterior region, which is capable of protrusion; but this apparatus is
absent in many burrowing and tubicolous forms. The sexes are separate, so
that there is no such complicated system of generative organs as occurs in
the Oligochaeta. The nephridia usually act as genital ducts. In the
majority of cases the egg develops into a larva, the "Trochosphere," which
leads a free life and undergoes a greater or less metamorphosis into the
adult condition.
The CLASSIFICATION OF POLYCHAETA adopted in this work is as follows:[312]—
{258}BRANCH A. PHANEROCEPHALA.
SUB-ORDER 1. _Nereidiformia_ [= _Errantia_, _auctt._ + _Ariciidae_].
Family 1. Syllidae see p. 306
" 2. Hesionidae " 308
" 3. Aphroditidae " 309
" 4. Phyllodocidae " 313
" 5. Tomopteridae " 315
" 6. Nereidae " 315
" 7. Nephthydidae " 317
" 8. Amphinomidae " 318
" 9. Eunicidae " 318
" 10. Glyceridae " 320
" 11. Sphaerodoridae " 320
" 12. Ariciidae " 321
" 13. Typhloscolecidae " 321
SUB-ORDER 2. _Spioniformia._
Family 1. Spionidae see p. 321
" 2. Polydoridae " 323
" 3. Chaetopteridae " 323
" 4. Magelonidae " 325
" 5. Ammocharidae " 325
SUB-ORDER 3. _Terebelliformia._
Family 1. Cirratulidae see p. 325
" 2. Terebellidae " 327
" 3. Ampharetidae " 330
" 4. Amphictenidae " 330
SUB-ORDER 4. _Capitelliformia._
Family. Capitellidae, see p. 331.
SUB-ORDER 5. _Scoleciformia._
Family 1. Opheliidae see p. 331
" 2. Maldanidae " 332
" 3. Arenicolidae " 333
" 4. Scalibregmidae " 334
" 5. Chlorhaemidae " 334
" 6. Sternaspidae " 335
BRANCH B. CRYPTOCEPHALA.
SUB-ORDER 1. _Sabelliformia._
Family 1. Sabellidae see p. 336
" 2. Eriographidae " 338
" 3. Amphicorinidae " 339
" 4. Serpulidae " 339
SUB-ORDER 2. _Hermelliformia._
Family. Hermellidae, see p. 341.
COMPARATIVE ANATOMY OF THE POLYCHAETA.
GENERAL SHAPE OF THE BODY.—The majority of the Polychaeta have an elongated
and very mobile body, like that of _Nereis_, consisting of an indefinite
and usually of a considerable number of segments; a few, however, have a
shorter body, with fewer segments, definite in number, for instance
_Aphrodite_ and _Polynoë_, which have thirty to forty segments; and some
Hesionids, with only some seventeen to twenty segments.
{259}In Aphroditidae and certain Amphinomidae the body is more or less oval
in shape. In _Lipobranchius_ and _Sternaspis_ it is grub-like, short, and
cylindrical, with rounded ends; in the former it is difficult to
distinguish head and tail, or dorsal and ventral surfaces.
The segments composing the trunk may be all alike, or may constitute two
more or less sharply marked regions, the _thorax_ and _abdomen_, differing
in the character of the chaetae, or in their arrangement, or in some other
way, as in the Sabelliformia and the Capitelliformia.
As peculiar cuticular structures, the curious shields of _Sternaspis_, and
of certain of the Maldanidae may be mentioned.
The posterior extremity is generally more or less narrowed, and most of the
Nereidiformia are provided with special elongated cirri, borne by the anal
segment. In the Maldanidae and others the body terminates in a funnel, at
the bottom of which is placed the anus. Only in a few cases is the anus not
terminal; in _Notopygos_ and other Amphinomidae, as well as in some species
of _Polynoë_, it is dorsal.[313] In _Sabellaria_ and _Pectinaria_ the
hinder end of the body undergoes great degeneration; in the former it is
achaetous, but cylindrical and bent forwards alongside the body (Fig. 131).
In _Pectinaria_ (Fig. 177), this region, which is called the "scapha," is
leaf-like, and serves to close the narrower end of the tube in which the
worm lives. _Arenicola marina_, and some Terebellids have no chaetae in the
hinder, narrower part of the body.
[Illustration: FIG. 131.—_Sabellaria alveolata_ L. × 3. (After Malmgren.)
_a_, Anus.]
The HEAD.—The prostomium is, in the majority of cases, rounded or conical,
though it may be square (_Nephthys_) or elongated and jointed (_Glycera_),
or even hammer-shaped (_Tomopteris_); or it may be fused with the
peristomium, and apparently absent (_Arenicola_). In the great group
Cryptocephala, the peristomium grows forwards so as to hide the prostomium
entirely.
In a few of the Nereidiformia the prostomium is compressed, {260}and in the
Amphinomidae it is provided with a dorsal ridge or "caruncle," which is a
leaf-like process overlapping three or more segments. In many Aphroditidae
(as well as in _Polydora_) there is a peculiar "frontal" ridge passing
forwards from the prostomial tentacle, and downwards into the mouth (Figs.
132, _c_, and 133, A, _x_).
[Illustration: FIG. 132.—_Aphrodite aculeata_ L. Ventral view of anterior
region, × 6. _a_, Prostomium; _c_, frontal ridge on prostomium; _d_,
neuropodial cirrus; _l_, lower lip; _m_, mouth; _p_, palp; _s_,
intersegmental groove; _t_, tentacle; I, foot of peristomium, which has
shifted forwards so as to lie in front of the mouth; II to V, successive
feet.]
In all the Nereidiformia, as well as in Sabelliformia and Chlorhaemidae,
the prostomium bears sensory processes of two kinds, viz. dorsal tentacles
and ventral palps. The latter are invariably two in number, and are
particularly well developed in Aphroditidae, Nereidae, Syllidae, some of
the Eunicidae, and in Chlorhaemidae. Even when they are apparently absent,
as in _Nephthys_, it is possible that they are represented by certain lobes
at the sides of the mouth, for in many Syllidae they are so fused with the
prostomium as to be scarcely distinguishable. In the Chlorhaemids the
palps[314] are grooved, and in the Sabelliformia they become considerably
branched, and extend round the prostomium so as to nearly meet dorsally and
ventrally. Each palp is, in this sub-Order, represented by a greater or
smaller number of long, mobile filaments, arising from a common base; they
are grooved along the inner side, ciliated, and provided with secondary
processes. The crown of "gills," in fact, is nothing {261}more than the
greatly subdivided and enormously elongated palps, as both Pruvot[315] and
Meyer[316] have shown. In such forms as _Haplobranchus_ and _Amphicorine_
the process of subdivision (branching) has only gone a short way. In all
the Sabelliformia each filament, in addition to its sensory function, aids
in conveying food to the mouth by the action of the cilia, and has a
blood-vessel within, thus acting as a respiratory organ. The filament may
carry compound eyes (Fig. 143) either at its apex (_Branchiomma_) or at
intervals along its course (_Dasychone_).
[Illustration: FIG. 133.—A, Anterior end of _Polydora_ enlarged. _a_,
Prostomium; _x_, frontal ridge; I, peristomium; _c'_, its long cirrus; II,
III, etc., the following segments; _c_, gill; B, head of Sabellid; _P_,
palps (branchial crown); _t_, position of tentacles; _l_, processes of
upper lip membrane; I, peristomium raised into a collar; II, III, IV,
following segments.]
In the family Serpulidae one (rarely two) of the most dorsally placed gill
filaments is enlarged terminally, and acts as a stopper or "operculum,"
which closes the mouth of the tube when the animal withdraws into it.
Further, in _Spirorbis_ this operculum is grooved on one side, and serves
as a brood pouch in which the eggs undergo development (Fig. 184, p. 341).
It will be seen, therefore, that the palps may be very important organs for
the life of the worm, and they are no less interesting to the comparative
anatomist, serving as they do as an excellent illustration of the various
uses which Nature finds for one and the same organ.
In the other sub-Orders the prostomium carries neither palps nor tentacles.
{262}[Illustration: FIG. 134.—Heads of various Polychaeta (diagrammatic).
A, Polynoid; B, Syllid; C, _Nephthys_; D, _Eunice_; E, _Phyllodoce_; F,
_Trophonia_: _a_, prostomium; _c_, normal cirrus; _c^1_, peristomial cirri;
_c^2_, cirrus of second segment; _c^3_, cirrus of third segment; _el^1_
point of attachment of elytron; _p_, palp: _s_, nuchal organ (ciliated
pit); _t_, tentacle; I, peristomium; II, III, IV, segments.]
The _tentacles_ in the Nereidiformia present a wide variation in number;
probably the typical number is three, one of which is median and two
lateral—as in Polynoids, Syllidae, and some Eunicidae. Further, there is a
certain amount of evidence in the nerve supply of the median tentacle to
show that it was originally double. The presence of four tentacles, then,
as in _Nephthys_, _Phyllodoce_, and _Glycera_, may be a primitive
condition. By the disappearance of the paired lateral tentacles the worm
possesses a single median one, as in _Aphrodite_ and Amphinomids;[317]
whilst a duplication of these lateral ones leads to the condition of
_Eunice_ and _Hyalinoecia_, which have five tentacles. In the Chlorhaemidae
the number is further increased to five or more on each {263}side,[318] and
in the Terebellidae these prostomial processes become very numerous.
In the Cryptocephala there is never more than a single pair of tentacles,
and these are generally reduced to a group of sensory cells, though in
_Sabellaria_ they retain a considerable size.
In a few genera, such as _Aphrodite_, _Nephthys_, _Capitella_, the first
postoral segment is distinguished from the succeeding segments only by its
position with regard to the mouth (Fig. 132) and by its smaller size. But
in the remainder of the Polychaeta, with here and there an exception, the
peristomium is achaetous in the adult.[319]
Except in the Nereidiformia, peristomial or tentacular cirri are rare,
being represented in the Spioniformia by the very long "tentacles." In the
Nereidiformia one or more of the following segments may be added to the
peristomium, and share in the "cephalisation," which is so characteristic a
feature in this group. In Amphinomids the first three or four chaetigerous
segments are incomplete ventrally, owing to the shifting of the mouth
backwards; these segments form lateral lips, but they are not otherwise
modified. In _Phyllodoce_, however, there are four cirri on each side of
the mouth, and from the arrangement in the Alciopids we are justified in
concluding that the segment which carries the four pairs of cirri is really
made up of three segments (Fig. 134, E). Among the Hesionids there are four
such "cephalised" achaetous segments with long cirri.
[Illustration: FIG. 135.—_Sabellaria alveolata_ L. Ventral view of anterior
region, × 10. _a_, Notopodial cirrus; _b_, notopodium; _c_, neuropodium;
_ch_, peristomial chaetae; _d_, neuropodial cirrus; _m_, mouth; _P_,
multifid palp (gill filaments); _P'_, ridges after removal of gill
filaments; _s_, ventral (tubiparous) gland shield; _T_, tentacle; I, hood
formed by peristomium; II to VI, following segments.]
{264}In a few cases, such as the Chlorhaemids and _Sternaspis_, and to a
slight degree in _Arenicola_, the "head" and even the anterior part of the
worm is capable of being withdrawn into the body.
THE PARAPODIA AND CHAETAE.—The typical parts of a parapodium have been
described in the preceding chapter; here it is only necessary to refer to
the series of diagrams (Figs. 136, 137) representing the parapodia of the
more common Polychaetes, and to add a few remarks about them.
[Illustration: FIG. 136.—Parapodia. A, _Nephthys_; B, _Amphinome_; C,
_Glycera_ (the unlettered lobe above _g_ is the notopodial cirrus); D,
_Syllis_; E, _Eunice_; F, _Phyllodoce_. _a_, Notopodial cirrus; _b_,
notopodium; _c_, neuropodium; _d_, neuropodial cirrus; _g_, special gill;
_n_, aciculum (omitted in B); _x_, cirriform lip of chaetigerous sac.]
In most Annelids the chaetae are in two bundles on each side, but there are
certain families in which the dorsal bundle, and even the notopodium
itself, is absent, as in the Eunicidae, Syllidae, and Phyllodocidae; or the
dorsal bundle may be absent only in certain regions of the body, as in the
hind-body of Terebellids. In some Amphinomidae and Aphroditidae the
notopodium is scarcely distinct as a separate lobe, being a slight tubercle
on the upper surface of the neuropodium; but the notopodial chaetae are
present, and indeed particularly well developed in many cases.
But whilst, in the Nereidiformia, the parapodia, whether {265}consisting of
two lobes or only one, are always well developed, and project to a more or
less pronounced degree from the sides of the body, it is otherwise in the
rest of the group, where the chaetigerous lobes are usually reduced to mere
tubercles or ridges, no doubt in relation to their burrowing or tubicolous
habits. In _Sternaspis_ the chaetae issue directly from the body-wall.
Amongst the Nereidiformia we find examples in which the parapodia, instead
of being more or less conical "legs," are flattened fore and aft so as to
serve as efficient "fins," as in the active swimmers, _Nereis virens_ and
_Nephthys caeca_, and in the pelagic Phyllodocids, Alciopids,
Typhloscolecids, and _Tomopteris_.
[Illustration: FIG. 137.—Parapodia. A, _Polynoë_; B, _Scoloplos_; C,
_Euphrosyne_. (Transverse section of body.) _a'_, Accessory cirrus; _y_,
doubtful branchiae; D, _Sabella_ (thoracic). _a_, Notopodial cirrus
("elytron" in A, "gill" in B); _b_, notopodium; _c_, neuropodium; _d_,
neuropodial cirrus; _n_, aciculum (accidentally omitted in C).]
Of the typical dorsal and ventral cirri, the ventral is only absent in some
Amphinomids amongst the Nereidiformia; the dorsal is absent in _Nephthys_
and degenerate in _Glycera_, whilst in a very large number of families of
the other sub-Orders neither cirrus is present. These cirri, though
originally filamentous and sensory, may, by virtue of special blood supply,
become "gills," and this occurs in several families of different
sub-Orders. Thus in _Eunice_ this gill is comb-like; in _Amphinome_ and in
_Arenicola_ (on certain segments) it is arborescent, as it is also in one
to three segments in Terebellids; whilst in Ariciidae, Spioniformia,
Cirratulidae, Opheliidae, and _Sabellaria_ it remains more or less
finger-shaped or filamentous. In the family Serpulidae the thoracic cirri,
both dorsal and ventral, {266}become flattened and extended
antero-posteriorly, and unite with one another to form the "thoracic
membrane."[320] In Phyllodocidae the cirri are foliaceous and natatory, and
they contain a great quantity of glands of a peculiar character. The
Aphroditidae are distinguished from other Annelids by the possession of
"elytra" or dorsal scales, which appear to be the dorso-ventrally flattened
cirri, retaining their sensory nature, but adding to this function several
others.[321]
The CHAETAE or bristles are mainly used in locomotion, but it is not
unreasonable to believe that some of the stronger, serrated kinds may be
used as weapons of offence and defence; certainly the Polynoids, bristling
as they do with stiff chaetae along each side, must be rather unpleasant to
their smaller enemies.
The various bristles may be placed in three chief groups, viz. (1) simple;
(2) jointed; (3) uncini (see Fig. 138).
(1) The simple chaetae may be smooth and hair-shaped, _i.e._ "capillary,"
such as are present in nearly all families: or they may be forked
(Amphinomidae), comb-shaped (_Eunice_), notched or serrated, or provided
with a series of frills at right angles to their length, as in
Aphroditidae; or fringed along one or both sides with a membranous
expansion, as in Terebellids and Sabellids. The simple chaetae may also be
short and spine-like, as in the ventral bundles of _Arenicola_; or they may
be slightly curved at the end and notched, forming what are generally
termed "crotchets," such as are common amongst Oligochaeta. These
"crotchets" may be simple, or have numerous denticulations at the end
(Maldanidae), or be provided with a membranous hood (Spioniformia,
Capitelliformia). In _Hermione_ peculiar sheathed, spear-like bristles
occur (Fig. 138, N).
(2) Jointed chaetae have already been described (p. 246); they are confined
to the sub-Order Nereidiformia, and occur only in certain families.
(3) The uncini are very short chaetae, which are simply embedded in the
skin, and do not extend beyond the body-wall into the body-cavity. An
uncinus is a sharply curved hook, which may have more or less numerous
secondary teeth on it. They are characteristic of the Sabelliformia and the
Terebelliformia.
The chaetae appear as solid, usually fibrillated structures, of a yellow or
golden tint, transparent and refringent. Chemically {267}they consist of
_chitin_, and each chaeta is the product of a single cell. The chaetae of
_Euphrosyne_ are hollow and calcareous, being peculiar in both characters.
[Illustration: FIG. 138.—Chaetae of various Polychaetes (the magnification
is not the same in all cases). A, Doubly-fringed capillary, from
Terebellid; B, hooded crotchet, from _Polydora_; C, a fork, from
_Euphrosyne_; D, jointed chaeta, from _Phyllodoce_; E, simple chaeta, with
serrated ridges or frills, from a Polynoid; F, jointed chaeta, from
_Eunice_; G, uncinus, from _Pomatoceros_ (Serpulid); H, one of the outer
series of paleae from the hood of _Sabellaria spinulosa_; I, jointed
chaeta, from a Syllid; J, multidenticulate crotchet, from a Maldanid; K,
comb-shaped chaeta, from _Eunice_; L, uncinus of a Sabellid; M, uncinus of
Terebellid (_Amphitrite Johnstoni_); _a_, edgewise; _b_, side view; _m_,
attachments of muscles into _ba_, basal plate; _x_, accessory teeth. N,
Sheathed spear of _Hermione_; _a_, the spear-shaped capillary removed from
its sheath; _b_, the same, with sheath.]
Certain modifications of the chaetae presented by various worms deserve
mention. In _Polydora_ (Fig. 133, A) and in _Chaetopterus_ (Fig. 173, p.
324) those of one segment are especially strong, but their significance is
uncertain. In _Capitella_ those {268}of the notopodium of the eighth and
ninth segments are specially modified; they are analogous to the copulatory
chaetae of Oligochaeta. In _Aphrodite_, in addition to the ordinary
locomotor chaetae, there are brilliant, iridescent bristles and peculiar
felting threads arising from the indistinct notopodium; these latter,
however, are not true "chaetae," but are separate chitinous filaments
similar to the constituent fibres of an ordinary chaeta.[322]
While the chaetae in the Nereidiformia and others are grouped in bundles,
those of many other families are in vertical, transverse rows, as in
Maldanidae and in _Arenicola_. The uncini are always embedded in such rows,
usually slightly raised from the general level of the body surface, each
being termed a "torus uncinigerus." These tori are usually limited to the
sides of the body, but in _Myxicola_ and in _Notomastus_ they encroach upon
the dorsal surface, and in _Chaetozone_, also upon the ventral, so as
nearly to encircle the body, recalling the "perichaetous" condition of some
earth-worms.
[Illustration: FIG. 139.—_Aphrodite._ Foot, × 2. _a_, Elytron; _b_,
notopodium; _c_, neuropodium; _d_, neuropodial cirrus; _n_, aciculum; 1,
iridescent bristles; 2, stiff chaetae; 3, felt.]
GILLS.—We have already seen that several different organs, _e.g._ the palps
in Sabelliformia, the prostomial tentacles of Chlorhaemidae, and the
notopodial cirri of sundry other Polychaetes, may take on a respiratory
function. There are, however, certain "gills" developed either on the
parapodium itself or elsewhere on the body which it is difficult to
homologise. Such are the _retractile gills_ on the parapodia of the
Glyceridae (Fig. 136, C); those of _Dasybranchus_, near the abdominal
neuropodia; those of _Mastobranchus_, near the notopodia. _Nephthys_ has a
sickle-shaped gill on the under surface of the notopodium. The long gill
filaments at the posterior end of _Sternaspis_, again, are only doubtfully
interpreted as the dorsal cirri of some of the posterior segments.
{269}Since primitively the whole skin of the worm is respiratory, any part
of the skin may become more or less specialised for this function, and
chiefly, of course, on the more actively moving parapodia. The
blood-vessels constituting the essential part of the "gill" may make use of
any already existing outgrowth (such as a cirrus or a tentacle), or may
push the body-wall out on their own account.
INTERNAL ANATOMY.
Probably those organs which have the greatest effect in modifying the shape
of the body are the SEPTA, for we find in the long, free-swimming worms
that these are regularly present throughout the body, and external
"segmentation" of the body is well marked. In burrowing and tubicolous
forms the septa are frequently incompletely developed, or more or fewer may
be absent; and the body becomes less distinctly segmented externally, tends
to vary greatly in diameter during movement, or becomes plumper. With the
disappearance of the septa there is also a diminution in the number of
NEPHRIDIA, as in _Arenicola_, with only six pairs. Further, there is
frequently a dimorphism of these organs; instead of all of them serving
equally as excretory organs and as genital ducts, some of the most anterior
in the Sabelliformia and Terebelliformia become greatly enlarged, and take
on practically the whole of the former function; whilst more or fewer of
the posterior nephridia dwindle in size, and become genital ducts. The
absence of septa allows a free communication between the successive
segments, and thus a freer flow of coelomic fluid for the distension of the
anterior end of the worm during burrowing.
The ALIMENTARY SYSTEM presents certain modifications of a systematic value.
In the Nereidiformia the muscular pharynx, which is always protrusible and
is preceded by an eversible buccal region, frequently encloses thickened
cuticular plates which serve as crushing and grasping organs. The form,
number, and arrangement of these "jaws" vary in the different families.
They form valuable fossil records of extinct worms.
In the Scoleciformia and Capitelliformia the buccal region exists, but
there are no jaws. In the Sabelliformia and Terebelliformia eversion does
not take place and jaws are absent.
{270}Amongst the Nereidiformia the jaws are absent in the Phyllodocidae and
Hesionidae; when present they are usually set in the direct course of the
food. There may be one small tooth used for stabbing, as in some Syllids
(Fig. 141, A); or a circle of such denticles (_Autolytus_, Fig. 140, D). To
these are added powerful grasping jaws in _Nereis_ (E); or the latter may
alone be present, as in _Glycera_ (F). In _Polynoë_ the four jaws are
carried by hard pieces, to which the muscles are attached (C and G). In
_Nephthys_ there is a dorsal and a ventral jaw.
[Illustration: FIG. 140.—Jaws of various Chaetopods. A, Transverse section
of the anterior end of _Eunice_; _a_, _b_, _c_, _d_, various parts of the
upper series of denticles lying in a special chamber; _g_, oesophagus; _k_,
lower jaw: B, the denticles of _Eunice_ separated; _U_, upper series; _a_,
grinder; _b_, forceps; _c_, rasping plates; _d_, grater; _L_, lower series;
_j_, tooth; _k_, base into which muscles are inserted: C, Polynoid; _U_,
upper, and _L_, lower jaws; _j_, tooth; _k_, base: D, Diagrammatic section
across pharynx of Autolytus; E, of _Nereis_; F, of _Glycera_; G, of
_Polynoë_.]
In the Eunicidae, however, the numerous denticles are carried in a special
pouch below the food tract, with which it communicates anteriorly.[323]
They are arranged in an upper and lower series. The lower series (L)
consists of a pair of flat plates (_k_) on each side partially embedded in
and acted upon by muscles, with a harder enamelled piece—the actual lower
"tooth" (_j_)—at its anterior end. The upper series (U) consists of several
pieces, varying in shape and size in the various genera of this
{271}family; but developmentally they result from modifications of two rows
of small, similar pieces.[324]
The _intestine_ is generally straight and cylindrical, and is usually
constricted by the septa, if these are present. In the Polynoids the
intervening sacculations become so long as to receive the name of "caeca,"
which, in _Aphrodite_, become enormously elongated (Fig. 142); there are
eighteen pairs of them (_c_), each being a slender tube bent upon itself,
giving off short branches and dilated distally, where it lies in the base
of the parapodium.
[Illustration: FIG. 141.—A, Alimentary canal of Syllid: B, transverse
section of pharynx of the same; _b_, buccal region; _d_, oesophageal
outgrowth; _g_, salivary glands; _i_, intestine; _j_, tooth; _p_, pharynx;
_s_, gizzard: C, alimentary canal of _Petta_ (after Wirén); _i_, intestine;
_o_, oesophagus; _r_, rectum; _s_, stomach.]
[Illustration: FIG. 142.—Alimentary canal of _Aphrodite_. × 1. (From
Gegenbaur.) _a_, Anus; _b_, pharynx; _c_, caeca; _o_, mouth.]
The intestine is looped in a few genera only, as in _Trophonia_, or coiled,
as in _Sternaspis_, _Petta_ (Amphictenid, Fig. 141, C), and _Ammotrypane_.
In the course of the tube there may be a thick-walled muscular _gizzard_,
with hard chitinous lining, as in certain Terebellids, where it appears to
replace, in function, the pharynx of the Nereidiformia; in the Syllidae the
gizzard is present in addition to the pharynx (Fig. 141, A).
Glandular appendages of the oesophagus are present in many {272}worms.
Amongst the Nereidiformia, the Syllidae and Hesionidae possess oesophageal
diverticula (Fig. 141, A, _d_), which are used, not for secreting a
digestive fluid, but as reservoirs for water and air swallowed by the
worms; and are provided with muscular walls, by which their contents can be
driven out. They appear, in fact, to be used like the swim-bladder of
fishes.[325] Many Chaetopods take in water by the anus—no doubt for
respiratory purposes—and pass it forwards along the intestine. In the
Capitelliformia a special groove conducts the water for some distance, then
the groove becomes closed to form a canal, which, after a course forwards
as a free tube below the intestine, again enters the latter, constituting a
"siphonal apparatus," similar to that of the Echiuroids and the sea
urchins.
SENSE ORGANS.—In addition to the prostomial _eyes_, which are present in
nearly all the Nereidiformia and Spioniformia, eyes may exist elsewhere on
the body: thus _Myxicola infundibulum_ and _Fabricia_ possess a pair on the
anal segment; in _M. aesthetica_ Clap. there is a pair to every segment; in
_Branchiomma_ there is a compound eye near the tip of each gill filament
(_i.e._ palp); whilst in _Dasychone_ a series occurs along each gill
filament. All these examples belong to the Cryptocephala, in which, owing
to certain peculiar modes of life, these sense organs are required in
correspondingly peculiar positions. It is usually stated that
_Polyophthalmus_ possesses, in addition to the usual prostomial eyes,
twelve pairs on as many successive segments; but the minute structure of
these organs points rather to their function as light-producing organs.
[Illustration: FIG. 143.—A gill filament, A, of _Branchiomma_, B, of
_Dasychone_. _a_, Axis; _f_, secondary filaments; _o_, compound eye; _x_,
lappets.]
The Capitelliformia and Opheliidae possess a pair of peculiar "ciliated
pits" or "nuchal organs" at the upper side of the head, {273}between the
prostomium and peristomium, and capable of eversion (Fig. 144). They are
most characteristically developed in the Capitelliformia, where each organ
abuts upon a special lobe of the brain. The function of these "ciliated
organs," which bear a great resemblance to those of the Nemertines, is a
matter of speculation. Similar organs, in the form of simple pits or
grooves, occur in many of the Nereidiformia, Terebelliformia, and
others.[326]
Otocysts are rare. _Arenicola_ possesses a pair at the base of the
prostomium, each of which in some species retains an opening to the
exterior.[327] They probably serve as "organs of direction" rather than of
"hearing." _Aricia_ and _Polyophthalmus_ likewise have such organs on the
prostomium; whilst _Fabricia_, _Myxicola_, _Terebella_, and a few others
possess them in the peristomium, or in some other segment of the body.
[Illustration: FIG. 144.—_Ammotrypane aulogaster_ Rathke, enlarged. (From
Cuningham.) Anterior end. _a_, Prostomium; _b_, everted buccal region; _c_,
notopodial cirrus; X, ciliated organ everted; I, II, III, first three
segments.]
REPRODUCTIVE PHENOMENA.—With a few exceptions mentioned below, the
Polychaeta are unisexual. The sexual cells are developed in all cases from
the lining epithelium of the body-cavity. The exact spot at which this
occurs varies in different cases; it may be, though rarely, on the floor of
the body-cavity; it is more usually on the wall of some blood-vessel,
either the ventral vessel or on branches of it; or on the many blind
blood-vessels of _Aphrodite_. The number of such genital organs is very
great in most worms, but in those presenting two regions of the body they
are confined to the posterior segments (Sabelliformia, Terebelliformia,
Capitelliformia). The number is very limited in _Arenicola_ and other worms
presenting but few nephridia: in the former genus there being six pairs, in
_Trophonia_ only one pair.
The following genera are hermaphrodite:—_Amphiglena_, _Salmacina_,
_Protula_, _Spirorbis_, belonging to the Sabelliformia, to {274}which must
be added some Hesionidae. In this family ova and spermatozoa are developed
around the same blood-vessel. But in the former group of worms (as also in
_Ophryotrocha_) the two kinds of cells are produced in different regions of
the body. Thus in _Protula_ the anterior abdominal segments are male, the
posterior ones female, while in _Spirorbis_ the reverse arrangement holds;
and in _Syllis corruscans_ the anterior segments of the body contain eggs,
whilst the posterior region contains spermatozoa, and this region separates
and becomes a male worm.
The eggs and spermatozoa in the Polychaeta are discharged into the sea
either by rupture of the body-wall or through the nephridia; the male and
female elements unite, and the resulting fertilised eggs undergo
development, either floating separately in the water, or embedded in jelly,
or attached to the body or to the tube of the worm.
The result of the segmentation of the egg is a free-swimming larva known as
a "Trochosphere," similar to that of _Polygordius_. The larvae of different
species present various more or less marked departures from this type, for
instead of the two girdles of cilia there may be only the anterior girdle,
or there may be several complete or incomplete girdles between the two
typical ones, or there may be (Chaetopterids) only a single girdle of cilia
about the middle of the body, the two typical girdles being absent.[328]
The postoral region, after elongation, generally becomes marked out into
three segments, and these segments develop chaetae, which are usually
temporary and specially long.
The little animal is thus equipped for an independent life: the provisional
chaetae help in keeping it balanced; and in some cases (Spionidae) serve to
protect the little soft creature, for when it is touched it curls up, and
its chaetae stick out at the sides, so that it looks like a hairy
caterpillar. But the larva is quite at the mercy of the sea, for it is
carried hither and thither by currents, and in this way the species is
disseminated. The larvae of the Polychaetes, like those of other animals,
occur at certain periods of the year in large quantities at the surface of
the sea, and serve as food for various larger animals.
{275}[Illustration: FIG. 145.—A, Trochosphere of _Nephthys_. × 65. _a_,
Anus; _b_, apical plate (brain); _c_, apical tuft of cilia; _c'_, girdle of
cilia; _i_, intestine; _m_, mouth; _st_, stomach. B, Larva of _Spio_, with
three segments, eight days old. × 100. _c_, Preoral girdle of cilia; _c'_,
preanal girdle; _ch_, long provisional chaetae; _pr_, prostomium with eyes.
(From Claparède and Metschnikoff.)]
These larvae are at first very different from the adult animal, and the
necessary changes to be passed through are more or less great according to
the species. It is not our intention to describe these changes in
detail.[329] The larva increases in size, the permanent chaetae make their
appearance in regular order, and the body exhibits segmentation, the new
segments always appearing just in front of the anal segment. The internal
organs gradually develop, and the prostomial and parapodial appendages grow
out in their turn. In the Sabelliformia the multifilamentous "gills" arise
by the continued branching of an at first simple process (the palp) arising
from the latero-ventral surface of each side of the preoral lobe.[330]
These gradually encroach dorsally and ventrally till the prostomium is more
or less encircled; meanwhile the peristomium grows forwards so as to
conceal the prostomium, which no longer increases at the same rate as does
the rest of the body.
Although most worms appear to discharge their ova directly into the sea and
take no further care of them, some make provision for their offspring
either by laying the eggs in a jelly, which will serve as food for the
young larvae—_Aricia_, _Ophelia_, _Protula_, _Phyllodoce_—or by attaching
them to their body. In certain Polynoids the eggs are attached by means of
a secretion to the back, under the elytra, where they undergo development
up to a certain stage. In _Exogone_ and some other Syllids they are
attached to the ventral cirri, or in _Grubea limbata_, all over the back.
In the female _Autolytus_ (_Sacconereis_) a ventrally-placed brood sac is
formed by the hardening of a {276}secretion; the eggs develop into embryos
inside the brood sac, and then become free, with head appendages and three
pairs of parapodia. Enormous numbers of such embryos may occur; for
instance, some 300 were counted in a brood sac of _Autolytus ebiensis_. In
the case of tubicolous worms, the eggs are frequently attached to the tube,
either inside or outside. In _Spirorbis_ and _Salmacina_ the operculum
serves as a brood pouch.
Only a very few species are known to be viviparous, viz. _Syllis vivipara_
Kr., _Cirratulus chrysoderma_ Clap., _Marphysa sanguinea_ Mont., and
_Nereis diversicolor_ Müll.
In most genera there is no external difference between a mature worm filled
with generative products and an immature one, except, it may be, in the
colour; for the yolk of the eggs is frequently tinted yellow, or pink, or
bluish, while the spermatozoa in mass are white; so that the normal
colouring of the worm may be modified when filled with these elements. But
in a few instances striking anatomical peculiarities are exhibited by the
mature worm.[331] In many species of _Nereis_, for instance, those segments
containing the generative products undergo more or less extensive changes,
while the anterior ones remain unaltered. The body of the ripe _Nereis_ is
then distinguishable into an anterior non-sexual region and a posterior
sexual region; and so great are these changes in certain species that the
mature worms were for a long time believed to belong to a different genus,
and received the name _Heteronereis_. But we now know their true relations,
thanks to the work of Claparède and others. The males in the Heteronereid
phase have fewer unaltered anterior segments than the females, so that
there is a sexual dimorphism.
[Illustration: FIG. 146.—Male "Heteronereis" of _N. pelagica_ L. × 1. A,
Non-sexual region; B, sexual, modified region. (From Ehlers.)]
The changes which _Nereis_ undergoes in its transformation affect chiefly
(_a_) the shape of the parapodia, and (_b_) the form of the chaetae of
these parapodia. Other organs may also be affected, though less noticeably;
thus the eyes become enlarged, the intestine may become so compressed by
the generative {277}products as to be functionless, and the tail develops
special sensory papillae.[332]
In the parapodia an increase in size and a sharper delineation of the
various parts take place; then flattened foliaceous outgrowths (Fig. 147,
_x_, _y_) arise from certain lobes of the feet, in which, too, the blood
supply becomes greatly increased. The old chaetae are pushed out by the
development of new ones of quite a different shape; these are jointed like
the old ones, but the appendix is, in many species at least, flattened and
oar-shaped (Fig. 123, C, p. 246); and the chaetae are arranged in a
fan-like manner. Both these modifications are in evident relation to the
free-swimming habit which the Heteronereid now adopts. The new foot serves
as a swimming organ, the old one was a walking appendage.
[Illustration: FIG. 147.—Parapodium of male "Heteronereis" of _N. pelagica_
L. × 10. (From Ehlers.) _a_, Notopodial cirrus; _b_, notopodium; _c_,
neuropodium with new chaetae; _c'_, foliaceous outgrowth; _d_, neuropodial
cirrus; _x_, _y_, foliaceous outgrowths.]
Whilst some species, such as the common British _N. diversicolor_, undergo
no change, and others become modified as just described, others, again, are
polymorphic. Claparède was the first to show that _N. dumerilii_ may occur
in at least five different mature forms; these differ from one another in
size, colour, mode of life, character of the eggs, etc. The immature forms
may become ripe and lay eggs while still retaining the "Nereid"
characteristics, or these immature forms may become "Heteronereids"[333]
whilst the sexual elements are ripening. There are then three different
kinds of males and of females in this one species, some being found at the
bottom of the sea, as the large Heteronereid form, while the small
Heteronereid swims on the surface. The relations of these various forms to
one another, and the causes leading to the {278}assumption of a
Heteronereid condition in some cases and not in others, are unknown.
A somewhat similar phenomenon is exhibited by members of the family
Syllidae.[334] In this family sexual reproduction is frequently accompanied
by the asexual modes of fission and gemmation. In some genera, such as
_Eusyllis_, _Odontosyllis_, and _Exogone_, there occur changes quite
similar to those characterising "Heteronereis"—that is, the posterior
segments in which the genital organs exist become altered, so that the worm
consists of two distinct regions, and is termed a "Heterosyllis." The most
marked change is the appearance of a dorsal bundle of long capilliform
chaetae in each of the genital segments (Fig. 148, I).
But in other genera the hinder genital region of the body becomes
_separated_, on maturity, from the anterior non-sexual region. Various
stages of this "schizogamy," or fission into a sexual and a non-sexual
zooid, have been observed in different genera. In the genus _Syllis_ the
first segment of the sexual zooid, after its separation from the asexual
zooid, proceeds to bud forth a head. The character of the head is alike in
both sexes, though different species present heads of different shapes; and
as the worms were originally described as distinct genera, the names then
given are retained as descriptive terms. Thus the "Chaetosyllis" form has
only two tentacles; the "Ioda" form has three tentacles and a pair of
palps. One and the same species (e.g. _S. hyalina_) may successively pass
through these stages.
With regard to the asexual portion, there is a regeneration of the tail
segments after the sexual zooid has separated; and the number of segments
so regenerated is usually equal to those that have become sexual. After a
time these newly formed segments will produce generative organs, and take
on the characteristic natatory chaetae, and this region will in its turn
separate.
But in other genera, such as _Autolytus_, the regeneration of segments may
commence _before_ the separation of the sexual zooid; and the head of the
sexual zooid becomes budded out _before_ separation from the asexual
portion. So that the animal now consists of two worms, each with its own
head, separated by {279}a region or zone of proliferation (Fig. 148, IV).
Moreover, in some species not only is the hinder part of the body
_converted_ into a sexual zooid, but the zone of proliferation becomes very
active, and produces by gemmation a large number of segments, which become
marked out, by the appearance of heads at intervals, into a number of
zooids, in which genital organs will later make their appearance. A chain
of as many as sixteen zooids may be formed in _Autolytus_ (Fig. 148, V)—the
hindermost by _conversion_ of the hinder part of the body of the original
"stock," the intervening zooids by _gemmation_.
[Illustration: FIG. 148.—Diagrams illustrating the various stages in the
asexual formation of a chain of zooids. (Modified from Malaquin.)
I, Heteronereid or Heterosyllid stage. A, Non-sexual; A', sexual region of
the body, with modified parapodia.
II, _Syllis_. The hinder sexual region, B, is similarly modified, and will
separate from the parent zooid, A, and become an independent zooid.
III, _Autolytus_. The hinder zooid, B, develops a head by budding before
separation.
IV, _Autolytus_, etc. A zone of budding (_z_) makes its appearance in front
of the head of B, and by its growth will give rise to a series of new
segments in the middle of the body.
V, _Myrianida_, _Autolytus_, etc. From this zone of budding a very large
number of segments have been formed, which have, further, become grouped so
as to form three individuals, C, D, E; B is the hindmost zooid, which is
either formed from the hinder segments of the parent zooid or is produced
by budding, like C, D, E.]
One original "stock," or asexual zooid, thus produces several sexual
zooids, but these are only of one sex for a given stock. The males differ
in several important characters from the females; so different, indeed, are
the two sexes that before their history was {280}worked out by Agassiz[335]
they were placed in different genera. The male zooid has thus come to be
known as _Polybostrichus_ (Fig. 149, B). It has three tentacles and two
bifid palps; there are two pairs of peristomial cirri; the testes are
confined to the four anterior segments, which are without natatory chaetae.
The female is termed _Sacconereis_, owing to the possession of a great
ventral brood sac; its head possesses no separate palps; the peristomium
carries only one cirrus on each side; ova occur in every segment of the
body, and may even extend into the hinder segments of the asexual zooid
(Fig. 149, C).
[Illustration: FIG. 149.—_Myrianida fasciata._ (From Malaquin.) The bright
red markings of the living animal are here represented black. A, An asexual
individual which has produced by budding from the zone (_z_) a chain of
twenty-nine zooids, the oldest being labelled 1, the youngest 29. B, A ripe
male zooid (_Polybostrichus_), with three tentacles and a pair of forked
palps (_p_). There are five unaltered anterior segments. C, A ripe female
zooid (_Sacconereis_) with the palps fused with the prostomium; _s_, the
ventral brood pouch projecting on each side; _t_, tentacles.]
A further development of this process of gemmiparity is exhibited by
_Myrianida_. Here, there is no conversion of the hinder segments, but the
normal preanal zone of proliferation gives rise to a large number of new
segments. After a time the most anterior of these becomes a head, and thus
a new zooid {281}is marked out. The zone of proliferation immediately in
front of the new head now proceeds to form new segments, and a second zooid
results. This process goes on till a considerable number of new worms have
been formed at the tail of the original one, the oldest of these new ones
being the most posterior, the youngest next the original "stock." In each
zooid there is a zone of activity which adds to its number of segments, so
that as we pass backwards the zooids increase in size. As many as
twenty-nine such zooids may be formed in this way entirely by gemmation;
and as each zooid becomes completed, genital organs make their appearance,
and when these are ripe the zooid separates from the "colony" and leads an
independent life. Here, as in _Autolytus_, the sexes are dimorphic, the
male and female resembling those of that genus.
The process of gemmation, as seen in _Autolytus_, closely resembles that
exhibited by certain Oligochaeta (_Naididae_), where there exists a
definite alternation of generations; the production of new individuals by
gemmation occurring throughout the greater part of the year, and sexual
reproduction recurring only at certain intervals. In the Polychaeta such
alternation exists in _Myrianida_; but it is only the terminal link of a
series, which takes its starting-point in the process exhibited by the
majority of Annelids, where no sexual character marks maturity. The next
stage is presented by "epigamous" forms like Heteronereis and Heterosyllis;
then "schizogamy" makes its appearance in certain Syllidae, resulting in
the formation of two morphologically and physiologically distinct
individuals which lead independent lives. The appearance of a head and of a
zone of proliferation leading to the formation of a chain of sexual zooids
is accompanied by a delay in the appearance of the genital organs, for in
_Autolytus_ these arise _during_ the formation of the new individuals, as
part of the general process of new formation; whilst in _Myrianida_ the
delay is prolonged, and the generative elements do not make their
appearance till _after_ the new individuals have reached some size.
More simple cases of the separation of the body into two parts, sexual and
asexual, occur also in some of the Serpulidae. Thus in _Filigrana_ and
_Salmacina_ the generative elements make their appearance in the hinder
segments, as they do throughout the Sabelliformia; and this hinder part of
the body separates {282}from the anterior region after the formation of a
new head between the two regions.[336]
Another modification of the process of budding and fission is exhibited by
_Syllis ramosa_, one of the most interesting forms of animal life which was
obtained by the "Challenger." This worm consists of a main stem, whence
arise a number of lateral branches, which may also branch so as to give
rise to an arborescent colony (Fig. 150). The branches of the first and
second and higher orders arise by budding from the sides of the original
form or branches of lower order; and some of these branches develop
generative products, and bud forth a head near the point of attachment.
These sexual branches, no doubt, separate from the colony and distribute
the ova. The worm lives in a Hexactinellid sponge, _Crateromorpha meyeri_,
living in depths of 95 to 140 fathoms in the Eastern seas.[337]
[Illustration: FIG. 150.—Portion of _Syllis ramosa_. (Reduced from
M‘Intosh.)]
REGENERATION OF LOST PARTS.—The process of budding and fission of the worm
into two parts is merely an extension of that resulting in the formation of
new segments when the worm is injured. In most of the Nereidiform
Polychaetes the number of segments forming the body continues to increase
throughout life by the formation of new segments between the anal segment
and the one in front of it; that is to say, there is normally a process of
budding taking place at this point. Now in many of the longer worms it may
be noticed that the segments of the hinder end suddenly become smaller than
the rest; these are segments newly formed to replace those lost by the
worm. But this "regeneration," though the same in principle as ordinary
growth {283}at the penultimate segment, is due to activity in a segment
(any segment) further forwards; in other words, in the less modified worms
every segment has the power of forming new tissues, just as each of the
joints of a crab's leg has the power of forming the remaining joints when
injured. It is not therefore surprising that a "zone of budding" arises in
an uninjured worm at certain seasons, viz. that of reproduction; it is a
property that each worm possesses, though generally it remains latent till
injury provides the stimulus.
Moreover, not only can new segments arise at the hinder end, but a new head
can be formed at the anterior end, as has been observed in worms belonging
to many families—in the less modified Syllidae,[338] in others of the
Nereidiformia, and even in Sabellids, where the greatly specialised gill
filaments can be reproduced. Thus Sir J. Dalyell[339] noted in _Dasychone_
that the crown of branchiae was regenerated in about a month in springtime,
while in winter the process occupied 116 days. He cut a _Dasychone_ into
three pieces; the hindermost produced a head, the anterior piece developed
an anus, and the middle portion formed both a head and tail!
These regenerated heads are of course at first smaller than the rest of the
body, but soon grow to a normal size. Naturally this extensive power of
regeneration is of extreme value to the Polychaetes, for if a fish or other
enemy bites the head off a worm, a new one can form; and it is not
difficult to see in this the origin of the reproduction by fission as a
normal process.
{284}CHAPTER XI
NATURAL HISTORY OF POLYCHAETES—GENERAL HABITS—CHARACTER OF TUBE AND ITS
FORMATION—COLOURING—PROTECTIVE AND MIMETIC DEVICES—PHOSPHORESCENCE—FOOD
—USES—ASSOCIATED WORMS—WORMS AS HOSTS—DISTRIBUTION—FOSSIL REMAINS.
All the many hundreds of species of Polychaetes are marine, with a very few
exceptions, which have been in recent years recorded from fresh (_i.e._
drinkable) water, viz. a species of _Nereis_ from a lake in Mingrelia,
another _Nereis_ and a _Lumbriconereis_ from running water in
Trinidad,[340] a Sabellid, _Manayunkia speciosa_,[341] from Philadelphia;
and another Sabellid, _Coabangia_,[342] from fresh water at Tonquin, which
lives in borings in shells of _Melania_; and it is by no means improbable
that other fresh-water Polychaetes exist in Lake Tanganyika in Africa,
where a Medusa has recently been discovered.
In brackish water of various densities many Polychaetes live; _Arenicola_
especially is regardless of the character of the medium, and _Nereis
diversicolor_ appears to withstand considerable admixture of fresh water.
The majority of the Polychaetes occur "inshore," that is, between
tide-marks and in shallow water down to 20 fathoms; but they occur at all
depths more or less abundantly, and some have been dredged from depths of
more than 3000 fathoms.
The nature of the soil composing the shore has a good deal to do with the
number of worms to be found there; thus in calcareous districts they are
fewer than in places where harder rocks, {285}such as granite, form the
shore line, for the chalk or limestone wears away more quickly, and exposes
to destruction the worms which may have sheltered in its crevices: further,
it does not give so permanent a place of attachment to seaweeds, on which
many Polychaetes feed. The calcareous rocks, too, are more likely to be
traversed by springs of fresh water, which is not to the taste of the
worms. The sand resulting from the destruction of the rocks, whether hard
or soft, is of itself unsuitable to the majority of worms, which are most
abundant where mud containing decaying vegetable matter is mixed with the
sand: this, which gives a firmer consistency to the soil, so that the
burrows retain their form better, supplies food for the burrowers.
GENERAL HABITS.—The division of the Polychaetes into the "Errantia" or
free-swimming and wandering forms, and "Sedentaria" or tubicolous and
sedentary forms, is a misleading mode of classification, for as a matter of
fact only a comparatively few forms are really free-swimming throughout
life; the majority, even if they do not form definite tubes, burrow
galleries for themselves in the soil, and these burrows are in many cases
only rarely left; this is true of both groups. Amongst the "errant"
Polychaetes nearly all the Eunicidae secrete a parchment-like tube, and
some Polynoids form mud tubes. Among the "sedentary worms" there are forms
which merely burrow; while _Myxicola_ readily leaves its gelatinous tube
and swims freely; _Pectinaria_ carries its house with it as it moves about,
and _Polycirrus_, a Terebellid, does not form any tube at all.
Owing to their sedentary habits, quite a representative collection of
genera may be made, especially at a spring tide, at any seaside place which
is provided with a sandy shore, and with rocks and seaweed. The larger
species, however, require to be dredged, and the best time is at night, for
then many forms which during the day are concealed in their burrows, will
be issuing forth to obtain food.
It may be useful to give instances of worms occurring in various situations
between tide-marks. Throughout pretty well the whole of the area left
uncovered by the tide, even up to nearly high-water mark in many parts of
the coast, the cylindrical "castings" of sand and mud, forming little
heaps, indicate the burrows of _Arenicola_, the common "lug-worm"; these
"castings" have passed through the worm's body, having been {286}swallowed
during the process of burrowing as well as for the purpose of obtaining
food, as in the case of the earthworms. Rather nearer the water may be seen
little tufts of sand-threads, about an inch high, springing from a short
piece of cylindrical, sandy tube rising up out of the sand; this is the
head end of the tube of _Terebella conchilega_ (Fig. 153).
Amongst the rocks may be found loose stones of different sizes; on lifting
them up, various kinds of worms may be brought to light, according to the
locality, the time of year, the position with respect to the sea, and so
on. _Polynoë_ is pretty sure to be present somewhere near low-tide mark;
the number of species is considerable, and their colouring very varied: but
as the worms have a habit of remaining still on the under surface of the
uplifted stone, the observer may easily overlook them.
Other worms occur below the stones, more or less buried in the sand or mud;
for instance, a small _Nereis_ may be lying in its temporary burrow
immediately underneath, and will at once withdraw from the now injured part
of the burrow; while deeper in the mud or sand, especially in rather
highly-smelling mud, little red worms are abundant, such as _Scoloplos_,
_Nerine_, _Capitella_, and others. By digging near low water one may find
_Nephthys_, _Glycera_, and others burrowing or hiding in the soil.
In rock pools, or sandy stretches amongst rocks kept moist and cool by
abundant _Fucus_, one may see under stones the red or yellow gill filaments
of _Cirratulus_ and of Terebellids protruding from their burrows and tubes,
while other worms are to be met with in clefts of the rocks, and amongst
the roots of _Laminaria_.
Still farther out, below low-water mark, where one must wade, can be seen
the beautiful branchial crowns of various Sabellids protruding from their
tubes; but care is necessary on approaching these worms, as eyes are, in
many cases, present on the branchiae and a shadow is readily perceived;
then the brightly-coloured tuft disappears, and only a piece of sandy or
muddy cylindrical tubing remains to tell where the _Sabella_ has withdrawn.
In order to obtain the worms one must dig quickly and deeply before they
have been disturbed; for the tube is of considerable length, and the
inhabitant withdraws to the bottom of it. Some of these soft-skinned worms
have the power of boring into hard rocks,[343] though by what means they do
so is {287}uncertain.[344] _Polydora ciliata_ makes a tube of mud
projecting from the mouth of U-shaped galleries in chalk, limestone,
shells, and even shale; it has no hard jaws or other structures sufficient
to account for the holes, but it is possible that the specially strong
chaetae on the sixth segment may be of some use in this work. Other
lithodomous worms are _Sabella saxicava_ and _Dodecaceria concharum_, which
is a common little borer, forming galleries in oyster-shells, etc.
The TUBES formed by these Polychaetes are very varied in constitution.[345]
In some cases a mucus, which hardens to form a firm protective envelope, is
secreted from special parts (_e.g._ the ventral gland shields of
Terebellids and Sabelliformia), or from the greater part of the general
surface of the body; in other cases the secretion serves to stick together
particles of mud or sand, or shelly fragments, so as to form a more or less
cylindrical tube (rarely branched), which is lined internally by the
hardened "mucus," having the appearance of silk.
[Illustration: FIG. 151.—_Clymene ebiensis_ in its tube (_t_) (from _Règne
Animal_). _a_, Anterior, _p_, posterior end, which is, however, injured.]
But the process of tube-making is not a simple one, for in many cases, at
least, the worms exhibit definite powers of choice. Thus some species of
_Sabella_ choose only the very finest particles of mud; _Terebella
conchilega_ chooses fragments of shell and grains of sand; _Onuphis
conchylega_ employs small stones more or less of a size; _Sabellaria_ makes
use only of sand grains. Whilst some worms, like _Terebella_, _Nicomache_,
and others, make a very irregular tube, _Pectinaria_ builds a most
remarkably neat house, open at each end, which it carries about with it,
the narrow end uppermost (Fig. 152); the grains of sand are nearly all of
the same size and only one layer in thickness, embedded in abundant
"mucus," and with the outer surface quite smooth.
Sir J. Dalyell[346] made some most interesting observations on the method
followed by sundry tube-formers in the building of {288}their tenements,
and these observations, though made nearly half a century ago, have
required very little addition or correction in modern times. In speaking of
_Sabella_, he writes as follows:—
[Illustration: FIG. 152.—The tube of _Pectinaria auricoma_. × 3. (From
M‘Intosh.) This is its natural position as carried about by the animal.]
[Illustration: FIG. 153.—The upper end of the tube of _Terebella
conchilega_. Slightly enlarged. (From M‘Intosh.)]
"Let a tall and ample crystal jar containing a _Sabella_ be emptied of its
contents and speedily replenished with sea-water; the animal, if in view,
has retreated during the short interval; the orifice of the tube is closed,
all is at rest. But soon after {289}replenishment it rises, to display its
branchial plume still more vigorously than before, and remains stationary,
as if enjoying the freshness of the renovated element, always so
grateful—the harbinger of health and strength to those whose dwelling is
there. The passing spectator would conclude that he now beholds only a
beautiful flower, completely expanded, inclining towards the light like
some of those ornaments of nature decorating our gardens. He pauses in
admiration. But if a drop of liquid mud falls amidst the element from
above, disturbing its purity, then, while the plume unfolds to its utmost
capacity, does the animal commence a slow revolution, the body also passing
around within the tube. Now are the thousands of cilia fringing the ribs
[_i.e._ the secondary filaments] of the branchiae discovered to be in
vigorous activity, and their office to be wondrous. A loose muddy mass is
soon afterwards visibly accumulating in the bottom of the funnel; meantime
the neck or first segment of the body, rising unusually high above the
orifice of the tube, exhibits two trowels beating down the thin edge as
they fold and clasp over the margin, like our fingers pressing a flattened
cake against the palm of the hand. [This refers to the lappets of the
peristomial collar.] During these operations muddy collections are seen
descending between the roots of the fans [right and left gills] towards the
trowels, while another organ, perhaps the mouth, is also occupied, it may
be, in compounding the preparation with adhesive matter. Still does the
partial or complete revolution of the plume above, and of the body within
the tube, continue; the bulk of the muddy mass diminishes, activity abates;
it is succeeded by repose, when the tube is found to have received evident
prolongation."
[Illustration: FIG. 154.—_Terebella conchilega_ Pall. Upper end of the tube
(_s_) showing the anterior end of the worm. _h_, Its head; _t_, tentacles
collecting sand grains (_y_) in their grooves; _x_, sand grains in mouth of
worm; _f_, filamentous fringe of tube. (After Watson.)]
The Terebellids use their numerous tentacles in searching {290}for
particles of sand, etc.; each tentacle is grooved along its ventral
surface, and the particle is conveyed along the furrow to the mouth. These
particles are actually taken into the mouth, and mixed with some sort of
secretion; on ejection again, each particle is placed by another tentacle
in its position at the edge of the tube, and by means of its lower lip the
Terebellid works it into place.[347]
But whereas the greater number of tubicolous worms make use of adventitious
material wherewith to strengthen the wall of their tube, the Serpulidae
secrete carbonate of lime from their tube-glands, and mould a tube of this
substance. Amongst the Eunicidae the secreted substance is of itself strong
enough to protect the animal; for in _Hyalinoecia_ and species of _Eunice_
the tube consists of a translucent, tough, parchment-like material.
Chemical analysis has been employed in a few cases to determine the
substance composing the tube. In the case of _Hyalinoecia_ (sometimes
erroneously called _Onuphis_) the material consists of a phosphoric salt
containing magnesia and a characteristic organic substance "onuphin"[348];
in _Spirographis_, a Sabellid, the name "spirographin" is given to its
special secretion, whilst in Serpulids the organic base of the calcareous
tube is "conchiolin."
[Illustration: FIG. 155.—_Eunice tibiana_ Pourt. × ½. The branching tube
(_t_) with the worm (_w_) protruding its head through one of several
openings. (From Ehlers.)]
The majority of worms are solitary, but there are a few instances of social
worms—not that there is any co-operation or distribution of labour amongst
the individuals, but they merely occur together in quantities; thus the
sandy tubes of _Sabellaria_ may form compact masses of several cubic feet,
which, left uncovered by the receding tide, look like rocks upon the shore;
as, for instance, at Paignton and Torquay. _Filigrana implexa_ and _Serpula
uncinata_ similarly intertwine their calcareous tubes to form masses.
{291}Whereas most worms live at the bottom of the sea, at various depths, a
few are to be found at the surface. Purely pelagic habits are confined to a
few families, viz. Tomopteridae, Typhloscolecidae, and the Alciopids and
others amongst the Phyllodocidae; though _Nectochaeta_, one of the
Polynoidae, and _Ophryotrocha_, one of the Eunicidae, are modified for this
mode of life.[349] Several genera become pelagic during the breeding
season. All these forms are excellent swimmers, and many of them are
transparent.
The COLOURING OF POLYCHAETES.—The majority of Polychaetes quickly lose
their colour in spirits, and become uniformly dull or light brown in
museums. There are a few, however, which retain their brilliancy, like
_Aphrodite_ and _Chloeia_, but in both cases the coloration is due to the
beautiful hair-like bristles ranged along each side of the animal; in the
former the colours of the rainbow flash from specimens which have been kept
in spirit for any length of time. The Polynoids, too, with their golden
chaetae and pigmented scales, retain to some extent their characteristic
colouring. But the colours of most Annelids are due to pigments in the
skin, together with the haemoglobin of the blood, which are soluble, or
otherwise changed, in alcohol; for instance, the bright greenish-blue tint
of the common _Phyllodoce_ of our coasts is changed to a rich chocolate
brown; but such cases are rare, most worms becoming more or less
decolorised.
The varied colouring in the Polychaetes, as in other animals, is due to a
variety of causes. The red is in many cases due to haemoglobin of the
vascular system showing through the transparent body; the green of the
tentacles of the Sabellids and Chlorhaemids is similarly due to
chlorocruorin. In other cases the contents of the intestine or the tint of
the coelomic fluid may affect the colour of the worm. In _Capitella_ the
coloured excretory products are regained in the skin; in an Eunicid living
in a yellow sponge, on which it feeds, the colouring matter is extracted
and stored in the skin; in the same kind of way green caterpillars may owe
their tint to feeding on green leaves. But many of the Polychaetes possess
distinct _pigments_ in the skin; thus in _Arenicola_ the dark pigment
{292}melanin has been recognised; in _Cirratulus_ and _Nereis_ certain
lipochromes; whilst _Eulalia viridis_ contains a pigment allied to
bonellein. These various pigments yield different absorption bands when a
solution is examined with the spectroscope; others, however, give no bands,
but are distinguished by different chemical reactions.[350] The colour of
the intestine of _Chaetopterus_ has been stated to be due to "modified
chlorophyll," but it is quite a different substance.
When seen in the living and healthy condition, however, these Polychaete
worms vie with the very butterflies in their brilliant and beautiful
colourings, and though our own worms are not lacking in beauty, many
tropical and southern forms exceed them in gayness of tint. Bright reds,
orange, yellows, greens, blues, rich violets, and sombre browns are all
displayed.[351]
The handsome _Terebella nebulosa_ of our own coasts is coloured bright red,
sprinkled with white spots. _Nicomache lumbricalis_ is pink, with red
girdles. Eunicids are frequently red or brown, and the red gills along each
side, together with a brilliant iridescence, render these worms very
beautiful. Nereids present a great range of coloration, from light green to
sundry tints of brown and red in various combinations. Amongst the
Serpulids our common _S. vermicularis_ is a very showy little worm, with
its orange body, its red gills splashed with orange, and its orange
operculum streaked with red; and a Southern form, _Placostegus coeruleus_,
occurring at the Cape of Good Hope, is provided with beautiful
lavender-blue gills. Our own Sabellids present examples of beautiful
markings on the gills, in different colours or in different shades of the
same colour. Amongst Polynoids, _P. leucohyba_, from the Antilles, has blue
elytra; _Hemilepidia erythrotaenia_, a long worm from the Cape of Good
Hope, has the anterior end of its body covered with light blue elytra,
whilst the uncovered part is orange, with a broad magenta-red band along
the dorsal surface.
The Phyllodocids are mostly very brightly coloured. The common _P.
lamelligera_ of our coast has a bluish-green body, with olive-green
parapodia; but _Lopadorhynchus erythrophyllum_, {293}from Jamaica, has a
blue body with red parapodia; whilst _Notophyllum myriacyclum_ has a brown
body with longitudinal dark-brown stripes and yellow parapodia. Both these
worms live in coral reefs, where brilliancy of colour is one of the
characteristic features of the fauna. Other worms are of various shades of
green: the dark green _Arenicola_ with red gills; the bright green _Eulalia
viridis_; the deep green _Amphinome smaragdina_, from Jamaica;
_Gnathosyllis diplodonta_, with its green and yellow body, serve as
examples.
Patterns or "markings" may be exemplified by _Lepidasthenia elegans_ (Fig.
156), and _Myrianida fasciata_, which has a bright red band on each segment
(Fig. 149, p. 280). From this brief list of examples it will be seen that
beautiful, and even brilliant, coloration is not confined to any particular
mode of life; many of the most typically tubicolous forms, like the
Terebellids and Serpulids, are as brilliantly coloured as the most
typically free-swimming genera, like the Phyllodocids. Carnivorous forms
like Amphinomids and Syllids present as wide a range of tint as the
limivorous forms like _Cirratulus_, _Sabella_, or Maldanids. Shore-lovers,
and deep-sea dwellers, and surface-swimmers, all exhibit equally bright or
equally sombre tints; it is therefore difficult and rash to dogmatise on
the "use" of these colourings to these animals, or to point to this worm as
being protectively, to the other as being warningly, coloured; for we are
too ignorant as to the habits of the worms.
[Illustration: FIG. 156.—_Lepidasthenia elegans_ Gr., × 2, to illustrate
colour-markings: the dark bands in the anterior part of the body occupy two
elytriferous, and the intermediate segments. In the hinder region, where
the elytra are in every third segment, this one is dark. _el._12, The
twelfth elytron.]
PROTECTIVE AND MIMETIC DEVICES.—From the point of view of "protection" in
the evolutionist's sense of the word, we can {294}say but little.
Protective resemblance there is undoubtedly amongst the Polynoids, for the
scales of these forms resemble more or less closely the stones or sand
amongst which they live; in the same species there is great variety in
coloration. This protective habit is carried still further in the case of
_Psammolyce_ by the attachment of sand grains to little cups on the elytra,
so that the back of the animal is concealed. Certain commensals, such as
_Polynoë arenicolae_, _P. pentactes_, are coloured so as to resemble their
associates. In a few cases it is possible that the gills of Sabelliformia
are protectively coloured; for in _Sabella pavonia_ they vary from a light
yellowish tint to a deep violet-brown, and the dark markings on them are
therefore more or less distinct. Spread out as the gills are in life, they
are in many cases difficult to recognise; it is rather their movement as
they are withdrawn that attracts one's attention to them, as the tubes of
these worms frequently serve for the attachment of brownish seaweeds, to
which the gills bear resemblance. But, as a matter of fact, little work has
been done in this direction, and speculation on the matter without evidence
is worthless. Many pelagic forms, being transparent, such as _Tomopteris_
and Alciopids, are no doubt protected by their lack of colour; yet these
forms present brightly-coloured spots,—the light-producing organs in the
parapodia of the former, and the large dark eyes of the latter.
Semper[352] mentions a case of possible mimicry in a species of _Myxicola_
which lives in the clefts of a coral, _Cladocora_. The branchial funnel,
when expanded, resembles very closely the expanded coral in size, colour,
etc.; but he points out that the species occurs in other situations, where
its colouring is not protective. Probably the "mimicry" is in other
instances merely accidental.
No doubt many Polychaetes may be "warningly coloured," but experimental
evidence is incomplete. _Polycirrus aurantiacus_ is bright red, with orange
tentacles; these worms were rejected by certain fish.[353] The animal has
given up living in tubes as all its allies do, and it is the tentacles
which appear to be distasteful to its enemies, for when irritated it coils
itself up and wraps {295}itself round with its tentacles. Moreover, when
the tentacles were cut off the fish did not reject the body of the worm.
The tentacles are thus coloured in such a way that fish recognise them, and
associate with the colour some distasteful property.
[Illustration: FIG. 157.—_Chaetopterus variopedatus_ Ren. × ½. On the left
the entire animal, with the three regions A, B, C. _c_, Peristomial cirrus;
_d_, "sucker"; _e_, the great "wings"; _f_, "fan"; _m_, mouth. On the right
the animal is represented in the dark, under stimulation, so as to exhibit
the phosphorescent portions of the body. (From Panceri.)]
PHOSPHORESCENCE.—Many worms of very different habits have the power of
emitting a light from some parts of the body, and they are then said to be
"phosphorescent."[354] Probably _Chaetopterus_ is most eminently
photogenic; the base of the great "wings," the "fans," and other parts
emit, on stimulation, an azure blue to greenish light, so bright that one
may read one's watch by it. Several species of _Polynoë_ exhibit a similar
phenomenon, each elytron, with the exception of the area of attachment,
being brilliantly illuminated. In these species the phosphorescent elytra
are frequently thrown off by the animal, so that possibly they deceive
enemies. _Polycirrus aurantiacus_ produces a beautiful violet
phosphorescence; usually its many tentacles alone show the light, but under
strong stimulation the entire body takes {296}part in the display, and no
doubt the phosphorescence has, like the colour, a "warning" purpose.
The production of the light in these various forms is apparently due to two
different processes. In some cases, _e.g._ _Chaetopterus_, Syllids,
Terebellids, it appears to be due to the oxidation of certain cell contents
which are discharged more or less freely on irritation of the nerves;
whilst in Polynoids the phenomenon is due to some purely nervous process,
for the elytra have no glands, but are provided with ganglia and a nervous
network.
In other worms, however, there are definite light-producing organs. In
_Tomopteris_ there is on each parapodium, above and below, a
brightly-coloured spherical organ, which for a long time was regarded as an
eye, but from its structure appears to be a "photogen" (Fig. 167, p. 315).
The same is very likely the true explanation of the segmental "eyes" of
_Polyophthalmus_, for their structure recalls that of the light-organs of
deep-sea fishes.
As many of the phosphorescent Polynoids are commensals, while
_Chaetopterus_ inhabits tubes, and close allies of other phosphorescent
worms have no power of emitting light, it is impossible to apply the same
explanation of its purpose to all cases alike; in some it may be
"accidental," though in others it may be of definite use in warning enemies
or in attracting prey.
The FOOD OF WORMS.—The Nereidiformia are mostly carnivorous, and feed on
small Crustacea, Mollusca, sponges, and other animals; and Polynoids are
even said to eat one another. Many worms do not disdain various seaweeds,
whilst the Spioniformia and Scoleciformia, which burrow in mud and sand,
and are without biting organs, swallow the mud and digest what animal or
vegetable débris it may contain. The Terebellids and Cryptocephala depend
on minute organisms which may be driven into the mouth by the action of the
cilia of the gills or tentacles.
In the case of deep-sea forms, it is an interesting fact that the
intestines are not unfrequently crammed with Radiolaria and Foraminifera in
a fairly fresh, uninjured condition, indicating that these Rhizopods do not
merely _sink_ to the bottom, but must actually _live_ there.[355]
The ECONOMIC PURPOSES to which Polychaetes are put are few; they are used
either as bait for fishes or as food for man.
{297}One of the commonest baits used for certain fish, as all who have done
any sea-fishing off the piers of our coasts know, is the common lug-worm
(_Arenicola marina_), whilst _Nephthys caeca_ and _Nereis fucata_ are also
used in some places; and for whiting _Nereis cultrifera_ and _N.
diversicolor_. _Marphysa sanguinea_, known to the fishermen in some parts
as "varme," is less frequently used.
A peculiar worm—_Palolo viridis_—is used as food by the natives of Samoa
and Fiji. The worm is similar to our Eunicid _Lysidice ninetta_, and lives
in fissures among corals on the reefs, at a depth of about two fathoms. At
certain days in October and November they leave the reefs and swim to the
shores of the above islands, probably to spawn; and this occurs on two days
in each of the above months—the day on which the moon is in her last
quarter, and the day before. The natives, who call the worm "Mbalolo," give
the name "Mbalolo lailai" (little) to October, and "Mbalolo levu" (large)
to November, thereby indicating the relative abundance of the worms in
these two months. The natives eat them either alive or baked, tied up in
leaves; and they are esteemed so great a delicacy that presents of them are
sent by the chiefs who live on shore to those living inland. A dark
green-blue Phyllodocid, which is called "A'oon," occurs in abundance off
Mota Island, amongst the New Hebrides, has similar habits, and is also
eaten.[356]
ASSOCIATED WORMS.—A considerable number of worms live in association with
other animals, either as commensals or as parasites, and it is not in every
case possible to decide in what relation the two animals stand.
_Labrorostratus parasiticus_, a Eunicid, is parasitic in the body-cavity of
_Odontosyllis ctenostomatus_ (Fig. 158); such an association between two
members of the same group of animals is peculiar; but still more
exceptional is the occurrence of _Haematocleptes terebellides_, as a
parasite in _Marphysa sanguinea_, for both parasite and host are members of
the same family, the Eunicidae. Another Eunicid, _Oligognathus bonelliae_,
occurs in the body-cavity of the Gephyrean _Bonellia_.
The Polynoid _Acholoe astericola_ and the Hesionid _Ophiodromus flexuosus_
occur as ectoparasites (or perhaps commensals) in the ambulacral grooves of
the starfish _Astropecten aurantiacus_. An Amphinomid is stated to live in
the branchial chamber of {298}the barnacle, _Lepas anatifera_. _Alciopina
parasitica_ lives, during the early stages of its life-history, within
_Cydippe_, and it is possible that most of the Alciopids thus make use of
Ctenophores as their nurseries.
A considerable number of the Polynoids are ectoparasitic: _P. castanea_
lodges in the peri-oral region of _Spatangus purpureus_, and in the
ambulacral grooves of _Astropecten_; _P._ (_Halosydna_) _bairdi_ lives
between the mantle and foot of the mollusc _Fissurella cratitia_; _P.
pentactes_ is found on the body of the Holothurian _Cucumaria pentactes_,
and appears to be protectively coloured. _P._ (_Antinoë_) _parasitica_
lives under the elytra of another Polynoid, and _P. acanellae_ on the coral
_Acanella normani_.[357]
[Illustration: FIG. 158.—_Odontosyllis ctenostomatus_, with (_L_)
_Labrorostratus parasiticus_ in its body-cavity. The parapodia and cirri
are omitted from the greater part of the body. (After St. Joseph.) × 4.]
As commensals there may be mentioned _Nereis fucata_, which lives in the
upper coil of whelk-shells which are inhabited by a hermit crab. The same
shell usually bears a particular sea-anemone, so that there are three
animals living together in or upon the cast-off house of a fourth.
_Siphonostoma_ is found in the "nests" made by the mollusc _Lima_. A
_Eunice_ is constantly associated with the coral _Lophohelia prolifera_,
amongst the branches of which the worm twines its tube; whilst another
Polychaete inhabits a tube formed by the interweaving of the fine branches
of the coral _Antipathes filix_,[358] found in the West Indian seas. A
species of _Polydora_ forms its tube in _Heliopora_. The Polynoids present
many instances of commensalism, a few of which may be here mentioned. _P.
johnstoni_ Marenz. is only found in the tubes of _Terebella nebulosa_;
other species occur in the tubes of other Terebellids. _P. marphysae_ lives
in tubes of the Eunicid _Marphysa sanguinea_. Two species live in the tubes
of _Chaetopterus_. _P. extenuata_ has been found in tubes of _Serpula
vermicularis_, while _P. arenicolae_ occurs on {299}the body of the common
lug-worm, with the colouring of which it closely harmonises.
WORMS AS HOSTS.—The Polychaeta serve not only as food for fishes,
Crustacea, and other predatory animals of larger size, but are also liable
to be the hosts of parasites[359] such as Gregarines, and even, as we have
seen, of other members of their own group. Sundry ectoparasitic Copepoda
have been found attached to worms between the parapodia or to the sides of
the feet, and an unnamed Copepod occurs attached, sometimes in considerable
numbers, to the sides of _Nereis cultrifera_. The Polychaeta also act as
protectors to other animals, for on the under surface of elytra of sundry
Polynoids may very frequently be found specimens of _Loxosoma_, which may
also be attached to gills of Eunicids; whilst below those of _Aphrodite
echidna_ and _Hermadion pellucidum_, _Pedicellina belgica_ occurs. Under
the felt of _A. aculeata_ the Sabellid _Branchiomma vigilans_ forms its
tube, and Vorticellids may be found on chaetae, gills, or other parts of
the body of sundry worms.
DISTRIBUTION.—Very little can be said in a brief way of the _geographical
distribution_ of these worms, for many of the genera are cosmopolitan,
although only a few species occur in all the great oceans, _e.g._ _Polynoë
imbricata_, _Hyalinoecia tubicola_, _Nerine_ (_Scolecolepis_) _cirrata_,
and _Terebellides stroemi_.
As for species, it can be said generally that the different oceanic areas
and even different coasts present different species, but we know
practically nothing of variation amongst Polychaeta, and many so called
species may be mere local varieties, for frequently the descriptions of
"new species" are scarcely intelligible. At any rate we know that certain
species occur at widely separated localities, for two or three species of
Polynoids occur in Japan, and again at Dinard on the French coast. A
considerable number of species are common to both sides of the North
Atlantic ocean, having been obtained off Norway and in the Gulf of the St.
Lawrence. A few of these which are common on our coasts may be
enumerated:—_Nereis pelagica_, _Nicomache lumbricalis_, _Glycera capitata_,
_Thelepus cincinnatus_, _Scoloplos armiger_, _Sabella pavonia_, _Ophelia
limacina_, _Aphrodite aculeata_, _Trophonia plumosa_, _Polynoë squamata_,
_Capitella capitata_, _Sthenelais limicola_.
{300}As for _bathymetrical_ distribution,[360] many genera occur at all
depths, though Polychaetes appear to be most abundant, as far as we know at
present, in "shallow water"—that is, down to twenty fathoms or so; but this
may be due to the greater facility of collection on shore and in these
slight depths, for the "Challenger" obtained considerable numbers of new
species at greater depths.
The "deep-sea" forms are chiefly tubicolous, and since these tubes are
fixed and partially embedded in the bottom, probably comparatively few are
brought up. Some genera occur at very great depths; thus the Terebellid
_Leaena abyssorum_ and the Serpulid _Placostegus benthalianus_ were brought
up from 3125 fathoms—the greatest depth from which Polychaetes were
obtained by H.M.S. "Challenger"; and it is interesting to note that species
of each of these two genera occur in shallow water, the Serpulid being
represented in our own coast fauna by _P. tricuspidata_.
Amongst our own fauna, a few examples may be given of the "replacement of
species."[361] The littoral _Sthenelais boa_ is represented by _S.
limicola_ in deeper water; _Sabellaria alveolata_ by _S. spinulosa_;
_Polynoë imbricata_ by several deep-water species. Similarly with genera:
the littoral _Pomatoceros_ is replaced by _Serpula_ in deeper water; and
the Hesionid _Psamathe_ by _Castalia_.
The limitation of species to certain regions, or to certain depths of an
ocean, may appear at first sight peculiar, in view of the unrestricted
communication between all its parts; but there are as efficient "barriers"
there as on land, for generally a particular worm can live only in a
certain temperature and at a certain pressure, and is dependent for its
food on particular organisms, which in their turn depend on the depth and
its accompaniments. It is, in fact, so much the more peculiar that certain
species are more or less cosmopolitan, or occur at widely distant points.
It is less peculiar, of course, to find different species of the same genus
at different depths or in different areas, for any slight variation in a
species advantageous to new conditions would readily be fixed, and give
rise to a new species.
The distribution of the Polychaeta depends probably on the pelagic larvae,
which are carried by currents from one part of an ocean to another. There
can be little doubt that many {301}Polychaetes are very "plastic," and can
adapt themselves to changed conditions of life with considerable ease; for
_Nereis diversicolor_, _Arenicola marina_, and others live equally well in
water of very different densities, and with a different food supply. The
great variety in the "habitats," and presumably therefore in their food
supply, etc., exhibited by many Polychaetes, as well as the great variation
observable in some species of Polynoina, and the close affinity of the
species and genera of this sub-family, lead us to the same conclusion.
EXTINCT POLYCHAETES.—The most numerous fossil records of the Polychaetes
are calcareous tubes of various shapes and sizes; they are irregularly or
spirally curved, and are very usually attached at one end, or by one
surface, to stones or to fossils. These tubes belong to the Serpulidae, and
are referred to the genera _Serpula_, _Spirorbis_, _Ditrupa_, and
others.[362]
_Spirorbis_ is the oldest unequivocal representative of the Polychaetes, as
its tubes are found more or less abundantly in the Silurian and other
Palaeozoic strata. In Palaeozoic times _Serpula_ was rare, as it was too in
the Trias and Lias, but in the Jurassic strata it becomes abundant. In the
chalk, _S. socialis_ may occur in masses like _S. uncinata_ of the present
day, forming "Serpulite chalk." In the older tertiaries the genus is
represented by _Spirulaea_.
_Terebella lapilloides_ occurs in the Lias as a cylindrical, more or less
curved tube of sand-grains.
[Illustration: FIG. 159.—_Eunicites avitus_ Ehl. A fossil worm from the
lithographic slate of Solenhofen: the jaws are seen in front, and the
acicula along each side. (From Ehlers.) Natural size.]
Amongst the Nereidiformia the remains are fewer, but the {302}acicula and
the hard jaws are preserved in certain rocks, and can be referred to
existing families. _Eunicites avitus_[363] is represented by a double
series of acicula, indicating the parapodia of the two sides; and by
remains of both upper and lower jaws (Fig. 159). Four different species of
the worm have been described from the lithographic slate of Bavaria, of
Jurassic age; and several upper jaws of other Eunicids have been discovered
in the Palaeozoic beds of Canada and Scotland, and have received the names
_Lumbriconereites_, _Oenonites_, and _Arabellites_, in reference to their
nearest allies amongst living genera.
There are, however, numerous remains, in the forms of tracks or casts, in
the earlier rocks, which have been referred to the Polychaeta. The names
_Crossopodia_, _Myrianites_, _Nereites_, _Phyllodocites_, have been given
to some of these traces, though they are open to numerous other
interpretations. Some of the "tracks" are similar to those made by living
Crustacea in walking over wet sand; others appear to be the casts of some
animals. Tubular burrows in rocks or fossils, some straight, others
U-shaped, have received such names as _Arenicolites_, _Scolithus_,
_Histioderma_; whilst under the name _Lumbricaria_ certain cylindrical,
coiled structures, resembling worm "castings," are met with in this same
lithographic stone of Solenhofen. Many of the tubes referred to Polychaetes
by the earlier palaeontologists have been transferred to other groups; thus
_Cornulites_ is now believed to be a Pteropod shell.
This very meagre geological record is quite insufficient to form any basis
for a phylogeny of the group. And this poor supply of remains is not
surprising, when we consider the soft nature of the tissues, the absence,
in the majority of families, of skeleton and of other parts which could
have been fossilised; yet we might have expected a greater abundance of
fossilised jaws than is represented at present. But it must be borne in
mind that the conditions of life of these soft-bodied animals are not
conducive to their leaving abundant fossilised remains.
{303}CHAPTER XII
CHARACTERS OF THE SUB-ORDERS OF POLYCHAETES—CHARACTERS OF THE
FAMILIES—DESCRIPTION OF BRITISH GENERA AND SPECIES—THE MYZOSTOMARIA.
SYSTEMATIC.—The Order Polychaeta may be divided into two branches, in one
of which, the PHANEROCEPHALA, the prostomium retains its ancestral
condition as a lobe overhanging the mouth, and frequently carries, in
addition to paired eyes, certain sensory processes of a simple structure,
the tentacles and palps; the body-segments are more or less alike, and
(except in some Spioniformia, some of the Terebelliformia, and the
Capitelliformia) do not present two sharply marked regions, owing to the
differential arrangement or character of the chaetae. In the second branch,
the CRYPTOCEPHALA, the peristomium grows forwards during development, so as
to compress or even hide the prostomium, which thus becomes a very
insignificant organ. The tentacles are reduced, but the palps become
greatly developed and take on sundry new functions. The body in this group,
by the character and arrangement of the chaetae, is distinguishable into a
thorax and abdomen, presenting certain internal differences.
These two branches may be supposed to have arisen from a common ancestor
having a general resemblance to a nereidiform worm, such as _Syllis_,
possessing palps and tentacles on the prostomium, definite parapodia and
cirri on the body, and internally, a well-marked and regular repetition of
organs.
The branch PHANEROCEPHALA contains the following five sub-Orders, though it
is possible that the Capitelliformia deserves a more important position in
the system:—
SUB-ORDER 1.—The _Nereidiformia_ have well-developed tentacles and palps;
the peristomium almost invariably possesses {304}special cirri; the
parapodia are well-marked locomotor organs, supported by acicula, and carry
dorsal and ventral cirri. The chaetae are usually jointed, though unjointed
ones may coexist with these; uncini are never present. An eversible buccal
region leads into a muscular pharynx, which in the majority is armed with
chitinous jaws; the septa and nephridia are regularly repeated throughout
the body. The worms lead a predaceous life, and are mostly carnivorous; a
few form tubes.
SUB-ORDER 2.—The _Spioniformia_ possess neither tentacles nor palps; the
peristomium usually carries a pair of long tentacular cirri, and extends
forwards at the sides of the prostomium. The parapodia project only to a
slight degree; the dorsal cirri may attain a considerable size, and act as
gills throughout the greater part of the body. The chaetae are unjointed;
uncini are only present in the aberrant _Chaetopterus_.[364] The body may
present two regions more or less distinctly marked externally, but without
corresponding internal differences. The buccal region may be eversible, but
there are no jaws. Septa and nephridia are regularly developed. The worms
are burrowers, or tubicolous.
SUB-ORDER 3. _Terebelliformia._—The prostomium is a more or less prominent
lobe (upper lip) with or without tentacles but without palps. The
peristomium may carry cirri or "tentacular filaments."[365] The parapodia
are feebly developed; there are no ventral cirri; the dorsal cirri may
exist and function as gills on more or fewer of the anterior segments. The
chaetae are unjointed, and uncini are usually present. The buccal region is
not eversible; there are no jaws. The septa are usually incomplete, with
the exception of one strongly-developed "diaphragm" anteriorly; the
nephridia are dimorphic, those of the anterior (prediaphragmatic) segments
are of large size and are excretory; the posterior series are mere funnels,
and act as genital ducts. These worms are burrowers or tube-formers, and in
the majority the tube-forming glands are grouped on the ventral surface of
the anterior segments to form "gland-shields."
{305}SUB-ORDER 4.—The _Capitelliformia_ have no prostomial processes, but
possess a pair of large retractile "ciliated organs." The parapodia do not
project; the chaetae are unjointed, and are hair-like in the anterior
segments and hooded "crotchets" posteriorly; this external division of the
body does not correspond with definite internal differences. There are no
cirri, though special "gills," often retractile, are frequently present.
The buccal region is eversible; there is no armed pharynx. An "accessory
gut" or "siphon" exists. The nephridia are small, and sometimes more than
one pair in a segment; special genital funnels exist in more or fewer of
the anterior segments of the hind body. There is no system of
blood-vessels; the coelomic corpuscles are red. The worms are burrowers.
SUB-ORDER 5.—The _Scoleciformia_ possess a prostomium, which rarely
(Chlorhaemidae) carries any sensory processes; the peristomium is without
cirri (except, perhaps, in the Chlorhaemidae). The parapodia are ill
developed, and may be absent; only rarely are dorsal cirri present, acting
as gills; ventral cirri are absent. The chaetae are unjointed; true uncini
are not present. The buccal region is eversible, but there is no armed
pharynx. The septa are not regularly developed, as more or fewer are
absent, and the nephridia are considerably reduced in number, it may be to
a single pair (Sternaspidae and some Chlorhaemidae), but they are all
alike.[366] The worms are mostly burrowers.
The branch CRYPTOCEPHALA contains two sub-Orders:—
SUB-ORDER 1. _Sabelliformia._—The prostomium is entirely hidden by the
forward extension of the peristomium; the tentacles are very small, being
frequently represented merely by small knobs of sense-cells; the palps, on
the other hand, are greatly developed, branched, and contain blood-vessels,
acting as respiratory as well as sensory organs. The peristomium never
carries cirri or chaetae, and it is usually raised up into a projecting
collar, used in fashioning the lip of the animal's tube. The parapodia are
but feebly developed; cirri are absent, except in the Serpulidae, where the
dorsal and ventral cirri become united to form the "thoracic membrane"
(Meyer). The chaetae are of two kinds—unjointed, hair-like, fringed
bristles and "uncini." {306}By their arrangement the body is divided into a
thorax of nine segments and an abdomen; in the former the capillary chaetae
are dorsal, and in the latter ventral. The buccal region is not eversible;
there is no pharynx. The septa are regularly developed in the abdomen, but
are absent in the thorax; the nephridia are dimorphic; there are two large
ones in the thorax opening by a median dorsal pore just above the brain;
those of the abdomen are small funnels, and act as genital ducts. The worms
are tubicolous; "gland-shields" are present on the thoracic segments.
SUB-ORDER 2. _Hermelliformia._—The peristomium (Fig. 135) is enormously
developed, and forms a bilobed hood capable of closing over the mouth; the
truncated free end of each lobe carries three semicircles of peculiar
chaetae, which act as an efficient protection when the worm is withdrawn
into its tube. The prostomium is very small, but retains a pair of
well-developed tentacles; the palps, which are subdivided as in the
Sabelliformia, have become fused with the ventral edges of the peristomium,
and appear as a series of ridges on each side, carrying numerous filaments.
The thorax consists of five segments, the notopodia of three of which are
well developed and bear strong chaetae; dorsal cirri are present along the
greater part of the body, and act as gills. The arrangement of the chaetae
and of the internal organs is as in the Sabelliformia. The worms form tubes
of sand.
BRANCH A. PHANEROCEPHALA.
SUB-ORDER 1. NEREIDIFORMIA.[367]
FAM. 1. _Syllidae._—These are small worms, the majority being less than an
inch long, so that they are not easily observed. {307}The body consists of
a fair number of segments.[368] In many genera a dorsal bundle of
unjointed, natatory chaetae makes its appearance at maturity. The palps,
which are grooved, are in some cases so united with one another and with
the prostomium as to be scarcely recognisable. (For head see p. 262, and
for feet see p. 264.) The pharynx is armed with one or more teeth. There is
a special gizzard, following the pharynx, and provided with thick, muscular
walls of peculiar structure. Following the gizzard, the oesophagus receives
in many genera a pair of T-shaped diverticula, that are used for storing
water, which is swallowed with food. These diverticula are absent in
_Autolytus_ and other free-swimming forms. The reproduction of the members
of this family is interesting, and has already been described (p. 278).
[Illustration: FIG. 160.—_Syllis armillaris_ Müll. × 2. (From Johnston.)
The head is towards the right.]
_Syllis._—The tentacles and cirri are moniliform; the palps large; there is
a single dorsal tooth, which is provided with a poison gland, the duct of
which opens near its apex; it is used rather for stabbing its prey than for
grasping and tearing. _S. krohnii_ Ehlers, is abundant under stones, and
forms tubes of sand; it is nearly an inch long, and consists of some
eighty-five to ninety-five segments marked with yellow bands. It may
readily be identified by longer dorsal cirri, terminally dilated,
alternating with shorter ones. _S. cornuta_ Rathke, has a translucent green
body, about half an inch long; no alternation {308}of cirri. Mediterranean,
Atlantic, on the Norwegian coast, off Spitzbergen, and on the Madeira
coast. _S. armillaris_ Müll. is very common at low water; it is pale
yellowish-brown, with a couple of dusky marks on each segment; and measures
2 inches. The dorsal cirri are quite short, consisting of only eight to ten
joints. In _Pionosyllis_ the tentacles and cirri are not moniliform; a
single dorsal tooth. _P. malmgreni_ M‘I. under stones. _Sphaerosyllis_.—The
dorsal cirri are swollen at the base, and are not moniliform; the long
palps are fused along nearly their whole extent. _S. hystrix_ Clap. is only
about one-eighth of an inch in length. _Exogone_ Oerst. _Grubea_ Qfg.
_Autolytus._—The small palps are entirely fused with the prostomium; the
pharynx, which is bent upon itself, is armed with a circle of denticles.
Dorsal cirri somewhat foliaceous. There are no ventral cirri. The male and
female differ from one another and from the asexual "stock" (see p. 279).
_A. pictus_ Ehl. is abundant under stones. It measures about two-thirds of
an inch in length, is darkly coloured with a median lighter band; the
anterior dorsal cirri are long. _A. prolifer_ Müll. is common.
_Myrianida fasciata_ Milne Edwards, with its foliaceous cirri, occurs off
our coasts (see Fig. 149, p. 280). Atlantic, Mediterranean.
FAM. 2. _Hesionidae._—The body is relatively short, with only a few
segments (sixteen to fifty, according to the genus); in the larger forms it
is cylindrical. The parapodium is usually uniramous; the dorsal cirri are
long and multiarticulate; the chaetae are jointed. The prostomium carries,
in addition to four eyes, two or three tentacles, and generally a pair of
jointed palps. The peristomium and two or more of the following segments
are achaetous, and carry long "peristomial" cirri. The pharynx is very long
but unarmed.
_Psamathe_ Johnston, has many segments; head with two tentacles and a pair
of three-jointed palps. _P. fusca_ Jnstn. occurs amongst coralline Algae,
to which it bears some resemblance, which is heightened by the moniliform
cirri. It is a small worm, less than an inch in length. Mediterranean.
_Castalia punctata_ Müll. is dirty green or brownish, with a narrow
purplish band on each side. It occurs in deeper water than the preceding.
In _Ophiodromus_ the head has three tentacles; the palps are two-jointed;
there are six pairs of peristomial cirri; the parapodia {309}are biramous.
_O. vittatus_ Sars is dredged in numbers off the Scotch coast, and is found
also at low tides. It measures 2 inches in length. A closely allied species
lives in the ambulacral grooves of the starfish _Astropecten_.
FAM. 3. _Aphroditidae._[369]—The most characteristic feature of this
family, and one by which its members are absolutely distinguished from all
other Chaetopods, is the possession of scales or "elytra" on the back.
These flattened dorsal cirri are of a somewhat horny texture, and are
carried, generally, on alternate segments of the body; filamentous cirri
occurring on the other segments. In the sub-families _Hermionina_ and
_Polynoina_ the elytriferous segments are 2, 4, 5, 7, 9, etc., up to 23;
then every third segment. The worms are usually short, with some
thirty-five to forty-five segments, though _Sthenelais_ and a few others
have many more. (For head see p. 262, and for parapodium see pp. 265, 268.)
The pharynx is very thick walled, and furnished with two pairs of jaws,
which are, however, not hardened in the sub-family _Hermionina_. The
intestine is provided with a number of paired longer or shorter caeca (Fig.
142). A considerable number of this family are commensal or parasitic (see
p. 297). The family is well represented on our own coasts, so that only a
few of the more readily distinguishable species can be here described.
[Illustration: FIG. 161.—_Polynoë squamata_ L. Nat. size. _c_, Notopodial
cirrus; _e_, elytron; _f_, parapodium; _p_, palp; _t_, tentacle. (From
Johnston.)]
Sub-Fam. 1. _Polynoina._—Body flattened, with nearly parallel sides,
usually short, more rarely worm-like; three tentacles; peristomium with
long dorsal and ventral cirri; the ventral cirri of the next segment are
also elongated. Jaws are present. Elytra, usually twelve to eighteen pairs,
the surface of which is more or less papillose, and may be "fringed" along
the outer border, with long processes. The colouring of the elytra is
characteristic in most cases, though liable to considerable variation in
some species. The chaetae are generally strong, and of bright golden
colour: they are all unjointed. The _Polynoina_ are generally but feeble
swimmers, {310}and are mostly found under stones at low tide. Some species
have a very wide geographical range.
_Polynoë._[370]—The body is short; none or only a few segments at the end
of the body are uncovered by the elytra, except in the long body of _P.
johnstoni_.
_A. With twelve pairs of elytra._—In _P. squamata_ Linn. the elytra
entirely cover the body and conceal the head, each elytron overlapping the
next posterior one, and those of the two sides overlapping. General colour
sandy-brown, speckled, lighter or darker. The fringed elytra are very
firmly fixed to the body. The notopodial chaetae scarcely project from
below the elytra. The worm is common between tide-marks and in the
coralline region, is about one to one and a half inches in length, and
about one-third of an inch in width. Atlantic. _P. clava_ Montagu, may
attain a larger size, though it is generally smaller. The elytra are dark,
usually grey, mottled with white or light grey, unfringed, and do not
overlap to so great an extent as in _P. squamata_, so that the middle of
the back and the hinder part of the body is more or less exposed. This is
never the case in the preceding species, but even here it is subject to
variation in extent, depending on the amount of food contained by the worm
or on the ripeness of the genital products. It occurs in the
Mediterranean.[371]
[Illustration: FIG. 162.—Elytra, A, of _Polynoë squamata_ L.; B, of _P.
clava_ Mont. × 10. _a_, Area of attachment; _e_, external margin; _f_,
fringe (the letter is at the posterior side of the elytron); _i_, internal
margin. (From Bourne.)]
_B. With fifteen pairs of elytra._—_P. imbricata_ L. is probably the
commonest species of the genus, occurring nearly everywhere under stones at
low tide. It is about an inch in length; the elytra are deciduous, and are
very variously coloured and marked; sometimes uniformly grey or even black,
sometimes {311}mottled with brown: in other specimens each elytron has its
outer half pale or white, while its inner half is darker, usually some tint
of brown or olive green, so that the worm appears to have a dark band along
the middle of its back. Other patterns occur. The body is entirely covered
by the elytra. The chaetae project considerably, and are nearly as long as
half the width of the body; those of the notopodium are brown and are
directed upwards, being nearly as long as the golden neuropodial chaetae.
This species has a very wide range, occurring on both sides of the
Atlantic, even on the shores of Nova Zembla, and reappearing again at
Japan. _P. semisculptus_ Leach is rather larger than the foregoing. The
elytra are very readily detached: they are light in colour, without a
fringe, but with large papillae near the margin. The notopodial chaetae are
thicker than those of the neuropodium. Several other species are also
common, but _P. johnstoni_ v. Marenz.[372] differs from the rest in having
an elongated body of some seventy segments, so that the posterior half is
uncovered by the elytra, which are small, greenish-grey, speckled, and have
no fringe. It is common and widely distributed, but appears to be only
found in the tubes of _Terebella nebulosa_.
[Illustration: FIG. 163.—Elytron of _Polynoë imbricata_. _a_, Area of
attachment to body; _e_, outer border; _i_, inner border.]
_C. With eighteen pairs of elytra._—_P. gelatinosa_ Sars, may attain a
length of 2 inches. The elytra are very faintly coloured, transparent and
soft, attached by rather long peduncles. In spirit they become swollen and
folded, giving the worm a very untidy appearance. The prostomium is partly
overlapped by a peculiar collar-like fold of the peristomium.
_D. With numerous pairs of elytra._—_Lepidasthenia_ has a very long body,
consisting of more than eighty segments. The elytra are quite small, and
occur throughout the body on the usual segments. There are no notopodial
chaetae. _L. elegans_ Gr. is a very elegantly marked worm, which, however,
has not been recorded from the British area; it occurs in the Mediterranean
(see Fig. 156, p. 293).
Sub-Fam. 2. _Hermionina._—The body is short, oval and {312}depressed; the
particularly strong notopodial chaetae are directed upwards and backwards
so as to protect the elytra. The neuropodial chaetae are also strong. The
prostomium carries a single tentacle and two long palps; the prostomial
ridge may be well developed. The peristomium is chaetigerous, with long
cirri. The jaws are represented merely by thickened prominences.
[Illustration: FIG. 164.—_Aphrodite aculeata_ L. (from _Règne Animal_).
Nat. size. _c_, Neuropodial chaetae; _p_, palps; 1, iridescent bristles; 2,
stiff chaetae; 3, felting bristles of notopodium.]
_Aphrodite._—The fifteen pairs of elytra, arranged as in _Polynoë_, are
concealed by a "felting" of hair-like chitinous threads arising from the
notopodium (Fig. 139, p. 268). _A. aculeata_ L.—The "sea-mouse" is one of
the most beautiful of the Polychaetes. The small tentacle is very readily
detached; the palps are very long; the parapodia of the peristomium are
directed forwards so as to form lateral lips; and its cirri are not
especially modified (see Fig. 132, p. 260). The body, which measures 3 to 6
inches, consists of thirty-five to forty segments, and is broadest in the
middle, the last dozen segments being very small; the body terminates in a
point. Some of the notopodial chaetae are brilliantly iridescent, and give
the worm its characteristic coloration. It is fairly common in the
coralline regions, and is frequently thrown ashore after storms. Atlantic
and Mediterranean.
In _Hermione_ the "felt" is absent, so that the elytra are exposed. _H.
hystrix_ Sav. occurs in ten to thirty fathoms of water all over the British
area and Mediterranean. It resembles in its general appearance a fat
Polynoid, with strong chaetae. _Laetmonice filicornis_ Kinb. also occurs on
our north-west coasts, and _L. producta_ Gr. has been dredged in 500
fathoms off the {313}west coast of Ireland; it has been recorded also from
Kerguelen and from Japan, so that it has a very wide distribution.[373]
Sub-Fam. 3. _Acoetina._[374]—The long, vermiform body has some thirty-nine
to ninety-three pairs of elytra, placed on every alternate segment
throughout. It is represented in the British area by _Panthalis_ from 75
fms., which forms a tube of black mud.
Sub-Fam. 4. _Sigalionina._—This sub-family includes forms with a long,
vermiform body; anteriorly the elytra are on alternate segments, up to the
twenty-sixth, and posteriorly on every succeeding segment; "gills" here
coexist with elytra; cirri are absent. The prostomium in _Sthenelais_ Kinb.
has a median tentacle, which is absent in _Sigalion_ Aud. and Edw. _Sth.
boa_ Jnstn. is common off our coasts near low-water mark, where it burrows
in the loose sand with rapidity. It is an elegant worm, and may attain a
length of 8 inches, though it is generally smaller; it is narrow, flat, and
only slightly tapering at each end; the elytra, which may be more than a
hundred pairs, are greyish or slightly brownish, some being lighter than
others; the margin is fringed with simple processes (which in _Sigalion_
are pinnate). Atlantic and Mediterranean. In _Psammolyce_ the elytra are
covered with sand grains. British and Mediterranean.
FAM. 4. _Phyllodocidae._—The members of this family make use of the
foliaceous cirri (Fig. 136, F) in their very active movements. The rounded
prostomium bears four or five tentacles; there are four long peristomial
cirri on each side (see Fig. 134, E).
Sub-Fam. 1. _Phyllodocina._—The body is elongated, with numerous segments;
the eyes are small; the chaetae are jointed; the dorsal and ventral cirri
are foliaceous; the pharynx is covered with papillae externally, but
contains no "jaws."
_Phyllodoce_ has a more or less depressed body; four prostomial tentacles;
four pairs of peristomial cirri. _P. lamelligera_ Jnstn.[375] (the
"paddle-worm") may reach a length of 24 inches, but is usually 8 to 12
inches long and ½ inch across. The general colour is bright bluish-green or
yellowish-green, with metallic iridescence; the parapodia olive-green or
brown, the sensory {314}processes yellow. It lurks, during day, under
stones and shells, etc., in the Laminarian zone. The green egg masses, so
frequently referred to as belonging to _Arenicola_, are laid by
Phyllodocids.[376]
[Illustration: FIG. 165.—_Phyllodoce paretti_ Blv. × ½. (From _Règne
Animal_.)]
In _Eulalia_ an additional (fifth) tentacle arises from the middle of the
back of the prostomium. _E. viridis_ Müll. is a dark green worm smaller
than the preceding; common between tide-marks, hiding in cavities and
tunnels in limestone rocks, which have been bored by the mollusc
_Saxicava_; it is rare where such rocks are absent. It might have been
thought that its vivid colour would harmonise with its surroundings, but it
is most abundant in regions where _Fucus_ abounds and _Ulva_ is absent. It
is evident then that the colour is not protective; it may perhaps be of
warning significance, for the mucus secreted in quantities by glands on the
cirri of the Phyllodocids is probably objectionable to their enemies.
_Phalacrophorus_ Grf. and _Pontodora_ Grf. may be mentioned as pelagic
genera.
Sub-Fam. 2. _Lopadorhynchina._—This includes small forms, _Lopadorhynchus_
Gr., _Pelagobia_ Grf., and other pelagic genera.
[Illustration: FIG. 166.—_Nauphanta celox_ R. Grf. × 4. (From Greef.) _e_,
The large eye.]
Sub-Fam. 3. _Alciopina._—These are surface forms, and, like most pelagic
animals, are colourless and transparent; the eyes, however, are very large,
and, with certain brown spots in each segment,[377] are the only coloured
parts in the body; in structure the eyes are much more complicated than
{315}those of other Polychaetes. The prostomium has five tentacles; there
are long peristomial cirri, and in general their anatomy agrees most
closely with that of Phyllodocids. _Alciope_, _Asterope_, _Vanadis_,
_Nauphanta_ are genera of the family;[378] none have been recorded from the
British area.
FAM. 5. _Tomopteridae._[379]—This includes but one genus, _Tomopteris_,
which is pelagic. The transparent, colourless body consists of only a few
(eighteen to twenty) segments; the parapodia are as long as the body is
wide, and carry no chaetae; each is bilobed, and fringed with a membrane;
each of these lobes contains a yellow rosette-shaped photogenic organ. The
only chaetae present in the worm are on the "head." The prostomium is
hammer-shaped, and appears to carry a pair of short filaments ventrally
(Fig. 167, _x_) each with a single chaeta within it; and a longer filament
laterally (_y_), supported by a long, very delicate chaeta. The mouth is
behind these, and they probably are the first pair of parapodia which have
shifted forwards. _T. onisciformis_ Eschscholtz is not unfrequently
obtained off our shores in the tow-net.
[Illustration: FIG. 167.—_Tomopteris rolasi_ Grf. × 10. From Guinea Isles.
_pr_, Hammer-shaped prostomium; _x_, first chaetigerous process; _y_,
second chaetigerous process; _c_, rosette (photogenic) organ on first two
parapodia; _b_, similar organ in the lobes of the following parapodia; _d_,
pigment spots; _f_, parapodium. (From Greef.)]
FAM. 6. _Nereidae (Lycoridae)._—This family contains a very large number of
species, differing from one another in small and not readily recognisable
characters, such as the relative lengths {316}of the various processes of
the head, of the lobes of the feet, the arrangement of the "paragnaths"
(see Fig. 125, _d_) and so forth. The general features of the family have
been already described. The genus _Nereis_ is represented by six fairly
common species on our coast, which are almost world-wide in distribution.
_N. diversicolor_ Müll. is about 3 to 4 inches in length, of a general
fleshy-red colour, though tending in some cases to yellowish-brown or even
greenish. It may be distinguished by two diverging brown bands, which start
on the peristomium and pass backward one along each side of the body for
several segments. The prostomium is broader than it is long. The worm
burrows in mud or sand, all round our coast between tide-marks. It has a
very wide distribution, being met with on this side of the Atlantic, and
off the coast of Greenland, and off Japan. It is even found in brackish
water at Bembridge, Isle of Wight.
_N. cultrifera_ Gr. is green or greenish-grey, with a series of small
rectangular light spots along the mid-dorsal surface, and oblique light
lines at the sides of each segment. Posteriorly the greenish pigment
becomes less and less till the hinder segments are flesh-coloured. The
prostomium is as long as it is broad. This species attains a length of 6
inches. Southern coasts: locally known as "Red Cat."
_N. dumerilii_ Aud. and Edw. is rather smaller and narrower than the two
preceding species; it is reddish-violet in colour, marked with darker
transverse lines in each segment. It is readily recognised by the two dark
brown spots on the upper surface of the base of the notopodium in most of
the segments, and by the great length of the peristomial cirri, the longest
of which reaches the fifteenth segment. It is sometimes found enclosed in a
cocoon-like tube of hardened grey mucus, more or less covered with foreign
particles, such as sand grains. Atlantic, Mediterranean, Japan.
[Illustration: FIG. 168.—_Nereis cultrifera_ Gr. × 6. Head with buccal
region everted, to show the arrangement of the jaws. (From Ehlers.) Cf. _N.
diversicolor_, Fig. 125, p. 248.]
_N. pelagica_ L. is red-brown or bronze in colour, and is generally larger
than the other species, from which it is distinguished by being widest
about the middle of the body (see Fig. 122, p. 246); whilst in the
preceding species the greatest {317}breadth occurs at the segments
immediately following the head. Further, the palps are long, the
peristomium is twice as long as the next segment, and the back of the worm
is strongly arched. At all depths on rocky and stony ground. Northern
coasts.
_N._ (_Nereilepas_) _fucata_ Sav. lives in the topmost whorls of empty
whelk shells and in those occupied by hermit crabs. The ground colour is
tile-red, with two milk-white bands along the dorsal surface. The dorsal
lobe of the foot is slightly foliaceous, glandular, and vascular.
_N._ (_Alitta_) _virens_ Sars. is a giant amongst Polychaetes, reaching a
length of 18 inches. Its name suggests its colour; it is very plentiful at
certain times at St. Andrews, and between tide-marks along the shore of the
Mersey estuary, as well as elsewhere. It forms a burrow in the clay, etc.,
of the shore, and lines it with mucus, which is abundantly secreted by the
great foliaceous lobes of the parapodia. These great leaf-like lobes of the
foot recall the modification which the foot of many species of _Nereis_
undergoes in transformation into _Heteronereis_: they are so greatly
developed that, at first sight, the worm might be mistaken for a large
_Phyllodoce_. The worm is known as the "Creeper," and is much esteemed as
bait on some parts of our coast.
[Illustration: FIG. 169.—Parapodium of _N. virens_ Sars. ×4. _a_,
Notopodial cirrus; _b_, notopodium; _c_, neuropodium; _d_, neuropodial
cirrus; _l_, foliaceous appendage. (From Ehlers.)]
FAM. 7. _Nephthydidae._—The elongated body is quadrangular in section, the
dorsal and ventral surfaces being almost flat. (For head see p. 262, and
for parapodium, p. 264.) The two lobes of the parapodium are widely
separated, and each is fringed with a membrane, while a sickle-shaped
"gill" hangs down from the under surface of the notopodium. The pharynx is
enormous. Of the genus _Nephthys_ two species, called the "Lurg" or "White
Cat" by fishermen, occur on our coasts. Their active movements and
beautiful mother-of-pearl tint are characteristic. _N. hombergii_ Aud. and
Edw. occurs on the shore, and down to 20 fathoms; it is 3 or 4 inches long,
and may be found burrowing in the sand; the chaetae exceed in length those
of _N. caeca_ {318}Fabricius, which occurs less frequently and in deeper
water, and is larger than the preceding. Both are Atlantic forms.
FAM. 8. _Amphinomidae._—The body in this family is either vermiform, as in
_Eurythoe_, or oval and flattened, as in _Euphrosyne_ and _Spinther_. The
head carries a peculiar sense organ, the "caruncle," consisting of a smooth
axis with the sides folded so as to look like a number of lamellae. The
parapodia carry gills. Most of the Amphinomids are tropical and Southern
forms.
_Eurythoe borealis_ Oerst. measuring 6 inches, occurs all round the British
area, from the Shetlands, where it occurs in deep water, to the Channel
Isles, where it lives on shore, under stones, etc. (For parapodium of
_Amphinome_ see p. 264.)
_Euphrosyne._—The body is short, oval, and flattened. The parapodia are not
distinct processes, but the chaetae extend from each side of each segment
nearly to the middle dorsal line, and are absent ventrally (Fig. 137, C, p.
265). The dorsal and ventral cirri are more or less filiform, and there is
an intermediate similar process on the back (? = lip of chaetigerous sac).
Amongst the chaetae are a number of curious branched processes—usually
called "gills."[380] The presence of these and of the chaetae give the
upper surface of the body a fluffy appearance. _E. foliosa_ Aud. and Edw.
is fairly common under stones on our southern shores. It is about an inch
in length and is of a cinnamon-red colour.
FAM. 9. _Eunicidae._—The elongated body is provided with parapodial gills
in more or fewer segments (except in _Lumbriconereis_). The "gills" may be
cirriform (_Hyalinoecia_), pectinate (_Eunice_, _Onuphis_), or more complex
(_Diopatra_). The notopodium is represented by a lobe (usually called
"cirrus") into which an aciculum projects; in some cases it even contains a
few chaetae; most of the neuropodial chaetae are jointed (Fig. 138, F). The
prostomial tentacles vary in number; they may be three or five, or five and
two short "frontal palps," or they may be absent. Peristomial cirri are
absent, though in _Eunice_, _Diopatra_, and _Onuphis_ "nuchal cirri" are
present on the dorsal surface of the second segment (Fig. 134, D). One of
the most characteristic features in the anatomy of the Eunicids is the
peculiar jaw apparatus (see p. 270). The majority of the genera form
permanent tubes of parchment-like consistency, {319}which may be further
strengthened by the addition of grains of sand, small pebbles, etc.; the
tubes may be branched.
_Eunice_ has five tentacles, two great palps, and a pair of nuchal cirri;
the gills are pectinate, and there are four anal cirri. _E. harassii_ Aud.
and Edw. is about 8 inches long. It is reddish-brown, with white spots down
the back, one to each segment, and others at the sides. The gills begin at
the sixth segment, and when fully developed have eleven branches. The
dorsal cirrus is not longer than the gill. _E. philocorallia_ Buch.[381]
forms its tube amongst the branches of _Lophohelia prolifera_, in 200
fathoms, off the west coast of Ireland.
_Marphysa_ resembles _Eunice_, but has no nuchal cirri. _M. sanguinea_
Mont. is a fine bronze colour, with bright red gills, which commence on the
twentieth segment, and have only four or five branches. The worm, which
measures 12 to 18 inches, and is as thick as one's finger, hides in clefts
in rocks and under stones below low water. Mediterranean. It is known as
"Rockworm" in the Channel Islands.
_Hyalinoecia_ Mgrn., in addition to the five prostomial tentacles and
palps, possesses a pair of small "frontal palps" arising from the anterior
border of the prostomium; there are no nuchal cirri, and the gills are
simple filiform processes. _H. tubicola_ Müll., about 3 inches long, is
yellowish-brown, and forms a transparent, parchment-like tube. Atlantic and
Mediterranean.
[Illustration: FIG. 170.—_Ophryotrocha puerilis_ Clap., Metsch. × 25. _ci_,
Bands of cilia; _cp_, ciliated pit (nuchal organ); _J_, jaws. (From
Korschelt.)]