found in South America and the West Indies, in tropical Africa, in India,
and in some of the islands of the Malay Archipelago. But it also occurs
(_Sparganophilus_ and _Criodrilus_) in Europe and in America. A good many
of the genera are aquatic. This is the case with the two already mentioned;
the genera _Glyphidrilus_ and _Annadrilus_ of the Malay Archipelago can
live in water. The family is easily definable if we take the more typical
forms; but at one end of the series it fades into the next family, that of
the Lumbricidae. _Criodrilus_ is one of the genera which is difficult to
place. As is the case with many Geoscolicidae, _Criodrilus_ has ornamented
chaetae not only upon the clitellum, but upon the other segments of the
body. This character was until recently unknown among the Lumbricidae; it
has been lately found in _Allolobophora moebii_ and _A. lonnbergi_. The
absence of spermathecae characterises _Criodrilus_ as well as other
Geoscolicidae; but here again the character is not by any means
distinctive, for in _Allolobophora constricta_ there is the same absence of
these organs. In _Criodrilus_ the male pores are upon segment 15, as in the
Lumbricidae, but a species of _Kynotus_, which is certainly a Geoscolecid,
has these pores upon precisely the same segment. The only point in which
_Criodrilus_ is definitely a Geoscolecid, or rather not a Lumbricid, is in
the forward position of the clitellum, which begins upon the fifteenth
segment, far earlier than it does {387}in any undoubted Lumbricid. The
peculiar elongated cocoon, which much resembles that of _Sparganophilus_,
is another character which favours its Geoscolecine affinities. Dr.
Michaelsen has proposed to unite _Criodrilus_ and _Alma_ into a family
intermediate between the Geoscolicidae and the Lumbricidae.
[Illustration: FIG. 198.—_Alma millsoni_ F. E. B. × 1.]
Perhaps the most remarkable genus in the whole family is _Alma_. One
species lives in the Nile mud; another is the "Yoruba worm" of West Africa,
whose habits have been described by Mr. Millson. The most marked character
of this genus, apart from the branchiae (see p. 352) which apparently may
be present or absent according to the species, is in the two enormous
processes of the body-wall, which are illustrated in Fig. 198. These
contain the sperm-ducts, which, however, open some way in front of the free
end; they are provided on the ventral surface with a series of sucker-like
structures and with peculiar chaetae. Another interesting genus is
_Pontoscolex_, which was originally described from the sea-shore of Jamaica
by Schmarda; there are only two species which are certainly characterised,
though a variety from the Hawaian Islands may be a "good" species. It
possesses the remarkable peculiarity that the chaetae at the end of the
body are disposed in a perfectly irregular fashion, which earned for it the
name of brush-tail at the hands of its discoverer, Fritz {388}Müller. This
worm, which is universal, or nearly so, in its range, doubtless having been
transferred accidentally from country to country, invariably shows a light
spot not far from the tail; when this is examined with the microscope it is
seen that the chaetae are here absent or very small, and that the muscular
structure of the body-wall is slightly different; it was thought that this
spot was a zone of growth where fresh segments could be added after the
fashion of some of the aquatic Oligochaeta, to which, it may be remarked,
the present genus shows a curious point of likeness in the bifid character
of the chaetae. It seems, however, that there are really no grounds for the
supposition, and it is possible that we have here a "weak" spot, such as
that in the foot of certain land snails, which readily gives way when the
worm is picked up by a bird, and allows the "better half" of the creature
to escape. The Bermudian genus _Onychochaeta_ offers a very strange
peculiarity in that the chaetae on the hinder segments of the body are
enormously larger than those in front, and end in strong hooks; it seems
likely that their function is to maintain a tight hold of the ground while
the worm is leaning out of its burrow, as every one has seen the common
earthworms of this country do. _Onychochaeta_ has the same irregular
arrangement of the chaetae upon the greater part of the body, as has
_Pontoscolex_. This family, like so many others, has its giants and its
dwarfs. At one extreme is the great _Antaeus_ of South America, several
feet in length; at the other the inch-long _Ilyogenia_ of Africa. The
American _Urobenus_ has a pair of intestinal caeca like those of
_Perichaeta_, and placed in the same segment.
FAM. 14.9 _Lumbricidae._[448]—This family is to be distinguished by the
following assemblage of characters.
The male pores are usually upon segment 15, and never behind that segment;
the clitellum commences some way behind the male pores. The gizzard, which
is invariably single, is equally invariably at the end of the oesophagus.
There are three pairs of calciferous glands. The nephridia are always
paired. The spermathecae never have a diverticulum.
This family only contains three well-known genera, viz. {389}_Lumbricus_,
_Allolobophora_, and _Allurus_. The American _Bimastos_ may be distinct.
_Tetragonurus_, not allowed by some, is at present unknown except as
regards external characters; it differs from the other Lumbricidae in the
fact that the male pores are upon the twelfth segment. In _Allurus_ they
are upon segment 13, and in the remaining genera upon the fifteenth.
_Lumbricus_ is to be distinguished from _Allolobophora_ by its prostomium,
which is continued by grooves on to the buccal segment, so as to cut the
latter in half. It has also median sperm reservoirs, as well as the paired
sperm sacs which are alone present in _Allolobophora_.
[Illustration: FIG. 199.—_Allolobophora chlorotica_ Savigny. × 4. The
clitellar segments are marked in Roman numerals. _t.p_, Tubercula
pubertatis; ♂, male pore.]
[Illustration: FIG. 200.—_Allolobophora putris_ Vejd. × 5. Lettering as in
Fig. 199. The black dots represent the chaetae in both figures.]
This is the only family of earthworms which, so far as is known, can brave
the ice and snow, and what is still more {390}difficult to understand, the
perpetually frozen undersoil of the Arctic regions. Eisen has described a
number of species from Spitzbergen, and Colonel Feilden recently sent me an
example of _Allolobophora octoedra_ from Kolguiev, where Mr. Trevor-Battye
also saw another specimen. The family is characteristic of the Nearctic and
Palaearctic regions, and though found beyond them, is probably elsewhere an
accidental importation (see p. 371). There are at least fifteen species of
this family found in England and Ireland, and probably more will be
identified.
There does not exist at present any comprehensive account of the British
species of earthworms, though all of them are included in Dr. Rosa's recent
revision of the family. Most of the British forms belong to the genus
_Allolobophora_, which may be divided into two series according to whether
the chaetae are quite close together or further apart. The extent of the
clitellum and the position of those swollen eminences which appear earlier
than the clitellum, and are known as tubercula pubertatis, offer further
characters. In the following tables, extracted from those of Rosa, the
known British species of this genus are grouped according to these three
characters. With the help of these tables and Figs. 199 and 200, any of the
species ought to be easily identified.
WITH CHAETAE DISTANT.[449]
---------------------+--+--+--+--+--+--+--+--+--+--+--+--+
|25|26|27|28|29|30|31|32|33|34|35|36|
---------------------+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | |
_A. putris_ |..|——|——|——|——|——|——|..| | | | |
| | | |..|——|——| | | | | | |
| | | | | | | | | | | | |
_A. constricta_ | |——|——|——|——|——|——| | | | | |
| | | | | | | | | | | | |
_A. veneta_ | |..|——|——|——|——|——|——|——| | | |
| | | | | |——|——| | | | | |
| | | | | | | | | | | | |
_A. octoedra_ | | | | |——|——| |——|——|..| | |
| | | | | | |——|——|——| | | |
| | | | | | | | | | | | |
_A. cyanea_ (subsp. | | | | | |——|——|——|——|——|——| |
_profuga_) | | | | | | |——|——|——|——| | |
| | | | | | | | | | | | |
_A. rubida_ | | | | | |——|——|——|——|——|——| |
| | | | | |——|——|——|——|——|——| |
| | | | | | | | | | | | |
_A. mammalis_ | | | | | | |——|——|——|——|——|——|
| | | | | | | | |——|——| | |
---------------------+--+--+--+--+--+--+--+--+--+--+--+--+
{391}WITH CHAETAE PAIRED.
-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|24|25|26|27|28|29|30|31|32|33|34|35|36|37|38|39|
-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | | | |
_A. rosea_ |..|——|——|——|——|——|——|——|——| | | | | | | |
| | | | | |——|——|..| | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. foetida_ |..|..|——|——|——|——|——|——|——| | | | | | | |
| | | | |——|——|——|..| | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. eiseni_ |..|——|——|——|——|——|——|——|——| | | | | | | |
| | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | |
_A. caliginosa_ | | | |..|——|——|——|——|——|——|——|..| | | | |
| | | | | | | |——|..|——| | | | | | |
| | | | | | | | | | | | | | | | |
_A. terrestris_ | | | |..|——|——|——|——|——|——|——|——| | | | |
| | | | | | | | |——|——|——| | | | | |
| | | | | | | | | | | | | | | | |
_A. chlorotica_ | | | | |..|——|——|——|——|——|——|——|——|——| | |
| | | | | | | |——| |——| |——| | | | |
| | | | | | | | | | | | | | | | |
_A. georgii_ | | | | |..|——|——|——|——|——|——|——| | | | |
| | | | | | | |——| |——| | | | | | |
-----------------+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
There are of course other points by which the different species can be
distinguished. Colour in a few cases enables a species to be named at once
without any further aid. One of the most striking of these cases is the
Brandling, so common upon dunghills, and so dear to some anglers. This worm
is ringed with brownish purple upon a yellowish ground. The greenish _A.
chlorotica_ is often found under stones, and curls itself round into nearly
a complete circle when disturbed. _A. cyanea_, of a bluish grey colour, is
one of the earthworms very commonly met with in the early morning in London
and the neighbourhood. More generally, however, the colour is of a paler or
darker red, verging towards and attaining brown, or even blackish brown;
and is so variable that nothing in the way of identification can be
attempted from the colour alone, even with the most elaborate description.
_Lumbricus_, as already mentioned, is distinguished from all
_Allolobophora_ except _A. eiseni_, by the complete dovetailing of the
prostomium into the first segment. There are five species in this country
which may be thus distinguished by the position of the tubercula
pubertatis. The most familiar species is the common _L. rubellus_.
Tubercula pubertatis are on
_L. rubellus_ Hoff. 28, 29, 30, 31
_L. castaneus_ Sav. 29, 30, 31, 32
_L. herculeus_ Sav. 33, 34, 35, 36
_L. papillosus_ Friend 34, 35, 36, 37
_L. festivus_ Sav. 35, 36, 37, 38.
{392}CHAPTER XIV
HIRUDINEA (LEECHES)
INTRODUCTION—ANATOMY—REPRODUCTION—CLASSIFICATION—RHYNCHOBDELLAE AND
GNATHOBDELLAE
"The external appearance of the Hirudinea," remarks Professor
Vaillant,[450] "permits us, save for rare exceptions, to recognise at once
the animals which belong to that group." The leeches are distinguished as a
rule by the possession of two suckers, one at each end of the body; their
general shape usually differs from that of other Annelids by its oval
contour and its dorso-ventral flattening. _Cyclicobdella lumbricoides_ of
Grube, which Blanchard has stated to be the same species as _Nephelis
tergestina_, has, however, almost the form of an earthworm by reason of its
cylindrical shape and the inconspicuousness of the suckers, while
_Lumbricobdella_ also resembles an earthworm and has no posterior sucker at
all.[451] The Oligochaet family Discodrilidae (see p. 376) agree with the
leeches in their parasitism, in their general shape, in the presence of two
suckers, and, furthermore, in the existence of jaws, which are found in no
other Oligochaet, but occur in a large number of the Hirudinea. These
facts, indeed, though not perhaps important by themselves, are indications
of the really close resemblance of the Hirudinea to the Oligochaeta, a
group which they approach not merely in such habits as the formation of a
cocoon in which the eggs are enclosed, but also in many important points of
internal and external structure. Indeed, the fundamental differences
between the two groups are not numerous, and are not of such importance as
has been given them by some writers.
Leeches are to be found in most parts of the world, in {393}situations
which are sufficiently damp for their comfort. But we do not at present
possess enough knowledge to state much as to the facts of their
distribution. The structure of leeches is not so well known as is that of
the earthworms; for they have not been to so great an extent collected in
extra-European countries. It would even be desirable to ascertain precisely
the species which inhabit these islands, the most recent enumeration (1865)
being that contained in the British Museum Catalogue of non-parasitical
worms by the late Dr. George Johnston. For Italy this has been lately done
by Dr. Blanchard, and a good many of the species are common to the two
countries. Johnston enumerates altogether (after subtracting what are
probably synonyms) twenty-one species, distributed among the genera
_Branchellion_, _Pontobdella_, _Piscicola_, _Nephelis_, _Trocheta_,
_Haemopis_, _Hirudo_, and _Glossiphonia_ (= _Clepsine_), which number will
be possibly still further reduced. The first two genera are marine, the
remainder being fresh water or terrestrial; _Trocheta_ has been probably
introduced.
[Illustration: FIG. 201.—Anterior end of _Macrobdella sestertia_, to show
eyes and sense bodies. (After Whitman.)]
The use of _Hirudo medicinalis_ is well known to many of us from personal
experience. So extensively was this leech formerly made use of that it is
now far from being a common species either in this country or in France.
Those who desire full information as to Hirudiniculture should consult the
work of Dr. Ebrard, published in 1857.[452] The former extensive use of the
leech has led to the transfer of its name to the doctor who employs it, the
authors of the sixteenth century constantly terming a physician a leech; it
has been suggested, however, that the term was applied rather by way of
analogy. The useful blood-sucking habits of the medicinal leech have been
wrongly attributed to the innocent horseleech (_Aulastomum_)—innocent, that
is to say, of the blood of Vertebrates, for it has been described as "a
cruel and greedy worm," engulfing earthworms and even smaller specimens of
its own species. {394}"Horsleches," said an old writer, "are wholesome to
drawe foorthe foule blood, if thei are put into a hollowe rede, and one of
their endes cutte of, whereby the blood maie run forthe." But it is clearly
not easy for a creature destitute of jaws and teeth to bite, and the
similarity of general aspect has doubtless led to a confusion with the
savagely biting medicinal leech.
[Illustration: FIG. 202.—Sense body of _Macrobdella sestertia_. (After
Whitman.) _ep_, Epidermis; _s_, clear cells. Highly magnified.]
[Illustration: FIG. 203.—Section through eye of _Haemadipsa japonica_.
(After Whitman.) _ep_, Epidermis; _n_, nerve. Highly magnified.]
The Hirudinea are all distinctly segmented animals, but the segmentation
differs from that of the Oligochaeta in two points. In the first place the
number of segments is much smaller in a leech than in an Oligochaete,
although the difference does not appear great at first sight.
A leech's body may seem to be composed of seventy, eighty, or one hundred
segments, a number quite as great as is found, for example, in the genus
_Perichaeta_ among the earthworms; but the apparent number of segments in
the leech is produced by a very marked annulation of the real segments; and
this is indeed the second point of difference referred to above. But there
are earthworms which show frequently a secondary annulation,—secondary
because it appears late and does not affect other organs {395}of the body.
A segment of an earthworm may indeed have five or six distinct annulations,
but it will be bounded internally by two septa, and will bear only one set
of chaetae externally. In the leech external clues to the definition of a
segment were until recently wanting. They appear now to have been found in
the sensory organs of the skin (Figs. 201 and 202), which are, according to
Whitman,[453] disposed in a perfectly metameric fashion. Judged by this,
and also by the nephridia and nerve-ganglia, the number of segments in a
leech does not appear to exceed twenty-six, independently of the sucker,
which may represent a few fused segments, seven (in the medicinal leech)
according to Leuckart.
The eyes, which are so useful in the systematic arrangement of the group,
appear to have been evolved from these sensory organs by a further
exaggeration of their peculiarities. Figs. 202 and 203 show this point
convincingly. The segmental sense organ is shown in Fig. 202; to the
outside of certain sense cells, below which are a mass of ganglion cells,
are certain peculiar transparent cells very similar to the clear cells
found in the interior of the eye (Fig. 203). The segmental disposal of the
sensory bodies and of the eyes is shown in Fig. 201.
[Illustration: FIG. 204.—_Branchellion torpedinis._ (From the "Règne
Animal.") ×1.]
Some Hirudinea are furnished with external branchiae; this is the case with
_Branchellion_, in which genus the branchiae (Fig. 204) have an arborescent
form; in _Cystibranchus_ there are a series of paired simple vesicles which
take the place of these more complicated respiratory organs of
_Branchellion_. The Hirudinea do not, save for one exception
(_Acanthobdella_), possess chaetae; but it must be borne in mind that the
Discodrilidae and the genus _Anachaeta_ among the Oligochaeta are in the
same condition. In _Acanthobdella_[454] there are two pairs of chaetae upon
each side of the anterior five segments of the body. According to the
figure which Grube, the {396}original describer of the genus, gives of
these chaetae, the part implanted in the body is straight, while the part
extending freely beyond the body is sharply hooked.
The body of the leeches is never ciliated externally; there is, as in the
higher Oligochaeta, a cuticle secreted by the underlying epidermis. The
Hirudinea have, like the Oligochaeta, a clitellum which, as in some of the
lower members of that group, is limited to a very few segments in the
immediate neighbourhood of the generative openings. It occupies in _Hirudo_
segments 9, 10, and 11. The epidermis gives rise to many unicellular glands
which either remain _in situ_ or get moved to a deeper position. In this
the leeches exactly resemble the earthworms. There is generally a great
deal of connective tissue in the body-wall. The muscles consist of
circular, longitudinal, and radial series. The individual fibres have the
same structure as those of the Oligochaeta, consisting of a soft and
undifferentiated core, round which is a radially-striated sheath of
contractile substance.
ALIMENTARY CANAL.—The leeches are divided into the Rhynchobdellae, which
have a proboscis but no jaws, and the Gnathobdellae, which possess a series
of jaws but have no proboscis. But the division is not a hard and fast one,
for we have Whitman's genus _Leptostoma_, which should belong to the jawed
division, but which has quite rudimentary jaws without the sharp denticles
so characteristic of _Hirudo_. The pharynx is furnished with salivary
glands. The oesophagus is followed by the proventriculus, which has a
varying number of pairs of caeca; then comes the intestine and the rectum.
The anus is, as a general rule, placed dorsally to the sucker, but there
are a few rare exceptions where the anus is within the sucker. The caeca
are totally absent in _Trocheta_.
VASCULAR SYSTEM.—As will be seen from Fig. 205, the vascular system of the
Hirudinea is constructed on a plan which closely resembles that of the
Oligochaeta. The diagram represents _Glossiphonia_, one of the
Rhynchobdellae, the group which comes nearer to the Oligochaeta in many
particulars than the Gnathobdellae. We can recognise a dorsal and a ventral
vessel, which are united in the anterior part of the body by three
perioesophageal rings, the elongation of which, particularly of the last
pair (_v_), from before backwards is very marked. In the region of the
sucker the dorsal and ventral vessels are united by fourteen shorter loops,
the number of which has an interesting {397}relation to the number of
segments out of which this portion of the body is possibly formed. It will
be observed also that the dorsal vessel is double in this region, a
condition which obtains along its whole length in _Branchellion_—a
repetition of what has been described in more than one species of
Oligochaete. In the region of the last pair of digestive caeca the dorsal
vessel has appended to it copious sinuses which embrace the intestine and
supply its walls with blood. In _Hirudo_ there are only a pair of lateral
vessels, and neither dorsal nor ventral vessels; in this leech and in the
Gnathobdellae generally there are intra-epidermic capillaries, a fact first
discovered by Professor Lankester, and now known to occur also in the
Oligochaeta.
The development of the blood-vessels shows that they have no relation
whatever to the coelom, in spite of their subsequent connexion with it. The
two longitudinal stems of _Hirudo_ arise as cavities in the somatic layer
of the mesoblast after the formation of the coelom. In _Nephelis_, but not
in _Hirudo_, Dr. Bürger thinks that there is some reason for regarding the
vascular system as the remains of the primitive segmentation-cavity of the
embryo, an opinion which is held in respect of the vascular system of many
other animals.
[Illustration: FIG. 205.—_Glossiphonia marginata_, vascular and alimentary
system. ×4. (After Oka.) _a_, Dorsal vessel; _g_, intestinal caecum; _v_,
one of the hearts.]
BODY-CAVITY.—One of the most marked differences between the leeches and the
Oligochaeta is in the body-cavity. In the latter there are a series of
cavities corresponding to the segments, which are bounded in front and
behind by the intersegmental septa, and in which all the viscera lie. In
leeches such an arrangement is not recognisable save in _Acanthobdella_,
where Kowalevsky[455] has quite recently described a typical coelom divided
by septa into twenty segments. In transverse sections the body of other
leeches appears at first sight to be solid, owing to the growth of the
muscles and connective tissue. A more careful study, however, has revealed
the {398}fact that there are considerable remains of the body-cavity or
coelom which form a complicated system of spaces and channels. What has
happened, in fact, in the leech is that the coelom has become gradually and
partially obliterated by proliferation of the cells in the interior of the
body, a process of obliteration which has already commenced in the
Oligochaeta. In many of the latter, some of the principal blood-vessels
have become surrounded by a space cut off from the general body-cavity,
while in the majority a special cavity surrounds the testes and the funnels
of the sperm-ducts. This process of the formation of separate cavities for
the inclusion of the several viscera culminates in the leeches with the
marked obliteration of the greater part of the coelom. This has become so
much reduced to the condition of narrow tubes that there has been a
tendency to confuse it with the vascular system, more especially perhaps in
those forms in which the blood is tinged with haemoglobin, and in which
there is a connexion between the two systems of spaces. This confusion has
been further increased by the plan of injecting the vascular system, a
method of investigation which must be employed with great care in
delicately-organised creatures whose tissues can be easily ruptured, and so
lead to a flow of the injecting fluid into places and in directions
impossible during life.
[Illustration: FIG. 206.—Coelomic canals of _Glossiphonia complanata_. ×
10. (After Oka.) _a_, Dorsal canal containing dorsal blood-vessel; _b_,
ventral canal containing ventral blood-vessel; _l_, lateral canal; _n_,
nerve-cord.]
In transverse sections of leeches it may be seen in successful preparations
that the various organs of the body are enclosed in spaces. The funnels of
the nephridia open into lacunae which could hardly in any case be regarded
as blood spaces, while the blood-vessels themselves with their muscular
walls cannot be confounded, at least in the case of the larger trunks, with
the spaces not having muscular walls which surround them. Furthermore, it
will be pointed out immediately that the reproductive organs are produced
on the walls of spaces which are the commencement in the embryo of the
reduced coelom of the adult worm. These spaces therefore conform in every
particular to the general {399}conditions which have been laid down about
the characters of a true coelom. As to the complexity of this system,
attention may be directed, to the accompanying diagram (Fig. 206) of the
coelom of a segment of _Glossiphonia_, which has been lately worked out in
detail.[456] It will be observed that there are four main longitudinal
sinuses which are connected by a complicated system of transverse tubes and
spaces. In the anterior part of the body, before the point where the
intestinal caeca arise, the dorsal and ventral lacunae fuse to form one
larger so-called median lacuna. The cavity of this is interrupted, in
correspondence with the segmentation of the body, by septa exactly
comparable to those of Oligochaeta; but the septa in _Glossiphonia_ are not
present at every segment. So far our account of the coelom is chiefly
derived from the genus _Glossiphonia_. In _Hirudo_, which is an example of
the Gnathobdellae, the coelom is still further reduced; the lateral sinuses
in them appear to be absent. But on the other hand there is formed a series
of cavities in a form of connective tissue which has been termed botryoidal
tissue. The cells of this tissue become hollowed out, and form channels
which are in communication on the one hand with the remains of the coelom
and on the other with the vascular system. This system has certain
analogies with the lymphatic vessels of Vertebrates, and, like them, is an
intermediary between the body-cavity and the blood. Originally, however,
these botryoidal vessels have nothing whatever to do with either the
vascular or the coelomic system; their connexion with both is a purely
secondary affair, and only appears, comparatively speaking, late in life.
The development of the spaces here spoken of collectively as coelom
confirms this interpretation of their nature. In the embryos of _Hirudo_,
_Aulastomum_, and _Nephelis_ there is a ventral space,[457] which includes
the nerve-cord. Into this open a series of paired and segmentally-disposed
lateral cavities, a pair to each segment. The ventral cavity itself is
formed by fusion of the median parts of the lateral cavities. There is here
clear evidence of a coelom, developed on a plan fundamentally identical
with that of the Oligochaeta in that it is formed as a paired series of
chambers corresponding to the segmentation of the body.
NEPHRIDIA.—The "segmental organs" or nephridia are seen in {400}their
simplest form in such a type as _Glossiphonia_—the Rhynchobdellae, to which
this genus belongs, being indeed in most particulars less specialised than
the Gnathobdellae. Here we have a distinct funnel opening freely into the
median or ventral coelomic space, which is immediately followed by a
rounded swelling termed by Oka[458] the capsule; this is filled with cells,
in the interstices of which the ductules are situated and meander. There is
in this capsule a very strong likeness to the glandular brownish swelling
which immediately follows the funnel in the nephridia of certain of the
aquatic Oligochaeta, for example the Naids, where, as Vejdovsky has shown,
there is a similar "rete mirabile" of the nephridial duct. After the
capsule is a single row of cells which are disposed in a complicated coil.
These cells are perforated by the duct, which is thus, as in the
Oligochaeta, intracellular. In the first set of cells the duct is single,
and gives off numerous branchlets into the interior of each cell, a
condition which has also been observed in many Oligochaeta. Afterwards the
cells are perforated by two, or even three, main ducts, for the duct
returns upon itself and traverses the row of cells more than once; there
are also branchlets developed from one or other of the main ducts. The
terminal part of the nephridium is a short invagination from the exterior,
which is lined by cells. There is clearly a close resemblance here with the
nephridium of an Oligochaete. The nephridium, however, except for the
funnel and the narrow tube immediately following it, does not appear to be
ciliated.
[Illustration: FIG. 207.—Nephridium of _Hirudo medicinalis_. × 10. (After
Bourne.) _f_, Funnel; _v_, distal vesicle.]
There is, however, some difference of opinion as to the portions of the
nephridium where there are two ducts in a single cell. Bourne[459] thinks
that where there are two ducts there are two {401}cells, one lying inside
the other, and that there is sometimes also a telescoping of cell within
cell where the duct is single. In _Hirudo_ the same writer has described
the nephridial funnel, which has lost the simple character of that of
_Glossiphonia_. The funnel is represented by a cabbage-head-like mass (Fig.
207, _f_) of ciliated cells with no single definite outlet to the exterior
as in _Glossiphonia_. It appears to be an organ which has lost its proper
function—a degeneration of the funnel being, as a matter of fact, not
unknown in the Oligochaeta, where it may be carried to absolute extinction
(_Chaetogaster_). In _Branchellion_ and _Pontobdella_ the simple metameric
arrangement of the nephridia is to some extent lost, owing to the formation
of a network continuous from segment to segment. It will be borne in mind
that the Oligochaeta are the only other Chaetopods in which such a
nephridial network has been stated to exist.
MALE REPRODUCTIVE ORGANS.—In _Hirudo medicinalis_ there are nine,
occasionally ten, pairs of testes, which are round white bodies arranged
segmentally, _i.e._ a pair to each segment. From each arises a slender,
somewhat sinuous tube, which enters the common collecting tube of its own
side; each of these is much contorted at the upper end, the coiled portion
being termed the epididymis. From this they enter a muscular penis which
can be protruded. This is the arrangement met with in all leeches, save for
the fact that the penis is absent in some; in _Glossiphonia_ (see Fig. 208)
this is the case. The number of pairs of testes also varies; and in
_Nephelis_ they are no longer arranged metamerically.
[Illustration: FIG. 208.—Nervous system and reproductive organs of
_Glossiphonia plana_. × 2. (After Whitman.) _gl_, Prostate glands; _n_,
nerve-cord; _o_, ovary; _t_, testes.]
The testes arise as local proliferations of the epithelium of the lateral
coelomic cavities, but from the somatic wall, not from the splanchnic, as
in the case of the ovaries to be described later. A portion of the tissue
which is to form the testis grows out laterally into a thin cord, which is
to become the vas efferens of that {402}testis. Later both testis and duct
become hollowed out with a common cavity. The main portion of the vas
deferens of each side, as well as the terminal copulatory apparatus, is an
ingrowth from the epidermis which meets the downgrowths from the testes.
That there are considerable differences between the reproductive organs of
the leeches and those of Oligochaeta will be apparent from the above
description. There are, however, to begin with, certain obvious
similarities. In the first place, the origin of the reproductive glands is
identical; in both groups also the efferent ducts consist of two
portions—an invagination from the outside, and a formation of the proximal
part of the ducts near to the glands. In _Moniligaster_,
where—exceptionally—the testes develop on the posterior wall of their
segment in close contact with the funnels of the sperm-ducts, there is no
very hard and fast line to be observed between the tissues of the two. The
hollowing out of the testis in the leech, and the continuity of the cavity
thus formed with the duct, is a specialty of the leeches not found among
the Oligochaeta.
Like many Oligochaeta, the leeches may form spermatophores in which the
sperm is packed for its conveyance from one individual to another.
_Glossiphonia_ (_Clepsine_) _plana_, where the structure in question has
been elaborately described by Whitman,[460] may be selected as an example.
The spermatophore (Fig. 209) is about 8 mm. long, and is clearly formed of
two halves, each of which is formed separately in one ductus ejaculatorius,
the soldering together being effected in the common part of the male ducts,
where also a basal portion with a single lumen is added. The spermatophore
has a double wall. It is deposited not in the neighbourhood of the
generative pores, but upon the back; and Whitman has discovered the
extraordinary fact that the spermatozoa find their way through the
body-wall of the leech into the interior of its body, where fertilisation
presumably occurs.
[Illustration: FIG. 209.—Spermatophore of _Glossiphonia plana_. × 22.
(After Whitman.)]
{403}FEMALE REPRODUCTIVE ORGANS.—The ovaries of the Hirudinea appear to
differ from those of the Oligochaeta in that the ovaries are continuous
with their ducts. In _Hirudo_, however, the real ovary of each side
consists of masses of germinal tissue lying freely within a sac which
communicates with a duct; the two ducts unite to form a much convoluted
tube which opens into a thick-walled vagina, itself opening again on to the
exterior by a median unpaired opening on the seventh segment. The muscular
vagina is not always present.
The median unpaired female aperture offers now no particular difficulty,
since in many earthworms, e.g. _Perichaeta_, this orifice is in the same
condition; nor does the fusion of the oviducts and the so-called ovaries;
for in _Eudrilus_, for example, and in many Eudrilidae, the ovary is
contained in a sac into which the oviduct also opens. It will be noticed
too that the existence of short oviducts as compared with the long
sperm-ducts is a further point of likeness to at any rate the higher
Oligochaeta. But a further comparison needs first to be based upon a
consideration of the development of the different sections of the apparatus
in the leech. The independence of the ovaries and their ducts has been
proved by several observers; quite recently Bürger has dealt with the
matter in _Nephelis_, _Hirudo_, and _Aulastomum gulo_.[461] He has found
that the ovaries arise from the splanchnic wall of the lateral coelomic
cavities; they are therefore proliferations of the coelomic epithelium, as
in Oligochaeta and all Coelomates so far as is known. The peripheral layer
of the mass of indifferent cells which constitutes the ovary becomes
somewhat modified; its cells are flattened, and it at length separates
itself and forms a capsule surrounding the other cells, which are in fact,
or become, the ovary. This capsule meets and fuses with the ducts, which
are invaginations from the exterior of the body.
There are clearly differences between the ovary of a leech and that of a
typical Oligochaete like _Lumbricus_. The only point of agreement, in fact,
is the origin of the reproductive gland itself from the walls of the
body-cavity. In _Lumbricus_ and allied forms, whatever may be held with
regard to their homologies, the oviducts as a matter of fact appear first
as funnels, which afterwards bore their way to the exterior. They are
{404}purely mesoblastic structures, not—except perhaps the very
extremity—derived from an ingrowth from the epidermis. We have, however,
other Oligochaeta in which there are closer resemblances to what has just
been described in the Hirudinea. In more than one point the aberrant family
of the Eudrilidae come nearer to the Hirudinea than any other Oligochaeta,
in spite of _Branchiobdella_ with its jaws and sucker. Now in _Eudrilus_
the ovary is enclosed in a capsule which becomes continuous with the duct
of the great sperm-holding pouch, itself an invagination from the exterior;
there is no reason in this Annelid why the ova should not reach the
exterior by this system of ducts, although there is no actual experimental
proof that they do so. In any case there is also an oviduct corresponding
to that of _Lumbricus_, which opens into the opposite side of the duct of
the spermathecal pouch on the one hand, and into the receptaculum ovorum on
the other; it has no direct connexion whatever with the sac containing the
ovary. If we were to cut off the receptaculum and the oviduct and reduce
the spermathecal sac to its duct, the result would be much the same as we
find it in the leech.
COCOON.—The practice of forming a cocoon for the shelter of the eggs and of
the developing young is shared with the Oligochaeta; but not all leeches
deposit their eggs in this manner. _Glossiphonia_, for instance, carries
its eggs upon the ventral face of the body, where the young remain for some
time after they are hatched attached by the posterior sucker to their
parent's body, and from which situation of safety they make short
excursions. Other leeches deposit their eggs singly, but agglutinated
together upon stones, etc. In the medicinal leech the cocoon is ovoid in
shape, and from the end, which is closed by a temporary plug, the young
when ready escape. This cocoon is deposited, as is that of an earthworm, in
soil near to the borders of a marsh or pond, so that the young, while
enjoying the requisite degree of moisture, may not be injured by a too wet
environment. On the other hand _Pontobdella_ and some other leeches lay
their cocoons attached to bodies actually submerged in the water. The
cocoon is secreted, as in Oligochaeta, by the clitellum, and as in them, is
drawn off over the head, the ova and sperm probably flowing into it during
the process. The elasticity of the slightly hardened mucus causes the two
ends of the cocoon to {405}close up when it is free from the body; it is
then whitish and soft, and the leech fixes it, and appears to polish the
surface with its buccal sucker. In a few hours the cocoon becomes amber
brown, a colour which characterises the cocoons of a great many earthworms
and other Oligochaeta. The medicinal leech forms its cocoon in the same way
as does _Nephelis_, to which the above description refers; but when the
cocoon is formed the leech covers it with another layer of mucus, which
Vaillant, from whose work the foregoing notes are extracted,[462] thinks
may be produced from the so-called salivary glands.
CLASSIFICATION.—The number of different kinds of leeches is at present
uncertain. Seeing that no less than sixty-four varieties of the common
_Hirudo medicinalis_—colour varieties, it is true—are said to exist, it is
not wonderful that the labours of some systematists have been severe, and
have provoked much criticism and alteration on the part of others.[463] As
to genera, Vaillant, in his recent continuation of de Quatrefages'
"Annelés" in the _Suites à Buffon_, which includes the literature up to the
year 1886, allows thirty-seven, some (three) of which, however, are
admitted to be _incertae sedis_. Blanchard, who has paid a great deal of
attention to the group, reduces these by six, which he considers to be
synonyms; but on the other hand he has added or rescued from oblivion six
or seven others, and Whitman has instituted several Japanese and Australian
genera. Most of these generic types are, however, only imperfectly known,
and from external characters only. It is quite problematical how many valid
genera should be retained; in the meantime those that are fairly well known
are divided by Blanchard[464] in the following way:—
SUB-ORDER 1. RHYNCHOBDELLAE.—Hirudinea with an exsertile proboscis, without
jaws, and with colourless blood.
{406}FAM. 1. _Ichthyobdellidae._—Body formed of two regions, a narrower
anterior portion and a wider "abdomen," both anterior and posterior suckers
distinct from the body.
These leeches are parasites of fishes and of some other animals such as
tortoises. The family contains a number of genera. _Branchellion_ (Fig.
204) has a series of leaf-like branchiae on both sides; in _Cystibranchus_
the respiratory organs are reduced to a series of round vesicles. The
latter genus occurs in Europe and North America, and is parasitic upon
marine and fresh-water fishes. _Piscicola_ is a common leech which confines
its attacks to the inhabitants of fresh water. _Pontobdella_ (Fig. 210) is
marine, and affects rays and sharks; the best known species, _P. muricata_,
is usually of a green colour.
To the family Ichthyobdellidae also belongs the large Chilian leech
_Macrobdella valdiviana_, of which there is or are also species in North
America. Philippi's figure of this leech[465] shows the distinct neck; and
as it has no jaws, it should be referred to the present family. It has got
an undue reputation for size, 2½ feet[466] having been assigned to it. As a
matter of fact Philippi's illustration depicts an Annelid of about 7 inches
in length, with a greatest diameter of about an inch. But doubtless when
extended in walking it would be longer—1½ feet, Philippi thinks. It has no
eyes, a failing which is not unusual among the leeches.
FAM. 2. _Glossiphoniidae._—Anterior sucker fused with the body, posterior
sucker distinct. No cocoon.
[Illustration: FIG. 210.—_Pontobdella muricata._ × 1. (After Bourne.)]
The members of this family all inhabit fresh water; they have the habit of
depositing the eggs separately, which are then fixed to the ventral surface
of the body, and the young when hatched are {407}still protected by the
parent, returning to its body for shelter. The type genus _Glossiphonia_ (
= _Clepsine_) is common in Europe, and has many species. The Mexican and
Amazonian _Haementeria_ contains a number of species, of which the Mexican
_H. officinalis_ is used in medicine; but according to Miguel Jimenez, its
use is apt to be attended with unpleasant symptoms. Drowsiness, a buzzing
in the ears, and the development of a painful rash are some of the effects
produced by its bite. It is disputed whether the animal's saliva or foreign
matter introduced by it into the wound are the cause of the symptoms.
_Mesobdella_ of Blanchard[467] is said to be intermediate between this
family and the next. Each segment has three annuli, as in this family, but
the leech has three jaws, as in _Hirudo_.
SUB-ORDER 2. GNATHOBDELLAE.—Hirudinea without a proboscis, generally with
jaws; the blood is red; the eggs are invariably deposited in cocoons.
FAM. 1. _Gnathobdellidae._—Pharynx with three denticulate jaws.
This family as well as the next is terrestrial or fresh-water in habit. It
contains a number of generic types, including the medicinal leech, _Hirudo
medicinalis_, and the horseleech, _Haemopis_ (_Aulastomum_) _gulo_. The
former can be distinguished from the latter by its power of contracting
itself into an oval olive-shaped form, which power is not possessed by the
horseleech; the latter has, moreover, only two caeca, while the common
leech has ten pairs of these appendages of the intestine. The genus
_Limnatis_ is called after the Greek word λιμνῆτις, which Theocritus
applied to the leech. It is found in the Nile, and caused serious
inconvenience to the army of Napoleon. His soldiers in drinking at pools
sucked up the small leeches not thicker than a horse's hair, whose presence
in the hinder part of the mouth cavity produced divers objectionable
results, such as spitting of blood and hindered respiration.
FAM. 2. _Herpobdellidae._—Pharynx without denticulate jaws, with three
unarmed chitinous plates.
A characteristic genus of this family is _Trocheta_, which is so common at
the Zoological Society's Gardens and in the Regent's Park, and which has
been met with in other places near London; it is in this country an
introduced species, but is found in many {408}parts of the continent. It is
a land-leech, and lives upon earthworms.
The genus _Haemadipsa_, which M. Blanchard places in a special sub-family,
contains a number of species which are for the most part land-leeches.
Land-leeches occur in many parts of the world, but chiefly in the
tropics—in India, Ceylon, Java, South America, etc. They lie in wait for
their prey, upon the ground as a rule; but they may ascend herbs and shrubs
to gain a better outlook when they are aware of an approaching footstep. A
vivid account of the ferocity of these tiny Annelids in Ceylon can be read
in Sir J. E. Tennent's _Natural History of Ceylon_. They have been said to
be so pugnacious and so poisonous that persons surprised in their sleep by
the pests have succumbed to their united efforts. A whole battalion of
English soldiers decamped on one occasion from a wood which was overflowing
with land-leeches. The familiar misquotation "lethalis hirudo" might well
be applied to this species. Professor Whitman has written much upon the
habits of the land-leech of Japan (_Haemadipsa japonica_), which bites so
softly that its presence cannot be detected except for the stream of blood
which trickles from the wound. While it is feeding it emits from the pores
of the nephridia a clear fluid, which, as it appears, is used to keep the
skin moist; when unduly dried the same phenomenon occurs. It is curious
that in this and other leeches the nephridia should play a part which in
the earthworm is played by the dorsal pores; in both animals the glands of
the skin are also concerned with the same duty.
The purely aquatic leeches swim by undulations, and also crawl by the help
of the two suckers, like a "Geometer" caterpillar. But when a land-leech is
dropped into the water it at once sinks to the bottom and crawls out; it
does not swim, but can survive immersion for a long period. In this it
resembles the earthworms, which can also survive a prolonged immersion, and
even in the case of some are indifferent to the medium, land or water, in
which they live; the land-leech, however, is entirely dependent upon damp
surroundings; a dry air is fatal to it. The land-leech of Japan leaves a
slimy trail behind it as it crawls, in this respect recalling the land
Planarian _Bipalium kewense_.
GEPHYREA AND PHORONIS
BY
ARTHUR E. SHIPLEY, M.A.
Fellow and Tutor of Christ's College, Cambridge
{411}CHAPTER XV
GEPHYREA
INTRODUCTION—ANATOMY—DEVELOPMENT—SIPUNCULOIDEA—PRIAPULOIDEA—
ECHIUROIDEA—EPITHETOSOMATOIDEA—AFFINITIES OF THE GROUP.
The animals included in the above-named group were formerly associated with
the Echinodermata. Delle Chiaje[468] states that Bohadsch of Prague in 1757
was the first to give an accurate description of _Sipunculus_ under the
name of _Syrinx_, but Linnaeus, who noted that in captivity the animal
always kept its anus directed upwards, re-named it _Sipunculus_.
Lamarck[469] placed the Gephyrea near the Holothurians; and Cuvier[470]
also assigned them a position amongst the Echinoderms. He mentions
_Bonellia_, _Thalassema_, _Echiurus_, _Sternaspis_, and three species of
_Sipunculus_, one of which, _S. edulis_, "sert de nourriture aux Chinois
qui habitent Java, et qui vont la chercher dans le sable au moyen de petits
bambous préparés."
The name Gephyrea[471] was first used by Quatrefages, who regarded these
animals as bridging the gulf between the Worms and the Echinoderms. He
included in this group the genus _Sternaspis_ (_vide_ p. 335), now more
usually classed with the Chaetopoda.
The Gephyrea are exclusively marine. They are subcylindrical animals, which
can either retract the anterior end of their body—the introvert—carrying
the mouth into the {412}interior; or are provided with a long flexible but
non-retractile proboscis. The latter is easily cast off. They usually bear
spines or hooks of a hard chitinous character, secreted by the epidermis or
outermost layer of cells. The mouth is at the base of the proboscis or at
the end of the protractile part, the anus is at the other end of the body
or on the dorsal surface. The nervous system consists of a ring round the
mouth and of a ventral nerve-cord. A vascular system is present as a rule.
Nephridia are found which act as excretory organs, and in most cases also
as ducts for the generative cells. The Gephyrea are bisexual, and the male
is sometimes degenerate.
The group may be divided into four Orders:—(i.) Sipunculoidea; (ii.)
Priapuloidea; (iii.) Echiuroidea; (iv.) Epithetosomatoidea; of these the
first is by far the largest, both in number of genera and of species.
THE ANATOMY OF SIPUNCULUS NUDUS.
EXTERNAL CHARACTERS.—The body of _S. nudus_ when fully extended may attain
a length of a foot, or even a little more; in this condition it is seen to
consist of two portions, the anterior of which is, however, retracted into
the other when the animal is disturbed. The retractile portion is sometimes
termed the proboscis, but as its nature is entirely different from that of
the proboscis of the Echiuroidea, it is better to refer to it as the
_introvert_. Special retractor muscles are attached on the one hand to the
body-wall about half-way down the body, and on the other hand are fused
into a muscular sheath which surrounds the gullet, just behind the mouth.
When these muscles contract, they withdraw the introvert into the rest of
the body or _trunk_ in much the same way as the finger of a glove may be
drawn into the hand, by a thread fastened to the inside of its apex. The
introvert is protruded by the contraction of the circular muscles of the
body-wall. These exert a pressure on the fluid which fills the body-cavity,
and by this means the sides of the introvert are forced forward until
finally the head is exposed.
The introvert occupies about one-sixth or one-fifth of the total body
length. It is somewhat narrower than the trunk, and is covered by a number
of small flattened papillae, some of which lie with their free ends
directed backward, overlapping {413}one another like tiles on a roof. In
some other genera, as _Phymosoma_, the introvert bears rows of horny hooks,
which are apt to fall off as the animal grows old.
The trunk has from thirty to thirty-two longitudinal furrows, the
elevations between which correspond with a similar number of muscles lying
in the skin. This longitudinal marking is crossed at right angles by a
circular marking of similar origin, the elevations of which correspond with
the circular muscles in the skin. These two sets of markings thus divide
the skin of the trunk into a number of small square areas, very regularly
arranged (Fig. 212).
The outline of the trunk is more or less uniform, but it is capable of
considerable change according to the state of contraction of its muscles.
The circular muscles, for instance, may be contracted at one level, thus
causing a constriction at this spot. The colour of _S. nudus_ is a somewhat
glistening greyish-white.
[Illustration: FIG. 211.—Right half of the anterior end of _Sipunculus
nudus_ L., seen from the inner side and magnified. _a_, Funnel-shaped
grooved tentacular crown leading to the mouth; _b_, oesophagus; _c_,
strands breaking up the cavity of the tentacular crown into vascular
spaces; _c'_, heart; _d_, brain; _e_, ventral, and _e'_, dorsal retractor
muscles; _f_, ventral nerve-cord; _G_, vascular spaces in tentacular
crown.]
The anterior end of the fully-expanded _Sipunculus_ may be termed the head;
here the skin is produced into a frayed fringe which stands up in the shape
of a funnel round the mouth. This fringe is grooved on its internal surface
with numerous little gutters, all of them lined with cilia, which by their
constant motion keep up a current which sweeps food into the mouth.
{414}The fringe may be in the form of a simple ring round the mouth, or the
ring may be folded in at the dorsal side so as to take the form of a double
horse-shoe (Figs. 211 and 212).
BODY-WALL.—The glistening appearance of _Sipunculus_ is due to the cuticle,
a chitinoid layer which is secreted by the external layer of cells, the
_epidermis_. Beneath this lies a layer of connective tissue, which is not
always present in other Gephyrea; within this lies a layer of circular
muscles arranged in bundles, then comes a very thin sheath of oblique
muscular fibres, then a thicker layer of longitudinal muscles, and finally
a layer of peritoneal epithelial cells, which in _Sipunculus_ are for the
most part ciliated.
Scattered over the surface of the body, and opening by narrow tubes which
pierce the cuticle, are a number of glandular bodies which may be either
bi- or multi-cellular. The glandular cells are apparently enlarged and
modified epidermal cells; they are arranged in a cup-shaped manner, with
their apices directed towards the orifice. They are crowded with granules,
which are presumably poured out over the cuticle, but the exact function of
the secretion is entirely unknown. They have a well-developed nerve supply.
DIGESTIVE SYSTEM.—The mouth lies in the centre of the fringe, and is not
provided with any kind of jaw or biting armature; it leads directly into
the thin-walled alimentary canal, the first part of which is ciliated. The
alimentary canal is not marked out into definite regions, but passes as a
thin-walled semi-transparent tube to the posterior end of the body, and
then turns forward again and opens to the exterior by an anus situated
about an inch below the junction of the introvert with the trunk, on the
median dorsal line. The descending and ascending limbs of the alimentary
canal are coiled together in a spiral, which may be more or less close in
different individuals. The whole is supported by numerous fine muscular
strands, which pass from the walls of the intestine to the skin, and by a
spindle-muscle, which runs from the extreme posterior end of the trunk up
the axis of the spiral and terminates in the skin close to the anus.
No glands open into the alimentary canal at any point of its course, but
near the anus a simple diverticulum, or pocket, of unknown function arises.
The size of this outgrowth differs {415}enormously in different
individuals. The alimentary canal near the anus also bears two tuft-like
organs, which, however, do not open into the intestine, but probably have
some function in connexion with the fluid in the body-cavity.
Along the whole course of the alimentary canal there runs a ciliated
groove, into which the food does not pass, but the cilia of which probably
keep in motion a current of water whose function may be respiratory.
[Illustration: FIG. 212.—_Sipunculus nudus_ L., with introvert and head
fully extended, laid open by an incision along the right side to show the
internal organs. × 2. _a_, Mouth; _b_, ventral nerve-cord; _c_, heart; _d_,
oesophagus; _e_, intestine; _f_, position of anus; _g_, tuft-like organs;
_h_, right nephridium; _i_, retractor muscles; _j_, diverticulum on rectum.
The spindle-muscle is seen overlying the rectum.]
VASCULAR SYSTEM.—On the dorsal surface of the anterior end of the
alimentary canal lies a contractile vessel, usually termed the heart. It is
a tube about an inch long, ending blindly behind, but opening in front into
a ring-shaped space surrounding the mouth and partially enveloping the
brain. From this ring-like vessel numerous branches are given off which
pass into the fringe round the mouth, and probably the chief function of
the heart is by its contraction to force fluid into this fringe, and so to
extend it. The heart contains a corpusculated fluid. {416}A similar but
shorter tube is found on the ventral surface of the anterior end of the
alimentary canal in the species in question; it also opens into the ring
which surrounds the mouth.
RESPIRATORY SYSTEM.—There are no special respiratory organs, and it has
long been a matter of dispute where the respiration of Gephyrea is carried
on. The oxygenation of the blood probably takes place to some extent
through the walls of the oral fringe, but the blood which receives its
oxygen at this spot is limited in its distribution, and could only supply
the brain and head. It seems probable that the remaining organs are
supplied with oxygen by the fluid of the body-cavity, which bathes them on
all sides. This might obtain its oxygen from the blood in the heart, or
more probably, through the thin walls of the intestine, from the stream of
water which is maintained by the ciliated groove described above. Quite
recently a form—_S. mundanus_, var. _branchiata_—has been described[472]
with thin-walled papillae covering parts of the skin. These papillae are
full of corpuscles, and are regarded by their discoverer as branchiae.
BODY-CAVITY.—The pinkish fluid of the body-cavity contains numerous
corpuscles, the products of the reproductive organs (either ova or
spermatozoa), and some curious unicellular bodies known as "urns." The
latter are shaped like a bowl with a ciliated rim, and are formed from the
budding of certain cells on the walls of the dorsal blood-vessel.[473]
Their function is unknown, but they resemble certain multicellular bodies
found in the body-cavity of _Phascolosoma_. The generative cells found in
the body-cavity are further considered below. The true corpuscles are
either biconcave round corpuscles coloured with a chemical substance, the
haemerythrin of Krukenberg, which apparently plays the same rôle as
haemoglobin in other animals; or amoeboid corpuscles, which, though rare in
_Sipunculus_, are very numerous in _Phascolosoma_.
NERVOUS SYSTEM.—The nervous system of _Sipunculus_ consists of a brain or
cerebral ganglion, a circumoesophageal ring surrounding the gullet, and a
ventral nerve-cord. The brain is a small bi-lobed nervous mass situated on
the dorsal surface of the oesophagus, in the angle between the right and
left dorsal retractor muscles close to their point of insertion. Numerous
{417}nerves arise from it, and pass to the fringe surrounding the mouth and
to neighbouring parts. At the sides, the brain is continued into two stout
nerve-cords which encircle the oesophagus, and meeting, fuse together in
the median ventral line to form the ventral nerve-cord (Fig. 211). The
latter is of the same diameter throughout, and shows no signs of
segmentation; it is oval in section, and consists of small ganglion cells
heaped up on the ventral surface, _i.e._ next the skin, and of numerous
fibres situated dorsally. The cord gives off many nerves, which usually
arise in pairs. These pass into the skin, and forming rings, run round the
body, and give off finer nerves as they go.
The nerve-cord is supported by numerous strands of muscle which pass to it
from the skin. These are especially long in the region where the introvert
joins the trunk, and thus allow free play to the nerve-cord when the former
is being protruded or retracted.
_Sipunculus_ is not well provided with sense-organs, but in an animal which
lives buried in sand we should not expect to find these very highly
developed. On the introvert there are certain patches of epithelium bearing
long stout cilia, which have been regarded as tactile in function, and
there is a tubular infolding reaching the brain, which almost certainly has
some sensory function. Ward[474] has termed this "the cerebral organ." It
consists of a duct lined with ciliated cells, which opens to the exterior
in the middle dorsal line outside the tentacular fringe. The duct leads
down to the brain, and expands at its lower end into a saucer-shaped space,
covering that portion of the brain where its substance is continuous with
the external epithelium. In _Phymosoma_ this cavity is produced into two
finger-shaped processes, which are sunk into the brain and are lined by
cells crowded with a dense black pigment.[475] They are probably
rudimentary eyes, perhaps distinguishing only between darkness and light.
The pits appear to be absent in _Sipunculus nudus_, but Andrews states they
are found, although without pigment, in _S. gouldii_.[476]
EXCRETORY SYSTEM.—The excretory organs or "brown tubes" are typical
_nephridia_, that is to say, they consist of tubes {418}with glandular
walls which open on the one side to the exterior, and on the other by means
of a ciliated funnel-shaped opening into the body-cavity. In Gephyrea one
wall of the tube is produced into a long diverticulum or sac which hangs
down into the body-cavity, and is usually supported by muscle-fibres
running to the body-wall. The lower end of the sac is broken up into a
number of crypts or pits, lined by large glandular cells crowded with brown
pigment. The pigment-granules are secreted into the cavity of the sac, and
leave the body through the external opening; they probably consist of the
nitrogenous excreta of the animal. The upper end of the sac, into which
both the external and internal orifices open, is usually enlarged, and its
walls are very muscular. As in so many other animals, the nephridia serve
as ducts through which the reproductive cells leave the body of the parent.
REPRODUCTIVE SYSTEM.—The Gephyrea are bisexual. In _Sipunculus_ the testes
and ovaries are found in the same position in the two sexes, and are
indistinguishable without microscopic investigation. They each consist of
small ridges situated at the lower end of the ventral retractor muscles,
just where the latter take their origin from the longitudinal muscles of
the skin. At this level the cells which line the body-cavity on the inside
of the skin are heaped up, and become modified in the one case into ova or
eggs, and in the other into the mother-cells of the spermatozoa. This
method of forming the reproductive organs from modified cells lining the
body-cavity is very common in the higher animals; but it is seen in its
simplest and least modified form in the Sipunculidae.
The eggs break away from the ovary in a very undeveloped condition, but
whilst floating about in the body-cavity they increase in size and secrete
a thick membrane around them. They have a well-marked nucleus, and are oval
in outline.
The mother-cells of the spermatozoa also break away in an immature
condition, and complete their development in the nutritive fluid of the
body-cavity. They divide into a number of spermatozoa, usually eight or
sixteen, which remain in contact. They each develop a tail, which projects
outwards, and aids the cluster in swimming along. These clusters of
spermatozoa are about the same size as the ova of the female, and, like
them, make their way into the "brown tubes." The exact way in {419}which
this is accomplished is not very clear, but the cilia on the funnel-shaped
internal opening of the tube seem to have some power of selecting the
generative cells when they come within their reach, and of passing them on,
whilst they reject the much smaller corpuscles of the perivisceral fluid,
which are never found in the nephridia.[477] Once inside the internal
opening, the clusters break up and the spermatozoa escape singly into the
sea. Here they meet with and fertilise the eggs which have escaped from the
body of the female.
[Illustration: FIG. 213.—Larva of _Sipunculus nudus_ L. × 150. (After
Hatschek.) _a_, Mouth; _b_, anus; _c_, excretory organ; _d_, glandular
appendage of oesophagus; _e_, wall of stomach over which the retractor
muscle runs; _f_, invaginated sense-organ at aboral pole.]
DEVELOPMENT.—Hatschek,[478] who investigated the development of _Sipunculus
nudus_ at Pantano, an inlet of the sea near Messina, states that the
spawning takes place during the night, and ceases about July 10. The rate
of development depends upon the temperature, but the larvae usually free
themselves from the egg-membrane during the third day. When hatched the
embryos lengthen out a good deal, and take the form represented in Fig.
213. The larva swims actively by means of a ring of stout cilia, which
encircle the body just behind the mouth. Other shorter cilia are found on
the head, continuing into the lining of the mouth, and a little bunch of
them is situated at the extreme posterior end. The alimentary canal is
already formed, and is twisted, so that the anus lies dorsally, but not so
far forward as it does in the adult. A glandular structure opens into the
mouth, and another body of unknown function is connected with the
oesophagus; both these disappear during larval life. A pair of excretory
tubules, the {420}forerunners of the brown tubes, are found, and the chief
muscle tracts are already established. The nervous system is still in close
connexion with the skin, from the outer part of which it is derived; the
cerebral thickening bears two eye-spots.
The fluid of the body-cavity contains corpuscles, which are kept in active
circulation by the constant contractions of the body-wall, and by numerous
tufts of cilia which are borne on the inner surface of the skin. The dorsal
blood-vessel is one of the latest organs to arise.
The larva swims actively about for a month, during which time it increases
greatly in size; it then undergoes a somewhat sudden metamorphosis. The
ciliated ring and the structures related to the oesophagus begin to
disappear, the distinction between the head and the rest of the body is
obliterated, and the head becomes relatively small. The mouth changes its
position, and becomes terminal instead of being somewhat ventral, and the
tentacular membrane begins to appear. At the same time the larva
relinquishes its free-swimming life, and sinks to the bottom; it begins
creeping amongst the sand by protruding and retracting the anterior part of
its body, and takes on all the characters and habits of the adult.
I. ORDER SIPUNCULOIDEA.
Besides the genus _Sipunculus_, the Order Sipunculoidea includes ten other
genera. A key to these, taken for the most part from Selenka's admirable
monograph, is given on page 424.
_Phascolosoma_ contains, in comparison with _Sipunculus_, only small
species, and it is easily distinguished by the fact that the longitudinal
muscles are fused into a continuous sheath. As a rule the skin is smooth. A
few species bear hooks, which are generally scattered irregularly and not
arranged in transverse rows, as in _Phymosoma_ (Fig. 214) and most of the
other genera.
The fold which in _S. nudus_ surrounds the mouth may be in the same species
bent in so as to take the form of a double horse-shoe, the opening of which
is always dorsal, just above the brain; in this case the mouth is
crescentiform. In other genera the fold is broken up into discrete
tentacles, and these are variously arranged; in _Dendrostoma_ they are
grouped together in four or six bundles round the mouth, but the more usual
{421}arrangement is the horse-shoe-like row of tentacles which overhang the
crescentiform mouth, as in _Phymosoma_ and some species of _Aspidosiphon_.
The ventral side of each tentacle is grooved and ciliated, and the grooves
are continued into the ciliated mouth. Their dorsal surface is pigmented,
and in the hollow of the horse-shoe lies a deeply pigmented epithelium
covering the brain.
A blood-vessel courses up each tentacle, and usually two channels return
the blood to the vascular ring which surrounds the mouth. In those forms
which possess tentacles on the dorsal side of the mouth only, the ventral
part of the vascular ring lies in the lower lip, which is tumid and
swollen. The brain supplies a nerve to each tentacle.
When the introvert is retracted the tentacular ring is withdrawn and to
some extent collapsed; in this condition it would be almost touching the
rough external surface of the introvert. In some species of _Phymosoma_ the
delicate appendages of the head are guarded from the hooks on the introvert
by a thin membrane or collar,[479] which completely ensheaths the retracted
head.
[Illustration: FIG. 214.—A, _Phymosoma granulatum_ F. S. Leuck. × 2. B,
Head of the same. × 4. _a_, Pigmented pit leading to brain. The
crescentiform mouth on the lower side of the figure is overhung by the
tentacles.]
When the introvert is fully extended the dorsal blood-vessel contracts and
sends its blood forward into the vascular ring, and thence into the
tentacles or tentacular fold, which are thus erected. In several species of
_Sipunculus_, as _S. nudus_, _S. norvegicus_, _S. robustus_, _S.
tesselatus_, there is a ventral blind tube as well as a dorsal, into which
the blood is withdrawn when the head is retracted. In many other species in
various genera, such as _Phymosoma {422}weldonii_ and _Ph. asser_,
_Dendrostoma signifer_, _S. vastus_, the lumen of the dorsal vessel is
increased by numerous hollow blind processes which it bears, hanging freely
into the body-cavity. Three very small genera of Sipunculids—_Onchnesoma_,
_Petalostoma_, and _Tylosoma_—are devoid of all trace of vascular system
and of tentacles; the mouth opens in the centre of the anterior end of the
introvert. In _Onchnesoma_ the dorsal part of the lip is somewhat produced,
so that the head has somewhat the shape of a Doge's cap, and in
_Petalostoma_ there are two leaf-like processes of the body-wall which
guard the mouth.
The extent to which the intestine is coiled varies very much even in the
same species; the axis of the coil is often supported by a spindle-muscle,
but this is sometimes absent. The caecum, which opens into the rectum of
_S. nudus_, is again a very variable structure, and when it is present
varies remarkably in size.
The food of Sipunculids seems to consist almost entirely of sand, and their
only nourishment must be such small microscopic organisms or particles of
animal and vegetable débris as are to be found mixed with the sand. The
alimentary canal is, as a rule, quite full of sand, and yet in spite of the
tenuity of its walls they never seem to be ruptured. If the contents of the
digestive tube be washed out with a pipette, it will be found that it
requires considerable force to dislodge many of the sand-particles lying
next the wall. These are more or less embedded in crypts or pockets of the
wall, and as the sand passes along the intestine they probably serve as
more or less fixed hard points, against which the sharp edges of the sand
particles are worn off. Amongst the sand are usually to be found pieces of
shell, sometimes with a diameter equal to that of the alimentary canal;
these are usually rounded, but their angles may have been removed by
attrition before they entered the mouth of the Sipunculid.
In _S. tesselatus_ the sand is to some extent held together by a mucous
deposit; in those cases where there is no sand in the intestine, there is
always a coagulum of mucus, and the walls are contracted and thick; when
full of sand the walls are tensely stretched and very thin. This thinness
of the wall of the alimentary canal seems ill-adapted to a diet of sand,
nevertheless it is also met with in other great sand-eating groups of
animals, such as the Echinids and the Holothurians.
{423}The enormous amount of sand and mud which passes through the bodies of
the Sipunculids shows that they must take a considerable part in modifying
the mineral substances which form the bottom of the sea. Just as
earthworms, as shown by Darwin, play a considerable rôle in the formation
of soil, so must these animals, in conjunction with Echinids and
Holothurians, effect considerable modifications in the sand and mud which
pass through their bodies. Mr. J. Y. Buchanan[480] is "led to believe that
the principal agent in the comminution of the mineral matter found at the
bottom of both deep and shallow seas and oceans, is the ground fauna of the
sea, which depends for its subsistence on the organic matter which it can
extract from the mud." The minerals at the bottom of the sea are exposed to
a reducing process in passing through the bodies of the animals which eat
them, and subsequently to an oxidising process due to the oxygen dissolved
in the sea-water acting on the minerals extruded from the animals' bodies.
The rate at which the sand passes through the body of _Sipunculus_ is
unfortunately unknown, but that at any one moment a considerable quantity
is contained in the intestine is shown by the fact that the average weight
of five specimens of _S. nudus_ from Naples, taken at random, was 19.08
grms., whilst the average weight of sand washed out of their alimentary
canal was 10.03 grms. The sand contained in five other specimens of the
same species measured respectively 6 c.c., 7 c.c., 6.5 c.c., 7.5 c.c., and
7.5 c.c., giving an average of 6.9 c.c. for each individual.
_Onchnesoma_ and _Tylosoma_ have only one retractor muscle; _Aspidosiphon_
and _Phascolion_ have, as a rule, two; _Phymosoma_ and _Sipunculus_ have
four, and perhaps this is the more usual number.
_Phascolion_, _Tylosoma_, and _Onchnesoma_ have but one "brown tube"; in
_Phascolion_ this is the right, in _Onchnesoma_ it is sometimes the right
and sometimes the left that persists. Most other genera retain two, but
there are many exceptions; for instance, _Phascolosoma squamatum_ has but
one, and so has _Aspidosiphon tortus_, and in both cases it is that of the
left side. No Sipunculid has more than two. It has been pointed out by
Selenka that those species which have but one brown {424}tube are, as a
rule, inhabitants of tubes or shells, and do not move actively about in the
sand.
The eggs of all members of the family, with the exception of the genus
_Phymosoma_, are spherical, but those of the last-named genus are
elliptical. They are always surrounded by a thick membrane, the "zona
radiata," pierced by numerous pores.
_Aspidosiphon_ (Fig. 215) is easily recognised by the presence of two
symmetrically-arranged cuticular shields, one at each end of the trunk.
These are formed by the fusion of minute cuticular plates, such as exist in
the skin of most Sipunculids. The posterior shield is radially symmetrical,
but the anterior is somewhat like the shell of a Pecten, and symmetrical
only about one plane. The introvert is protruded from the acute angle of
the anterior shield, and when extended lies almost at right angles to the
trunk, instead of being, as is usually the case, in the same straight line
with it. In many specimens, and these seem as a rule to be the older ones,
a deposit of calcium carbonate takes place over these shields, covering
over and concealing their external markings.
_Cloeosiphon_ (_Echinosiphon_) has a calcareous ring, consisting of four or
five rows of lozenge-shaped calcareous bodies forming a close mosaic,
arranged round the base of the introvert, which when extended is in the
same straight line as the trunk. Each piece bears a brown spot, which is
said to be the pore of a gland (Fig. 217). _Golfingia_ Lankester, has a
cylindrical horny thickening at the anterior end of the trunk and another
at the posterior.
[Illustration: FIG. 215.—_Aspidosiphon truncatus_ Kef. × 2. _a_, Introvert
partially extended, but not sufficiently to show the head.]
KEY TO THE GENERA OF SIPUNCULOIDEA.[481]
I. The longitudinal muscles in the body-wall divided into 17-41 distinct
bundles. Four retractor muscles.
A. Body covered with papillae. Numerous filiform tentacles which seldom
(or never?) surround the mouth, but stand above and dorsal to it in a
horse-shoe, with the opening dorsal. No rectal {425}caecum. Hooks
usually present. Four retractors (in _Ph. Rupellii_ only two?). Heart
almost always without caeca. Eye-spots always present. Eggs oval, flat,
reddish. Almost entirely small tropical species 1. PHYMOSOMA
B. Body devoid of papillae. Tentacular membrane surrounds the mouth in
a circlet. Rectum with one or more caeca (except _S. edulis_?). Hooks
absent except in _S. australis_. Eggs spherical. The tentacular
membrane contains a vascular network. A ventral contractile vessel
usually present in addition to the heart. Mostly large forms. Found in
all seas 2. SIPUNCULUS
II. The longitudinal muscles in the body-wall form a continuous sheath,
and are not split up into bundles.
A. Two brown tubes. Numerous tentacles form a wreath round the mouth.
Alimentary canal forms a complete spiral, free behind except in _Ph.
Hanseni_. Spindle-muscle usually present. One or more ligaments
present, but only on the anterior convolutions of the intestine.
Adhesive papillae always absent. Hooks very frequently absent. Eggs
spherical. Found in all seas. 3. PHASCOLOSOMA
B. Two free brown tubes. Only four or six plumed tentacles. A complete
intestinal spiral, not attached behind. Spindle-muscle always present.
One or more ligaments present, but only on the anterior convolutions of
the intestine. Hooks are present, but sometimes fall off early in life.
Heart usually bears caeca. Found only in the tropics. 4. DENDROSTOMA
C. Only one brown tube, that of the right side, present; it is attached
to the body-wall throughout its entire length. Numerous tentacles form
a circle round the mouth. The alimentary canal forms no spiral, or an
incomplete one. No spindle-muscle, but the intestine is attached to the
body-wall throughout its length by numerous ligaments. Adhesive
papillae often present. Not more than two retractors. Spherical eggs.
Inhabits Mollusc shells or tubes. Found in all seas 5. PHASCOLION
III. At both ends of the trunk a distinct horny shield, or tube-like
cornification, or a calcareous ring at the anterior end of the trunk.
Hooks sometimes present. Longitudinal muscles continuous or split up into
bundles.
A. A shield at both ends of the trunk. Introvert excentric, arising
from the ventral side of the anterior shield. Tentacles small and few
in number, arranged in a horse-shoe above the mouth. A spindle-muscle,
which arises from the posterior end of the body, traverses the
intestinal coil. Two retractors only, these are the ventral; they are
frequently fused together from their point of origin. 6. ASPIDOSIPHON
B. A calcareous ring surrounds the anterior end of the trunk, from the
middle of which the introvert is extruded. Longitudinal muscles
continuous. Hooks bifid. Tropical. 7. CLOEOSIPHON
C. A corneous ring, from which the introvert issues, surrounds the
anterior end of the trunk, and the posterior end of the trunk is
{426}produced into a corneous spike. Six pinnate tentacles encircle the
mouth. Four retractors. Hooks present on the introvert. Longitudinal
muscles continuous. Intestine not coiled throughout in a spiral nor
fastened posteriorly. Spindle muscle present. 8. GOLFINGIA
IV. No tentacles, but two leaf-like extensions of the body-wall guard the
mouth. Four retractors. Few intestinal loops, quite free. No vascular
system. 9. PETALOSTOMA
V. No tentacles, no vascular system. One retractor, and one segmental
organ.
A. Introvert long. Body small, pear-shaped. 10. ONCHNESOMA
B. No introvert (?). Body cylindrical, thickly covered with papillae,
which are larger and more crowded at both ends of the trunk.
11. TYLOSOMA
SPECIES OF SIPUNCULOIDEA.—The genus _Phymosoma_ (Fig. 214) contains more
species than any other genus of Sipunculoidea, and they are all of fair
size. Twenty-seven species are known, of which seventeen occur in the Malay
Archipelago, thirteen being found there alone. _Phymosoma_ affects shallow
water, the deepest specimens being taken at a depth of about 50 fathoms;
this may be due to the fact that they flourish only in comparatively warm
water. With very few exceptions, they are found only in tropical seas, very
often living in tubular excavations made in soft coral rock.
The genus _Sipunculus_ contains sixteen species. They are the largest and
the most conspicuous members of the group. They have a very wide
distribution, some species, as _S. nudus_ (Fig. 212) and _S. australis_,
being almost cosmopolitan. They are most common in temperate and tropical
seas, but _S. norvegicus_ and _S. priapuloides_ are found far north, but
always at considerable depths, 100 to 200 fathoms.
The following account of the habits of _Sipunculus gouldii_ is taken from
Mr. Andrews'[482] paper on that species:—
"This _Sipunculus_ is very abundant in certain small areas of compact, fine
sand darkened by organic matter and not laid bare at ordinary low tide. In
such places, only a few square metres in extent, they pierce the sand in
all directions to a depth of more than half a metre, making burrows with
persistent lumen running from the surface downward and then laterally, but
with no regularity in direction.
"Kept in aquaria, the dependence of the animal upon the {427}nature of the
sand and its method of locomotion may be readily observed. A vigorous
individual buries itself in a few moments in the following manner: Running
out the introvert to nearly its full extent, and applying it to the surface
of the sand till some spot of less resistance is found, the animal still
further expands the introvert so that it penetrates the sand, provided this
is not too dense and firm, for then the body is merely shoved backward.
When the introvert is inserted, the contraction of the longitudinal muscles
of the body-wall brings the whole body forward somewhat, in case the
introvert is fixed in the sand. In case soft ooze was present, this
fixation did not take place, and the introvert was merely pulled out again,
but when the sand was of the right consistency the introvert was fixed by
becoming much swollen at the tip, and then constricted just posterior to
this swollen area. This bulb-like area exerts lateral pressure on the sand,
as could be seen by movements of the grains. The swelling of the anterior
end of the introvert is brought about by the body-wall contracting
elsewhere, and forcing in liquid to distend that end. Owing to the curved
form assumed by the body in the normal contracted state when first removed
from its burrow, the entrance of the introvert may often be nearly
vertical, and hence the entire body is soon raised nearly upright in the
water above the sand. If the body has thus been warped forward sufficiently
to become somewhat fixed in the sand, the introvert is rolled in and again
thrust forward from this new point of resistance, and so on till the animal
is entirely buried. This locomotion increases in speed as the creature
becomes more completely surrounded by sand, and is the only means of moving
from place to place.
"On a smooth surface, or on one not presenting the right degree of
resistance, the _Sipunculus_ does not change its position, but remains till
death finally occurs, rolling its introvert in and out and contracting its
body-wall to no purpose.
"The essential factors in the mechanism bringing about this hydrostatic
locomotion are an elongated contractile sac filled with liquid, and some
means of definitely co-ordinating the contractions of the sac.
"In natural environment the animals are found with sometimes one, sometimes
the other end nearer the surface of the sand: in the aquaria the same was
observed, but when the {428}water became stagnant and impure the anterior
end with expanded branchiae was often protruded somewhat above the surface
of the sand."
The genus _Phascolosoma_ contains at least twenty-five species, for the
most part small. _Ph. margaritaceum_, however, measures[483] 10 cm. in
length, and _Ph. flagriferum_, 13 cm. The latter is produced at the hinder
end of its trunk into a long whip-like process, which recalls the horny
spike of _Golfingia_. Most species live free, but a few inhabit the shells
of dead Gasteropods or of _Dentalium_, or the abandoned tubes of worms.
They occur in practically all seas.
[Illustration: FIG. 216.—Specimens of the Coral _Heteropsammia cochlea_,
with _Aspidosiphon heteropsammiarum_ or _A. michelini_ living in a state of
commensalism with them. (From Bouvier.)]
_Dendrostoma_ contains but five species, which are all found within the
tropics in the Pacific or in the West Atlantic. They are shallow-water
forms, and some are found between tide-marks.
_Phascolion_ is a smaller genus, containing but ten species, which may have
been derived independently from different species of _Phascolosoma_, and in
this case the genus should be broken up. The members of this genus live in
Mollusc shells, such as _Dentalium_, _Turritella_, _Buccinum_, _Chenopus_
(_Aporrhais_), _Nassa_, _Strombus_, and generally acquire the coiled shape
of their host. They are usually attached to the shell by means of certain
{429}adhesive papillae found on their posterior end. _Ph. strombi_ fills
its shell with mud, which must be kept together by some secretion of the
animal. The body lies in a tube in this mud, and the introvert projects
from the small round opening at the end of the tube, and explores the
ground in every direction. They are found in all seas, but more especially
in the colder waters.
[Illustration: FIG. 217.—_Cloeosiphon aspergillum_ Quatr. × ½. _a_,
Introvert covered with spines and partially extended, but not sufficiently
to show the head; _b_, calcareous plates surrounding the point of origin of
the introvert.]
The genus _Aspidosiphon_ includes nineteen species, which are, with few
exceptions, exclusively confined to the Indian Ocean and neighbouring seas,
including the Red Sea. The exceptions are _A. armatus_ from the Norwegian
coast, and _A. mülleri_ from the Mediterranean and Adriatic. _A. truncatus_
is also stated to occur at Panama, the Bahamas, and at Mauritius. The
remaining species almost all occur in the Malay Archipelago and
neighbouring islands, and as was the case with _Phymosoma_, this part of
the world seems to be the headquarters of the genus. _A. mülleri_ lives in
the interstices of rocks and stones, and occasionally in disused Mollusc
shells.
Two species of _Aspidosiphon_ have been described by Bouvier[484] living in
a state of commensalism with two species of Madreporarian corals,
_Stephanoceris rousseaui_ and _Heteropsammia cochlea_, which live on and
surrounding the shells of certain Molluscs at Aden (Fig. 216). Apparently
the Gephyrean takes up its abode within its house at a tender age, and
according to Bouvier, it provides for its increasing bulk by secreting a
coiled calcareous tube, the outer surface of which affords space for the
growth of the coral.
The genus _Cloeosiphon_, the _Echinosiphon_ of Sluiter, includes three
species: _C. aspergillum_ (Fig. 217), _C. molle_, and _C. javanicum_. The
first named occurs at Mauritius, the Malay Archipelago, and neighbouring
islands; the others are {430}confined to the last-named area, which thus
again forms the headquarters of a genus.
_Golfingia_, described by Lankester from a single specimen, was dredged in
St. Andrews Bay, at the depth of 10 fathoms.
_Petalostoma_ comprises but one species, _P. minutum_, which is found in
the English Channel.
_Onchnesoma_ comprises two species, _O. steenstrupii_ and _O. sarsii_, both
found off the coast of Norway at considerable depths between 200 and 300
fathoms.
_Tylosoma_ comprises one species, _T. lütkenii_, also from the Norwegian
coast. It is dredged from stony ground in 50 to 80 fathoms.
II. ORDER PRIAPULOIDEA.
ANATOMY.—This Order consists of the two genera _Priapulus_ and
_Halicryptus_. Both are cylindrical animals with the mouth at one end and
the anus at the other. The introvert is short, and is covered with rows of
chitinous spines, which are continued to some extent over the body.
The skin is folded in a series of rings, and the body is usually somewhat
swollen posteriorly. _P. caudatus_ bears a curious caudal appendage, beset
with a number of hollow lobes somewhat grape-like in appearance. This is
situated ventral to the anus; its lumen is continuous with that of the
body-cavity, but it can be separated from it by the action of a sphincter
muscle. Two such appendages exist in _P. bicaudatus_.
There cannot be said to be any head in the Priapuloidea; they have no
tentacles or tentacular fringe, no proboscis, and no distinct brain; simply
a round aperture, the mouth, which is surrounded by a groove in the skin,
at the bottom of which the circumoesophageal nerve-cord lies. The mouth
leads into a very muscular pharynx lined with stout chitinous teeth; this
passes into an intestine, which is as a rule straight, but in _P.
glandifer_ it has a single loop.
The Priapuloidea possess no vascular system and no brown tubes. Their skin
has in the main the same structure as that of the Sipunculids, with spines,
glandular bodies, and papillae with sensory hairs which resemble similar
structures on _Phymosoma varians_. Retractor muscles arise from the
longitudinal muscles {431}of the skin, and are inserted into the pharynx;
they are short and not constant in number.
The nervous system has retained throughout its primitive connexion with the
epidermis. In almost all animals the nervous system is formed from the
epiblast or outermost cellular layer of the embryo; it usually, however,
breaks away from this and sinks into the body. Thus in _Sipunculus_ it lies
within the body-cavity, and has retained its primitive connexion with the
outer layers of the skin only in the region of the brain; but in the
Priapulids the nervous system, which consists of a ring round the mouth and
of a ventral cord, lies embedded in the skin, and the nerve cells are
directly continuous with the cells of the epidermis. The nerve-ring lies at
the base of a groove in the skin, which forms a kind of gutter round the
mouth; the ventral nerve-cord is visible exteriorly as a light line which
marks the ventral surface of the animal. In no place is the ring or cord
differentiated in any way, and there cannot be said to be any brain or
special sense-organs. Numerous nerves are given off from the ring to the
pharynx and intestine, and from the cord to the body-wall.
[Illustration: FIG. 218.—_Priapulus caudatus_ Lam. Nat. size. _a_, Mouth
surrounded by spines.]
The sexes are distinct, but they differ from the other Gephyrea in the
nature of their reproductive organs. In mature specimens the ovaries or
testes are easily recognisable, lying to the right and left of the
alimentary canal. The reproductive glands are continuous with ducts, which
act as oviducts and vasa deferentia respectively. Both glands and ducts are
attached to the body-wall by a mesentery.
The excretory function is performed in the Priapuloidea by the ducts of the
generative organs. These are primarily connected with a number of branching
canals of small size which project into the body-cavity. According to
Schauinsland,[485] one or more pear-shaped cells are found at the end of
each branch, and each is {432}continued into a long cilium which hangs down
into the lumen of the canal, and by its movement produces a flickering
motion. Beyond the free end of the large cilium the canal is lined with
ciliated cells. The remarkable resemblance this form of excretory organ
presents to that of the Platyhelminthes (_vide_ p. 25) and of certain
Chaetopods is worthy of attention. In the young Priapuloidea the duct with
its branching canals is not masked by the generative organs, but as the
animals become mature, diverticula from the duct arise, and the cells
covering these become modified into ova in the female, and into spermatozoa
in the male. The presence of these follicles masks the excretory part of
the gland. The ova and spermatozoa escape through the ciliated ducts which
open to the exterior one on each side of the anus, and, contrary to what is
the case with other Gephyrea, leave the body without having ever been in
the body-cavity.
Nothing is known of the embryology of either member of this family, but
both genera appear to be sexually mature from the end of May until October.
CLASSIFICATION.—The two genera which make up the Order Priapuloidea are
characterised as follows:—
_Priapulus._—The body is continued into one or two caudal appendages, beset
with hollow papillae; these are ventral to the anus. The introvert forms ¼
to ⅓ of the total body-length; it is covered with spines in conspicuous
longitudinal rows, the rest of the body being ringed. The retractor muscles
are numerous, and are attached to the body-wall, some anteriorly and some
posteriorly.
The genus includes the following five species:—
_P. caudatus_ Lam. (Fig. 218). _Hab._ Coasts of Greenland, Norway, Great
Britain, the North Sea, and the Baltic.
_P. bicaudatus_ Dan. _Hab._ North Sea and Arctic Ocean.
_P. glandifer_ Ehlers. _Hab._ Coast of Greenland, North Sea.
_P. brevicaudatus_ Ehlers. _Hab._ North Sea and Baltic, from ten fathoms.
_P. tuberculato-spinosus_ Baird. _Hab._ Falkland Islands.
_Halicryptus._—No caudal appendages. Introvert ⅒ to 1/12 of the total body
length, with numerous spines arranged in close circles. Retractors numerous
and all attached to the body-wall anteriorly.
_H. spinulosus_ v. Sieb. (Fig. 219). _Hab._ North Sea, Arctic Ocean, and
Baltic, in from two to fifty fathoms.
{433}It will be noticed that with the exception of _P.
tuberculato-spinosus_, described by Baird from a single specimen, the whole
family is confined to northern seas.
HABITS.—The newly-captured specimens of both _P. caudatus_ and _H.
spinulosus_ are of a flesh colour, with a somewhat metallic sheen.
According to Apel, the latter lived in an aquarium for more than five
months, whilst the former died during the first month. When first
introduced into the aquarium they immediately began to busy themselves in
the mud or sand at its bottom, and very seldom showed themselves above it.
They forced their way into the sand by alternately contracting and
extending their introvert, and the _Priapulus_ arranged itself so that a
portion, often a very small one, of its caudal appendage was exposed to the
water; this fact supports the view that the appendage is respiratory in
function. When the animal buries itself deeply, the appendage does not
relinquish its position at the surface of the sand, but stretches itself
until it in some cases surpasses the length of the body. On the other hand,
_Halicryptus_ (Fig. 219), according to the same observer, lies with the
anterior end, the mouth, projecting from the surface of the sand, or else
it curves itself, so that both ends project into the water.
[Illustration: FIG. 219.—_Halicryptus spinulosus_ v. Sieb. × 6. _a_, Dark
line indicating the position of the ventral nerve-cord; _d_, mouth
surrounded by spines.]
Leckenby, who described specimens of _P. caudatus_ which were found by
fishermen searching for worms for bait in the outer harbour at Scarborough
at half tide, states that they live in sandy clay in U-shaped tubes, at a
depth of about 9 inches, the tubes opening at each end on to the surface of
the sand. The fishermen of this district call them "sea mushrooms."
_Halicryptus_ casts its cuticle in May and September; it becomes loose
first at the hinder end, and the split between it and the skin grows
forward until the animal lies free in a cuticular mantle. After some days
this is split, and the animal frees itself from it; the cast-off cuticle
includes for a short {434}distance the lining of the mouth, the anus, and
the two generative pores.
III. ORDER ECHIUROIDEA.
ANATOMY.—The most striking peculiarity of the Echiuroidea, as opposed to
the other two families of the Gephyrea, is the presence of a solid dorsal
outgrowth of a portion of the head, forming the proboscis. The nature of
this proboscis is something quite different from that of the introvert of
the Sipunculoidea; it would appear to correspond to an extension, in the
members of the last-named Order, of that part of the head which is dorsal
to the mouth and is covered by a peculiar pigment-epithelium, often in
continuity with the brain. In its outgrowth this portion of the body has
carried with it the nerve-ring and the vascular ring, which both surround
the mouth. The proboscis is found in all the genera with the exception of
the aberrant genus _Saccosoma_.
[Illustration: FIG. 220.—A, _Bonellia viridis_ Rol., ♀; B, _B. fuliginosa_.
Both nat. size. _a_, Grooved proboscis; _b_, mouth; _c_, ventral hooks;
_d_, anus.]
The body of the female _Bonellia viridis_, one of the best known species of
Echiurids, is shaped like a small sausage, and is usually about 2 inches
long. The proboscis arises from the anterior end, and is extremely
extensible. At the distal end the proboscis splits into two short arms,
which are often recurved; along the whole ventral surface runs a groove
lined with cilia, which by the approximation of its edges can be converted
into a tube. At the bottom of the proboscis the groove opens into the
mouth. {435}_Echiurus_; _Thalassema_, and the female _Hamingia_ have short
proboscides, which do not bifurcate but otherwise resemble those of the
female _Bonellia_.
[Illustration: FIG. 221.—View of a female _Bonellia viridis_ Rol., opened
along the left side, × 2. _a_, Proboscis cut short; _b_, a bristle passed
through the mouth into the pharynx; _c_, convoluted intestine; _d_, anal
tufts or vesicles; _e_, ventral nerve-cord; _f_, ovary borne on ventral
vessel running parallel with _e_; _g_, position of anus; _h_, points to
position of external opening of nephridium; _i_, nephridium. This line is
on a level with the internal funnel-shaped opening.]
The green colour of _B. viridis_ is due to a special pigment, "Bonellein,"
which at one time was thought to be identical with chlorophyll. A similar
green colour is found in _Hamingia arctica_, _Thalassema baronii_, and the
larvae of many forms.
A short distance behind the mouth, on the ventral surface, the female
_Bonellia_ and both sexes of _Thalassema_ and _Echiurus_ bear two incurved
stout chitinous hooks; these gave the name {436}Gephyrea Armata to the
above-mentioned genera. In addition to these, _Echiurus_ has a row of
chitinous bristles surrounding the posterior end of the body; the row is
single in _E. unicinctus_, double in _E. pallasii_. These bristles are
formed, like the hooks on the introvert of the Sipunculoidea, by epidermal
cells; those of _B. minor_ and of the posterior rings in _Echiurus_ are
said to arise each from a single cell, just as the bristles do in
Chaetopods.
The skin consists of very much the same layers as does that of
_Sipunculus_; the cuticle is thin, the epidermis is modified into numerous
glandular cells, papillae, and pits, from which the bristles arise. A third
layer of oblique or circular fibres is usually found inside the
longitudinal muscle-layer. The proboscis is solid, and contains much
connective-tissue and numerous muscle-fibres running in all directions; the
ventral groove is ciliated.
The alimentary canal in the Echiuroidea consists of a long thin-walled tube
with numerous convolutions; it is not coiled as in Sipunculids, but the
loops are irregularly arranged, and are supported by numerous fine muscular
strands which run from the skin. There is a ciliated groove running along
one side of the intestine, as in the Sipunculids. The anus is terminal. The
most striking peculiarity of the alimentary canal of the Echiurids is the
existence of a collateral intestine or "siphon." This is a narrow tube
which arises from the main canal not very far from the mouth, and re-enters
it again lower down. A similar structure occurs in some Echinids, and in
the Capitelliformia (pp. 272, 305). Its function is not certainly known.
Another characteristic feature of the Echiurids is the presence of "anal
vesicles," branching structures which unite into a common stem opening into
the intestine close to the anus. The free end of each of the branches
terminates in a ciliated funnel-shaped opening. The function of these
structures may be excretory, or they may control the amount of fluid in the
body-cavity.
A closed vascular system exists in Echiurids, consisting of a contractile
dorsal vessel running along the dorsal surface of the anterior end of the
alimentary canal, and continued along the axis of the proboscis. At the tip
of the proboscis it bifurcates, and each branch descends along the edge
until it reaches the base where, having encircled the oesophagus, the two
unite, and are continued as the ventral vessel which runs along the dorsal
surface of the nerve-cord, and eventually ends blindly. There is also a
vessel which {437}passes from the ventral vessel and encircles the
intestine, opening into the posterior end of the dorsal vessel. In
_Echiurus_ the same vessel encircles a stout muscle which runs from the
base of one of the ventral bristles to the other. In _Thalassema_ Lankester
states that the fluid within the vessels is colourless, and does not
contain corpuscles similar to those in the body-cavity fluid.
The "brown tubes" or nephridia vary in number in the Echiurids. In the
female _Bonellia_ there is but one; in _B. viridis_ the right, in _B.
minor_ the left usually persists. In shape, colour, contractility, and
minute structure they closely resemble those of _Sipunculus_. _Hamingia_ is
said to have a pair of brown tubes; _Echiurus_ has two pairs, except _E.
chilensis_, which has three; their internal openings are produced into long
coiled slits in some genera. _Thalassema gigas_ has one pair; _Th.
neptuni_, _Th. baronii_, _Th. formosulum_, and _Th. exilii_, two; whilst
_Th. vegrande_, _Th. moebii_, _Th. erythrogrammon_, _Th. caudex_, and _Th.
sorbillans_ have three pairs.
The nervous system consists of a ventral cord lying in the body-cavity, as
in the Sipunculoidea, but attached to the skin, and of a circumoesophageal
ring. With the growth of the proboscis this ring is drawn out, and the two
branches run along the sides of the proboscis and unite at the tip. There
is no specialisation of brain, nor are any special sense organs present,
but the ventral cord gives off paired nerves at regular intervals, which,
uniting dorsally, form rings in the skin in some and probably in all
species.
The perivisceral fluid is of a dark brown colour in _Thalassema_,
containing numerous spherical corpuscles deeply impregnated, according to
Lankester, with haemoglobin, and also containing granules of a brown
pigment. Haemoglobin is also found in certain of the muscles and in part of
the epithelial lining of the body-cavity. Lankester also describes the
presence of haemoglobin in the corpuscles of the perivisceral fluid in
_Hamingia_.
The genital glands are, like those of the Sipunculoidea, formed by a
special development of the cells lining the body-cavity. These cells are
massed together along the wall of the ventral blood-vessel. In _Echiurus_
and in _Thalassema_ the cells break off and float in the body-cavity,
developing into ova and spermatozoa. In _Bonellia_ each cell does not
become an egg, but a mass of cells breaks off, one of which increases in
size at the expense of the {438}others and forms the ovum. The mature
sexual cells leave the body through the nephridia.
[Illustration: FIG. 222.—An adult male _Bonellia viridis_ Rol. The original
was 1.5 mm. long. The nervous system is not shown. (After Selenka.). _a_,
Generative pore with spermatozoa coming out; _b_, anterior blind end of
intestine attached to the parenchymatous tissue by muscular strands; _c_,
green wandering cells containing chlorophyll; _d_, parenchymatous
connective-tissue; _e_, epidermis; _i_, intestine; _j_, vas deferens; _l_,
internal opening of vas deferens; _m_, the left anal vesicle; _n_,
spermatozoa in the body-cavity.]
_Bonellia_ and _Hamingia_ present very interesting cases of sexual
dimorphism. In both genera the female is an animal of considerable size
with the normal structure of the Echiuroidea, but the male (Fig. 222) is a
microscopic Planarian-like animal, which lives in the mouth and in the
nephridia of the female. Both in _Bonellia_[486] and in _Hamingia_ the male
is provided with a pair of hook-like ventral bristles; these are wanting in
the female _Hamingia_. The surface of the male is ciliated, and the skin
{439}contains circular and longitudinal muscle-fibres. The body-cavity is
developed, but does not reach to either end of the body. The alimentary
canal is closed, neither mouth nor anus existing; it is supported by
regularly arranged dorso-ventral muscle strands. A nerve-ring and a ventral
cord exist. There are also two rudimentary organs corresponding with the
anal vesicles of the female, and a single nephridium which acts as a duct
for the spermatozoa; the latter arise from modified cells lining the
body-cavity.
In both sexes the larvae develop to a certain stage without showing any
trace of sexual differentiation, but after this stage, the development of
the male is to a certain extent arrested; in some respects, indeed, it
undergoes retrogressive changes. At this time it is found clinging to the
proboscis of the female, thence it makes its way to the mouth, where it
undergoes its final change; and then creeping out, finds its way into the
nephridium of the female, and spends the rest of its life there in a
special recess cut off by a fold from the excretory part of this organ. In
_Hamingia_, however, Lankester, who first described the male, did not find
any in the nephridia, but found five specimens, each 1/12 inch long, within
the dilated pharynx of the female.
DEVELOPMENT.—In _Bonellia_ and _Hamingia_ it seems probable that the ova
are fertilised in the nephridium of the female; in the other genera they
are fertilised in the water after leaving the body of the mother.
In _Thalassema_ and _Echiurus_ the growth of the embryo results in the
formation of a typical Trochosphere larva, a type widely spread in the
animal kingdom, being found in the Chaetopoda (Fig. 145, A), Polyzoa (p.
510), and Mollusca. The large prae-oral lobe persists in the Echiuroidea as
the proboscis; the mouth is ventral in position, with usually a ring of
cilia encircling the body in front of and behind it; the anus is posterior
and terminal. A pair of larval excretory organs are present, and a special
nervous aggregation of cells at the apex of the prae-oral lobe is usually
indicated by the presence of a bunch of long cilia.
The trunk of the Trochosphere is unsegmented, and in certain groups of
animals it remains so, but in Chaetopods, and in _Echiurus_ and
_Thalassema_, it elongates and becomes divided {440}up into a series of
somites or segments. Of these there are fifteen in _Echiurus_, and
apparently eleven in _Th. mellita_; in this stage the Gephyrean larvae have
again so close a resemblance to the segmenting Chaetopod larvae as to be
easily mistaken for them. The segmentation is shown in the following way:
(i.) the middle layer of cells or mesoblast is typically segmented, and
forms septa, which separate each segment from its neighbours; (ii.) the
ventral nerve-cord arises as segmentally-arranged thickenings of the
epiblast, which fuse together, but retain their segmented appearance for
some time; (iii.) the skin shows the segmentation of the body both by the
arrangement of the pigment and by bands of cilia. The latter are replaced
in the adult by rows of spines, and on the fourteenth and fifteenth
segments in _Echiurus pallasii_ by the two peri-anal circles of bristles.
Each bristle, like those of Chaetopods, originates from a single cell.
The anal vesicles arise quite late in the development; when they have
acquired their openings into the body-cavity, they seem to take in water.
In _Thalassema_, as described by Conn, this is accompanied by remarkable
changes, amounting almost to a metamorphosis. The body increases in bulk
fourfold, the cilia of the prae-oral ring disappear, and the animal now
moves only by means of its proboscis; the pigment is absorbed, and all
traces of segmentation disappear. A similar intaking of water is described
by Spengel in _Bonellia_. In this genus the larva, which is coloured bright
green, and has two brown eye-spots, is not such a typical Trochosphere as
is that of _Echiurus_ and _Thalassema_.
[Illustration: FIG. 223.—_Echiurus pallasii_ Guér. × ½. _a_, Mouth at the
end of the grooved proboscis; _b_, ventral hooks; _c_, anus.]
Nothing is known of the development of _Hamingia_ or of _Saccosoma_.
SPECIES OF ECHIUROIDEA.—_Echiurus._ Proboscis not bifurcated at the end.
Two ventral hooks and a single or double peri-anal ring of bristles. The
body is to a varying extent marked {441}by rings bearing spines. Two or
three (_E. chilensis_) pairs of nephridia, their external orifice often
lengthened and spirally coiled. Both sexes alike.
Greef[487] mentions six species of _Echiurus_, viz. _E. pallasii_, _E.
forcipatus_, _E. sitchaensis_, _E. chilensis_, _E. carabaicus_, and _E.
chrysacanthophorus_; to these must be added _E. unicinctus_. It seems
probable that _E. forcipatus_ of Reinhardt is identical with _E. pallasii_,
although bigger, whilst _E. sitchaensis_, _E. carabaicus_, and _E.
chrysacanthophorus_ are inadequately described. The distribution of the
remaining three species is as follows:—
_E. pallasii_ Guérin (Fig. 223). North Sea, Atlantic, English Channel.
_E. unicinctus_ Drasche. Japan.
_E. chilensis_ Max Müller. Chili.
_Thalassema._—Proboscis rather pointed at the end, not bifurcated. No
peri-anal bristles, but two ventral hook-like bristles placed anteriorly.
One to three or four pairs of nephridia. The sexes resemble each other.
Greef mentions eight species of _Thalassema_ and Rietsch thirteen; three of
these, however, _Th. grohmanni_, _Th. lessonii_, and _Th. pelzelnii_, were
not seen by either author, and their description is taken from Diesing.
There is some reason for thinking that the two first-named species are
identical with _Th. neptuni_. Conn has established a new species for the
specimens whose embryology he worked out at Beaufort, Virginia, and Selenka
described a new species from the Challenger material.
With the exception of the three doubtful species mentioned above, the list
of species of _Thalassema_ is as follows:—
_Th. neptuni_ Gaertner (Fig. 224). English Channel (Devonshire coast),
Concarneau, Mediterranean (Gulf of Marseilles), Irish coast (Dungarvan).
_Th. gigas_ Max Müller. Trieste.
_Th. vegrande_ Lampert. Philippine Islands.
_Th. baronii_ Greef. Canary Islands (Lanzarote).
_Th. formosulum_ Lampert. Shanghai and Philippine Islands.
_Th. exilii_ Fr. Müller. Brazil (Desterro).
_Th. moebii_ Greef. Mauritius.
_Th. erythrogrammon_ Max Müller. Red Sea and East Indies (Billiton).
_Th. caudex_ Lampert. Red Sea and Indian Ocean.
_Th. sorbillans_ Lampert. Philippine Islands.
_Th. mellita_ Conn. West Atlantic (Beaufort).
_Th. faex_ Selenka. North of the Faroe Islands.
{442}_Bonellia._—Proboscis very extensible and bifurcated at the end. The
body and proboscis are coloured a bright green. Two ventral hook-like
bristles, but no peri-anal ring. A single nephridium. The above applies to
the female; the males are degenerate, and live in the nephridium or pharynx
of the female.
Three (or four?) species of this genus are known.
_B. viridis_ Rolando (Fig. 220). Mediterranean, Adriatic, North Sea
(Bergen).
_B. minor_ Marion. Mediterranean (Gulf of Marseilles, Naples).
_B. suhmii_ Selenka. Off Nova Scotia. Male not known.
_B. fuliginosa_ Rolando? (Fig. 220). Mediterranean (Naples).
_Hamingia._—Proboscis not bifurcated, about as long as body. No ventral
hook-like bristles. One or two nephridia, which open at the apex of one or
two well-marked papillae. The above applies to the female; as in the genus
_Bonellia_, the male is minute and parasitic. It has two well-marked
hook-like bristles situated behind the genital pore.
This genus was first described by Koren and Danielssen as _H. arctica_. Two
specimens were afterwards described by Horst as _H. glacialis_. Later
Lankester described two other specimens; he was the first to find the male
in the pharynx of the female. He is of the opinion that all three
descriptions apply to the same species, and for this the original name _H.
arctica_ must be retained.
_Hamingia arctica_ K. and D. Two hundred miles north of North Cape and in
the Hardanger Fjord.
_Saccosoma._—No proboscis. The body is flask-shaped. The mouth and anus are
terminal. The ovary is anterior, and there is only one nephridium. No
bristles.
Our knowledge of this remarkable Gephyrean is very incomplete, but such as
it is, it is due to the careful investigations of Koren and Danielssen, who
had only a single specimen at their disposition.
_Saccosoma vitreum_ K. and D. North of the Faroe Islands.
HABITS OF THE ECHIUROIDEA.—As a rule the members of this group conceal
their bodies in clefts and fissures of rocks and stones, keeping up
communication with the outer world by means of their proboscis.
Rietsch[488] describes a specimen of _Bonellia minor_, which he placed in
an aquarium, exploring with its {443}proboscis the nature of the bottom;
when the animal had found a convenient crevice, it fixed its proboscis in
it by means of the bifurcated end, and by its contraction drew the body up,
and entered the hole, proboscis first. It then turned round, and during
this operation doubtless the ventral hooks came into play; and then
stretching out its proboscis, it began to explore the neighbourhood. The
proboscis is evidently very sensitive, and in addition to being a locomotor
organ, it is also used for the prehension of food. If cut off near the
mouth, the animal does not long survive, but if a considerable portion is
left the scar heals, and the lost part is probably regenerated. In
captivity the animals frequently change their place of residence.
Eisig some years ago described the great extensibility of the proboscis of
_B. viridis_ when confined in the tanks of the Zoological Station at
Naples. When contracted the proboscis was but a few inches long, but at
times it was extended till it reached the length of 1½ metre, shining
through the water as a transparent green thread. The body of the _Bonellia_
was hidden under stones, but the proboscis could be seen seizing between
its two ends the bodies of certain Ascidians which covered the inside of
the tank, tearing them off the walls, and conveying them to the mouth along
its grooved ventral surface.
The food of the Echiuroidea consists of organic matter, in the main of
animal nature, but the group differs from the Sipunculoidea in not eating
sand.
Rietsch describes _Thalassema neptuni_ as being more active in its
movements and less sedentary than _B. minor_. The proboscis is still the
chief organ of locomotion, but the trunk plays a greater part in the
movements of the animal than it does in the last-named species. _Th.
neptuni_ is found in cavities of stones or in the chambers worn out by the
Mollusc _Gastrochaena_; when withdrawn from its house the body is found to
be covered by a thick layer of tenacious viscid mucus.
[Illustration: FIG. 224.—_Thalassema neptuni_ Gaert. × 2. A, The animal
lying on its ventral surface. B, Ventral view of the anterior end, showing
the grooved proboscis ending behind in the mouth, and the ventral hooks.]
{444}_Th. mellita_ was so named by Conn because it is found sheltering in
the Echinid _Mellita_. "It enters the shell at the oral opening while yet
very small, but once within its house it grows to its adult size, and is
obliged therefore to remain during the rest of its life a prisoner." Each
shell thus inhabited acquires a reddish brown horse-shoe-shaped marking,
which affords a conspicuous signal that the shell contains a _Thalassema_.
_Thalassema_ is seldom found living in sand, and _Bonellia_ never, but
_Echiurus_ is almost always found in U-shaped tubes or passages in the
sand, which it digs out for itself by the rapid contractions of its
body-wall aided by its bristles. It, like the other two genera named above,
does not long remain in the same hole, but frequently changes its home. As
a rule the _Echiurus_ sits near the mouth of its tube, which is often a
foot or even two in depth, and sends out its proboscis in every direction;
at the least sign of disturbance it withdraws into the deeper recesses. The
walls of the tube are kept from falling in by a layer of mucus, which makes
a smooth lining to the passage. The peri-anal bristles, which can be
withdrawn or protruded at will, enable the animal to fix itself at any
level in the tube.
The Echiuroidea are sometimes used by fishermen as bait. In _Echiurus
pallasii_ Greef found three parasites, all of them new species. One, a
Gregarine, he named _Conorhynchus gibbosus_; the others were
Platyhelminthes, and were named by him _Distomum echiuri_ and
_Nemertoscolex parasiticus_ respectively.
IV. ORDER EPITHETOSOMATOIDEA.
This Order includes the single Family _Epithetosomatidae_, which was
established by Koren and Danielssen to contain the remarkable Gephyrean
they described in 1881 under the name _Epithetosoma norvegicum_ (Fig. 225).
Unfortunately only two specimens were at their disposition, and these were
badly preserved, so that many details of their structure could not be made
out. The animals are of an olive-green colour, and consist of a trunk about
12 mm. long, and of a proboscis 30 mm. in length; the latter differs
essentially from the proboscis of the Echiuroidea inasmuch as it is hollow,
and seems to be a whip-like tubular extension of the skin. Its lumen opens
into the body-cavity. Ventral to the base {445}of the proboscis is the
mouth; the intestine is straight, and terminates in the anus, which is
posterior. The nervous system lies between the circular and the
longitudinal muscles of the body-wall, and contains a tube, the nature of
which is obscure. No vascular system is known. The ovary is attached to a
mesentery ventral to the anterior part of the alimentary canal, and there
is a single nephridium. No anal vesicles exist.
The most remarkable feature of the genus is a series of pore-like openings,
which are stated to lead from the outside into the body-cavity (Fig. 225,
_a_). These are arranged four on each side, at the bottom of two slit-like
depressions in the skin, which lie one on each side of the base of the
proboscis, slightly dorsal to it.
These remarkable structures are without parallel amongst the Gephyrea, and,
together with the peculiar character of the proboscis, justify the
Norwegian naturalists in adding a new family to the group.
[Illustration: FIG. 225.—A, _Epithetosoma norvegicum_ K. and D., magnified.
_a_, _a_, Right and left slits leading to the pores; _b_, mouth; _c_,
proboscis: B, the same animal opened dorsally; _a_, pores; _b_, oesophagus;
_c_, proboscis; _d_, brown tube. (After Danielssen and Koren.)]
AFFINITIES OF THE GEPHYREA.
Before considering to what other groups of animals the Gephyrea may be
allied, it is advisable to discuss the relationship of the four Orders
which compose the group.
Quatrefages, in the year 1865, divided the Gephyrea into I. _Gephyrea
Armata_, with which he included the Echiuroidea and _Sternaspis_,[489] and
II. _Gephyrea Inermia_ or Sipunculoidea. The Gephyrea Inermia, sometimes
called the Achaeta, have been extended to include the Order Priapuloidea,
and opposed to the smaller sub-group the Gephyrea Armata or Chaetifera. In
my opinion, however, these names now are no longer in accordance with our
knowledge of the structure of the {446}animals they attempt to describe,
and they should be given up. Both names had reference to the presence or
absence of the two hook-like bristles described on the ventral surface of
some of the Echiuroidea, but of the five genera of this family, two,
_Saccosoma_ and _Hamingia_ (the latter in the female or normal form), are
without these bristles, and can therefore be described neither as _Armata_
nor as _Chaetifera_. On the other hand, hook-like chitinous bristles of
somewhat the same nature, though smaller in size and varying in position,
are very common on the introvert of Sipunculoidea and on the body of the
Priapuloidea.
Again, the association of the two last-named Orders in one sub-group is, to
my mind, an error. The Priapuloidea have little in common with the
Sipunculoidea; almost the only real point of resemblance is the power of
protruding the anterior part of the alimentary canal, and withdrawing it by
the aid of retractor muscles. But in the Priapuloidea this power exists to
a very small extent, and it is a power shared by very many animals besides
the Gephyrea. The terminal anus of the former is a feature shared by the
Echiuroidea and by _Epithetosoma_, but these have little else in common
with the Priapuloidea. On the other hand, the entire absence of any head
appendages, such as the proboscis of the Echiuroidea and the tentacles or
tentacular membrane of the Sipunculoidea, the absence of a vascular system,
of nephridia or anal vesicles, taken together with the straight intestine
which occurs elsewhere only in _Epithetosoma_, the persistent connexion of
the nervous system with the epidermis, the unique character of their
excretory system and of the reproductive organs, are all features in which
the Priapuloidea differ from the more normal members of the other three
Orders. These constitute a list of peculiarities which are at least as
important, and probably even more important, than those which characterise
the Sipunculoidea and the Echiuroidea. Thus the Priapuloidea should, I
think, be regarded as a distinct Order, which occupies a very isolated
position in the group.
Until we know something about the development of _Halicryptus_ and of
_Priapulus_, it will be difficult to say whether the Order is more nearly
allied to one or the other of the two great Orders of Gephyrea, whether it
is very primitive or very specialised. The connexion of the entire nervous
system with the epidermis and the absence of a vascular system are both
{447}rather primitive features, and so is the Platyhelminthine character of
the excretory organs. With regard to the vascular system, however, it
should be pointed out that it arises very late in the larva of those
Gephyrea whose development is known, and that it does not seem to
correspond with the vascular system of other animals; it has no fine
vessels or capillaries connected with it, and apparently does not act so
much as the channel of the circulatory medium, but more as a mechanism for
the expansion of the head appendages, the tentacles in the Sipunculoidea
and the proboscis in the Echiuroidea; moreover, it is absent in some genera
of the former, such as _Onchnesoma_, _Tylosoma_, and _Petalostoma_, where
there are no tentacles.
The conclusion of the whole matter seems to be that the Priapuloidea are an
isolated Order retaining many primitive features, and having no closer
affinities to the Sipunculoidea than to the Echiuroidea.
Hatschek came to the conclusion, from his work on the development of
_Echiurus_, that the Echiuroidea are true "Annelids," and from the presence
and mode of formation of the bristles, that they are related to the
Chaetopods. In this view he is confirmed by Conn, who worked out the
development of _Thalassema_. This relationship is further confirmed by the
discovery of Sluiter's that _Sternaspis_, the genus of Chaetopods which in
other respects most nearly resembles the Gephyrea, has in one of its
species (_S. spinosa_) a well-marked bifid proboscis, which, like that of
the Echiuroidea, is thrown off at the least disturbance. Thus it seems
fairly well established that the Echiuroidea are closely connected with the
Chaetopoda, for although the only traces of segmentation they retain in the
adult are the serially-repeated nephridia of _Thalassema_ and _Echiurus
pallasii_, and the two rows of peri-anal bristles in the latter, and
possibly the circular nerves given off from the ventral cord, yet the larva
is fully segmented, and in other respects is almost typically Chaetopodan.
The relationship of the Sipunculoidea to the Echiuroidea is a more doubtful
point. Hatschek is inclined to separate them, and in this he is again
supported by Conn. Embryology unfortunately does not help us much. The
early stages and larvae of _Sipunculus nudus_ and of _Phascolosoma
elongatum_ have been investigated by Hatschek and by Selenka respectively.
In neither genus is there any trace of segmentation or of Annelid
{448}features, with the possible exception of the bristles on the larval
_Phascolosoma_. On the other hand, it must be remembered that the
development of _Sipunculus_ is remarkably abbreviated, and that such stages
may have dropped out, the larvae hardly differing more from the
Trochosphere of _Echiurus_ and _Thalassema_ than does that of _Bonellia_,
an undoubted Echiurid. Still the facts that there is never a head-kidney
present, that there is no trace of segmentation, and that at no stage is
the anus terminal, must have a certain weight.
If we leave out of account the larval history, which, although pointing to
a difference in the nature of the two families, is by no means decisive,
and consider the adult structures, we find very considerable evidences of
affinity. Taking firstly the main points of difference, we find these to be
(i.) the nature of the cephalic appendages, either a proboscis or some
modification of tentacles; (ii.) the position of the anus; (iii.) the
presence of anal vesicles; (iv.) the number of the nephridia, never more
than one pair in Sipunculids; and (v.) the difference in origin of the
chaetae. Of these most undoubtedly the first is the most important. The
Echiuroidea have retained the prae-oral lobe of the larva in the form of a
solid outgrowth of the body, which outgrowth has carried with it the
nerve-ring and vascular ring which surround the mouth. This has been lost
in the Sipunculoidea, but is, I think, represented by a modified patch of
epidermis which lies dorsal to the mouth and just above the brain. A solid
extension of the skin in this region, which involved the nervous and
vascular systems, would bring about the same relation of parts as is found
in the Echiuroidea. The tentacular membrane or tentacles of the
Sipunculoidea have such a variety of form and arrangement, whilst all
subserving the same end, that I am inclined to believe that they have
originated within the limits of the family.
The position of the anus in the Sipunculoidea is one common to very many
animals which live embedded in sand or in tubular holes; it is probably not
primitive, as in the larva of _Sipunculus_ it is near the posterior end,
and becomes more dorsal as the larva elongates.
The anal vesicles of the Echiuroidea probably have no representative in the
Sipunculoidea. In appearance and position they are very like the little
tufts which are found on the rectum of {449}_Sipunculus_, but since these
open neither into the body-cavity nor into the alimentary canal, it is
hardly fair to compare them.
The resemblances between the Orders seem to me, on the whole, to outweigh
the differences. The general structure of the skin, the coiled alimentary
canal, with its ciliated groove, supported by strands of muscles, the
vascular system which gives off no capillaries, the structure of the brown
tubes, the existence of chitinous hooks or bristles, the nervous system
with its single unsegmented ventral cord, the formation of the generative
organs, all point to a sufficiently close resemblance to justify us in
classing the two Orders together. In addition to these there are
considerable histological resemblances which cannot be discussed here, but
which have a certain weight.
To sum up, it seems probable that the Echiuroidea are derived from the
Chaetopoda, and that their nearest ally in this group is _Sternaspis_; and
that the Sipunculoidea are allied to the Echiuroidea, but have further
departed from the Annelid stock, and have lost even those traces of
affinity with the parent group which have been preserved in the development
of _Echiurus_ and _Thalassema_.