Cephalopod.
_Pal._ eyelid; _Co._ cornea; _Co.ep._ epithelium of ciliary body;
_Ir._ iris; _Int._ _Int1_ ... _Int4._ different parts of the
integument; _l._ lens; _l1._ outer segment of lens; _R._ retina;
_N.op._ optic nerve; _G.op._ optic ganglion; _x._ inner layer of
retina; _N.S._ nervous stratum of retina.]
The supra-oesophageal ganglia appear always to develop within the
region of the velar area. This area corresponds with the præ-oral lobe
of the Chætopod larva, at the apex of which is developed the
supra-oesophageal ganglion. Embryology thus confirms the results of
Comparative Anatomy in reference to the homology of these ganglia in
the two groups.
Optic organs[113]. An eye is present in most Gasteropods and in many
larval Pteropods. Although its development has not been fully worked
out, yet it has clearly been shewn by Bobretzky and other
investigators that it originates as an involution of the epidermis,
which first forms a cup and eventually a closed vesicle. The posterior
wall of the vesicle gives rise to the retina, the anterior to the
inner epithelium of the cornea. The external epidermis becomes
continued over the outer surface of the vesicle.
[113] For a fuller account of this subject the reader is referred
to the chapter on 'The Development of the Eye.'
The lens is formed in the interior of the vesicle, probably as a
cuticular deposit, which increases by the addition of concentric
layers. Pigment becomes deposited between the cells of the retina.
Fig. 122 B is a diagrammatic representation of the adult eye of a
Gasteropod.
The Cephalopod eye is formed, as first shewn by Lankester, as a pit in
the epiblast round which a fold arises (fig. 123 A) and gradually
grows over the mouth of the pit so as to shut it off from
communication with the exterior (fig. 123 B).
[FIG. 123. TWO SECTIONS THROUGH THE DEVELOPING EYE OF A CEPHALOPOD
TO SHEW THE FORMATION OF THE OPTIC CUP. (After Lankester.)]
The epiblast lining the posterior region of the vesicle gives rise to
the retina, that lining the anterior region to the ciliary body and
processes. It is important to notice that the condition of the eye
just before the above pit becomes closed is exactly that which is
permanent in Nautilus (_vide_ fig. 122 A). After the pit has become
closed a mesoblastic layer grows in between its wall and the external
epiblast.
The lens becomes formed in two independent segments. The inner and
larger of these arises as a rod-like process (fig. 124) projecting
from the front wall of the optic vesicle into the cavity of the
vesicle. It is a cuticular structure and therefore without cells. By
the deposition of a series of concentric layers it soon assumes a
spherical form (fig. 125, _hl_). The condition of the eye, with a
closed optic vesicle and the lens projecting into it, is that which is
permanent in the majority of Gasteropods (_vide_ fig. 122 B). At about
the time when the lens first becomes formed a fold composed of
epiblast and mesoblast appears round the edge of the optic cup (fig.
124, _cc_), and gives rise to a structure known in the adult as the
iris. Shortly afterwards this becomes more prominent (fig. 125, _if_),
and at the same time the layers of cells of the ciliary region in
front of the inner segment of the lens become reduced to the condition
of mere membranes (fig. 125 B); and in front of them the anterior or
outer segment of the lens becomes formed as a cuticular deposit (fig.
125 B, _vl_). At a still later period a fresh fold of epiblast and
mesoblast appears round the eye and gradually constitutes the anterior
optic chamber (_vide_ fig. 122 C, _Co_). In most forms this chamber
communicates with the exterior by a small aperture, but in some it is
completely closed. The fold itself gives rise to the cornea in front
and to the sclerotic at the sides. At a later period another fold may
appear forming the eyelids (fig. 122 C, _Pal_).
[FIG. 124. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED EMBRYO
OF LOLIGO. (After Bobretzky.)
_vd._ oesophagus; _gls._ salivary gland; _g.vs._ visceral ganglion;
_gc._ cerebral ganglion; _g.op._ optic ganglion; _adk._ optic
cartilage; _ak._ and _y._ lateral cartilage or (?) white body; _rt._
retina; _gm._ limiting membrane; _vk._ ciliary region of eye; _cc._
iris; _ac._ auditory sack (the epithelium lining the auditory sacks
is not represented); _vc._ vena cava; _ff._ folds of funnel.]
Auditory organs. A pair of auditory sacks is found in the larvæ of
almost all Gasteropods and Pteropods, and usually originates very
early. They are placed in the front part of the foot, and on the
formation of the pedal ganglia come into close connection with it,
though they receive their nervous supply in the adult from the
supra-oesophageal ganglia.
In a very considerable number of cases amongst Gasteropods and
Pteropods the auditory organs have been observed to develop as
invaginations of the epiblast, which give rise to closed vesicles
lying in the foot, _e.g._ Paludina, Nassa, Heteropods, Limax, some
Pteropods (Clio).
This is no doubt the primitive mode of origin, but in other cases,
which perhaps require confirmation, the sacks are stated to originate
from a differentiation of solid thickenings of the epidermis or of the
tissues subjacent to it.
The auditory sacks are provided with an otolith, which according to
Fol's observations is first formed in the wall of the sack.
In Cephalopods the auditory organs are formed as epiblastic pits on
the posterior surface of the embryo, and are at first widely separated
(fig. 113, _ac_). The openings of the pits become narrowed, and
finally the original pits form small sacks lined by an epithelium, and
communicating with the exterior by narrow ducts, equivalent to the
_recessus vestibuli_ of Vertebrates, and named, after their
discoverer, Kölliker's ducts. The external openings of these ducts
become completely closed at about the same time as the shell-gland,
and the ducts remain as ciliated diverticula of the auditory pits. The
widely separated auditory sacks gradually approach in the middle
ventral line, and are immediately invested by the visceral ganglia
(fig. 124, _ac_). They finally come to lie in contact on the inner
side of the funnel.
On the side opposite Kölliker's duct, an epithelial ridge is
formed--the _crista acustica_--the cells of which give rise to an
otolith connected with the crista by a granular material. At a later
period of development three regions of the epithelium of the sack
become especially differentiated. Each of these regions is provided
with two rows of cells, bearing on their free edges numerous very
short auditory hairs. The cells of each row are placed nearly at right
angles to those of the adjoining row.
[FIG. 125. SECTIONS THROUGH THE DEVELOPING EYE OF LOLIGO AT TWO
STAGES. (After Bobretzky.)
_hl._ inner segment of lens; _vl._ outer segment of lens; _a_ and
_a´_. epithelium lining the anterior optic chamber; _gz._ large
epiblast cells of ciliary body; _cc._ small epiblast cells of
ciliary body; _ms._ layer of mesoblast between the two epiblastic
layers of the ciliary body; _af._ and _if._ fold of iris; _rt._
retina; _r´´._ inner layer of retina; _st._ rods; _aq._ equatorial
cartilage.]
Muscular system. The muscular system in all groups of Molluscs is
derived entirely from the mesoblast.
The greater part of the system takes its origin from the somatic
mesoblast. In almost all Gasteropod and Pteropod larvæ there is
present a well-developed spindle muscle attaching the embryo to the
shell. This muscle appears to be absent in the Cephalopoda.
Body cavity and vascular system. The body cavity in Gasteropods and
Pteropods originates either by a definite splitting of the mesoblast,
or by the appearance of intercellular spaces. It becomes divided into
numerous sinuses which freely communicate with the vascular system.
Very different accounts have been given by different investigators of
the development of the heart in the Gasteropoda and Pteropoda.
It would seem however in most cases to arise as a solid mass of
mesoblast cells at the hind end of the pallial cavity, which
subsequently becomes hollowed out and divided into an auricle and
ventricle. Bobretzky's careful observations have fully established
this mode of development for Nassa.
In Pteropods the heart is formed (Fol) close to the anus, but slightly
dorsal to it (fig. 108, _h_). The pericardium is formed from the
mesoblast at a considerably later period than the heart.
A very different account of the formation of the heart is given by
Bütschli for Paludina. He states that there appears an immense
contractile sack on the left side of the body. This becomes
subsequently reduced in size, and in the middle of it appears the
heart, probably from a fold of its wall. The original sack would
appear to give rise to the pericardium.
In connection with the vascular system mention may be made of certain
contractile sinuses frequently found in the larvæ of Gasteropoda and
Pteropoda. One of these is placed at the base of the foot, and the
other on the dorsal surface within the mantle cavity immediately below
the velum[114]. The completeness of the differentiation of these
sinuses varies considerably; in some forms they are true sacks with
definite walls, in other cases mere spaces traversed by muscular
strands. They are found in the majority of marine Gasteropods,
Heteropods and Pteropods. In Limax a large posteriorly placed pedal
sinus is well developed, and there is also a sinus in the visceral
sack. The rhythmical contraction of the yolk-sack of Cephalopods
appears to be a phenomenon of the same nature as the contraction of
the foot sinus of Limax.
[114] Rabl holds that there is no contractile dorsal sinus, but
that the appearance of contraction there is due to the
contractions of the foot.
In Calyptræa (Salensky) there is an enormous provisional cephalic
dilatation within the velum which does not appear to be contractile.
Similar though less marked cephalic vesicles are found in Fusus,
Buccinum and most marine Gasteropods.
In Cephalopods the vascular system is formed by a series of
independent (?) spaces originating in the mesoblast, the cells around
which give rise to the walls of the vessels. The branchial hearts are
formed at about the time at which the shell-gland becomes closed. The
aortic heart (fig. 127, _c_) is formed of two independent halves which
subsequently coalesce (Bobretzky).
The true body cavity arises as a space in the mesoblast subsequently
to the formation of the main vascular trunks.
Renal organs. Amongst the Gasteropods and Pteropods there are present
provisional renal organs, which may be of two kinds, and a permanent
renal organ.
The provisional organs consist of either (1) an external paired mass
of excretory cells or (2) an internal organ provided with a duct,
which is not in all cases certainly known to open externally. The
former structure is found especially in the marine Prosobranchiates
(Nassa, etc.) where it has been fully studied by Bobretzky. It
consists of a mass of cells on each side of the body, close to the
base of the foot, and not far behind the velum. This mass grows very
large, and below it may be seen a continuous layer of epiblast. The
cells forming it fuse together, their nuclei disappear, and numerous
vacuoles containing concretions arise in them. At a later stage all
the vacuoles unite together and form a cavity filled with a brown
granular mass.
The provisional internal renal organ is found in many pulmonate
Gasteropods--Lymnæus, Planorbis, etc. It consists of a paired V-shaped
ciliated tube with a pedal and cephalic limb. The former has an
external opening, but the termination of the latter is still in doubt.
It consists, according to Büschli's description (No. 244), in the
freshwater Pulmonata (Lymnæus, Planorbis) of a round sack, close to
the head, opening by an elongated and richly ciliated tube in the
neighbourhood of the eye. From the sack a second shorter tube passes
off towards the foot, which seems however to end blindly. The cells
lining the sack contain concretions, and there is one especially large
cell in the lumen of the sack attached on the side turned towards the
eye. It coexists in Lymnæus with provisional renal organs of the type
of those in marine Prosobranchiata.
A somewhat different description of the structure and development of
this organ in Planorbis has recently been given by Rabl (No. 268). It
consists of a V-shaped tube on each side with both extremities opening
into the body cavity. The one limb is directed towards the velar area,
the other towards the foot. It is developed from the mesoblast cells
of the anterior part of the mesoblastic band. The large mesoblast (p.
227) of each side grows into two processes, the two limbs of the
future organ. A lumen in the cell is continued into each limb, while
continuations of the two limbs of the V are formed from the hollowing
out of the central parts of the adjoining mesoblast cells.
In Limax embryos Gegenbaur found a pair of elongated provisional
branched renal sacks, the walls of which contained concretions. These
sacks are provided with anteriorly directed ducts opening on the
dorsal side of the mouth. This organ is probably of the same nature as
the provisional renal organ in other Pulmonata.
_Permanent renal organ._ According to the most recent observer (Rabl,
No. 268), whose statements are supported by the sections figured, the
permanent renal organ in Gasteropods is developed from a mass of
mesoblast cells close to the end of the intestine. This is first
carried somewhat to the left side, and then becomes elongated and
hollow, and attaches itself to the epiblast on the left side of the
anus (fig. 108, _r_). After the formation of the heart the inner end
opens into the pericardium and becomes ciliated, the median part
becomes glandular and concrements appear in its lining cells, and the
terminal part forms the duct.
Previous observers have usually derived this organ from the epiblast;
according to Rabl this is owing to their having studied too late a
stage in the development.
In Cephalopoda the excretory sacks or organ of Bojanus are apparently
differentiations of the mesoblast[115]. At an early stage part of
their walls envelops the branchial veins. From this part of the wall
the true glandular section of the organ would seem to be formed. The
epithelium forming the inner wall of each sack is at an early age very
columnar.
[115] I conclude this from Bobretzky's figures.
The development of the organ of Bojanus in Lamellibranchiata has been
studied by Lankester. He finds that it develops as a paired
invagination of the epiblast immediately ventral to the anus.
Generative glands. The generative glands in Mollusca would appear to
be usually developed in the post-larval period, but our knowledge on
this subject is extremely scanty.
In Pteropods Fol believes that he has proved that the hermaphrodite
gland originates from two independent formations, one (the testicular)
epiblastic in origin, and the other (the ovarian) hypoblastic.
These views of Fol do not appear to me nearly sufficiently
substantiated to be at present accepted.
The generative glands in Cephalopoda appear to be simple
differentiations of the mesoblast. They are at first very closely
connected with the aortic heart (fig. 127, _kd_), but soon become
completely separated from it.
Alimentary tract. The formation of the archenteron, and the relation
of its opening to the permanent mouth and anus, has already been
described and needs no further elucidation. It will be convenient to
treat the subject of this section under three headings for each
group--viz. (1) the mesenteron, (2) the stomodæum, and (3) the
proctodæum.
_The mesenteron._ In the Gasteropoda and Pteropoda the mesenteron, as
has already been mentioned, forms a simple sack, which may however,
owing to the presence of food-yolk, be at first without a lumen. Of
this sack an anterior portion gives rise to the stomach and liver, and
a posterior to the intestine. This latter portion is the first to be
distinctly differentiated as such, and forms a narrowish tube
connecting the anterior dilatation with the anus. In the meantime the
cells of a great part of the anterior portion of the mesenteron
undergo peculiar changes. They enlarge, and in each of them a deposit
of food material appears, which is often at any rate derived from the
absorption of the albumen in which the embryo floats. The cells on the
dorsal side, adjoining the oesophageal invagination, and the whole of
the cells on the ventral side do not however undergo these changes.
There thus arises an anterior and ventral region adjoining the
oesophagus, which becomes completely enclosed by small cells and forms
the true stomach. The part behind and dorsal to the stomach is lined
by the large nutritive cells and forms the liver. It opens into the
stomach at the junction of the latter with the intestine, which in the
later stages becomes bent somewhat forwards and to the right. Still
later the hepatic region becomes branched, the albuminous contents of
its cells are replaced by a coloured secretion, and it becomes bodily
converted into the liver. The stomach is usually richly ciliated.
The various modifications of the above type of development of the
alimentary tract are to be regarded as due to the disturbing influence
of food-yolk. Where primitively the hypoblast cells are very bulky,
though invaginated in a normal way, the wall of the hepatic region
becomes immensely swollen with food-yolk, _e.g._ Nautica. In other
cases amongst certain Pteropods (Fol, No. 249) where the hypoblast is
still more bulky, part of the archenteric walls becomes converted into
a bilobed sack opening into the pyloric region, in the walls of which
a large deposit of food material is stored, which gradually passes
into the remainder of the alimentary tract and is there digested. The
bilobed nutritive sack, as it is called by Fol, is eventually
completely absorbed, though the liver in some, if not all cases, grows
out as a fresh sack from its duct.
The formation of the permanent alimentary tract, when the hypoblast is
so bulky that there is no true archenteric cavity, has been especially
investigated by Bobretzky (No. 242).
In the case of a species of Fusus the hypoblast, when enclosed by the
epiblast, is composed of four cells only. The blastopore remains
permanently open at the oral region, and around it the oesophagus
grows in a wall-like fashion. The protoplasmic portions of the four
hypoblast cells are turned towards the oesophageal opening, and from
them are budded off small cells which are continuous at the blastopore
with the epiblast of the oesophagus. These cells give rise posteriorly
to the intestine and anteriorly to the sack, which becomes the stomach
and liver. This sack always remains open towards the four primitive
yolk cells. The cells of the posterior part of it become larger and
larger and form the hepatic sack, which fills up the left and
posterior part of the visceral sack, pushing the yolk cells to the
right. The cells lining the hepatic sack become pyramidal in shape,
and each of them is filled with a peculiar mass of albuminous
material. The cells adjoining the opening of the oesophagus remain
small, become ciliated, and form the stomach. They are not sharply
separated off from the cells of the hepatic sack. The yolk cells
remain distinct on the right side of the body during larval life, and
their food material is gradually absorbed for the nutrition of the
embryo.
A modification of the above mode of development, where the food
material is still more bulky and the blastopore closed, is found in
Nassa, and has already been described (_vide_ p. 233).
_The stomodæum._ The stomodæum in most cases is formed as a simple
epiblastic invagination which meets and opens into the mesenteron.
When the blastopore remains permanently open at the oral region the
stomodæum is formed as an epiblastic wall round its opening. In all
cases the stomodæum gives rise to the mouth and oesophagus. At a
subsequent period there are developed in the oral region of the
stomodæum the radula in a special ventral pit, and the salivary
glands--the latter as simple outgrowths.
The oesophagus is usually ciliated.
_The proctodæum._ Except where the blastopore remains as the permanent
anus (Paludina) the proctodæum is always formed subsequently to the
mouth. Its formation is usually preluded by the appearance of two
projecting epiblast cells, but it is always developed as a very
shallow epiblastic invagination, which does not give rise to any part
of the true intestine.
In the Cephalopods the alimentary tract is formed, as in other
cephalophorous Mollusca, of three sections. (1) A stomodæum, formed by
an epiblastic invagination, which gives rise to the mouth, oesophagus
and salivary glands. (2) A proctodæum, which is an extremely small
epiblastic invagination. (3) A mesenteron, lined by true hypoblast,
which forms the main section of the alimentary tract, viz. the
stomach, intestine, the liver, and ink sack[116].
[116] The following description applies specially to Loligo.
[FIG. 126. LONGITUDINAL VERTICAL SECTION THROUGH A LOLIGO OVUM WHEN
THE MESENTERIC CAVITY IS JUST COMMENCING TO BE FORMED. (After
Bobretzky.)
_gls._ salivary gland; _brd._ sheath of radula; _oe._ oesophagus;
_ds._ yolk-sack; _chs._ shell-gland; mt. mantle; _pdh._ mesenteron;
_x._ epiblastic thickening between the folds of the funnel.]
_The mesenteron._ The mesenteron is first visible from the surface as
a small tubercle on the posterior side of the mantle between the
rudiments of the two gills (fig. 111 B, _an_). Within this, as was
first shewn by Lankester, a cavity appears.
This cavity is as in Gasteropods open to the yolk-sack, and only
separated from the yolk itself by the yolk membrane already spoken of.
It is at first lined by indifferent cells of the lower layer of the
blastoderm, which however soon become columnar and form a definite
hypoblastic layer (fig. 126, _pdh_). Between the hypoblast and
epiblast there is a very well marked layer of mesoblast. As the
mesenteric cavity extends, its walls meet the epiblast, and at the
point of contact of the two layers the epiblast becomes slightly
pitted in. At this point the anus is formed at a considerably later
period (fig. 127, _an_).
On the ventral side of the primitive mesenteron an outgrowth appears
very early, which becomes the ink sack (fig. 127, _bi_).
[FIG. 127. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF
LOLIGO. (After Bobretzky.)
_os._ mouth; _gls._ salivary gland; _brd._ sheath of radula; _ao._
anterior aorta; _ao1._ posterior aorta; _va._ branch of posterior
aorta to shell sack; _ma._ branch of posterior aorta to mantle; _c._
aortic heart; _oe._ oesophagus; _mg._ stomach; _an._ anus; _bi._ ink
sack; _kd._ germinal tissue; _eih._ shell sack; _vc._ vena cava;
_g.vs._ visceral ganglion; _g.pd._ pedal ganglion; _ac._ auditory
sack; _tr._ funnel.]
The mesenteric cavity, still open to the yolk, gradually extends
itself in a dorsal direction over the yolk-sack, but remains for some
time completely open to it ventrally, and only separated from the
actual yolk by the yolk membrane. There early grow out from the walls
of the mesenteron a pair of hepatic diverticula.
As the mesenteric cavity extends it dilates at its distal extremity
into a chamber destined to form the stomach (fig. 127, _mg_). At about
this time the anus becomes perforated. Shortly afterwards the
mesenteron meets and opens into the oesophagus at the dorsal extremity
of the yolk-sack, but at the time when this takes place the hypoblast
has extended round the entire cavity, and has shut it off from the
yolk. The yolk membrane throughout the whole of this period is quite
passive, and has no share in forming the walls of the alimentary
tract.
_The stomodæeum._ The stomodæum appears as an epiblastic invagination
at the anterior side of the blastoderm, before any trace of the
mesenteron is present. It rapidly grows deeper, and, shortly after the
mesenteric cavity becomes formed, an outgrowth arises from its wall
adjoining the yolk-sack, which gives rise to the salivary glands
(figs. 126 and 127, _gls_). Immediately behind the opening of the
salivary glands there appears on its floor a swelling which becomes
the odontophore, and behind this a pocket of the stomodæal wall forms
the sheath of the radula (figs. 126 and 127, _brd_). Behind this again
the oesophagus is continued dorsalwards as a very narrow tube, which
eventually opens into the stomach (fig. 127).
The terminal portion of the rudiment of the salivary gland divides
into two parts, each of which sends out numerous diverticula which
constitute the permanent glands. The greater part of the original
outgrowth remains as the unpaired duct of the two glands[117].
[117] In Loligo only a single pair of salivary glands is present.
In the larva observed by Grenacher the anterior pair of salivary
glands originated from independent lateral outgrowths of the floor of
the mouth, close to the opening of the posterior salivary glands.
_The yolk-sack of the Cephalopoda._ The yolk, as has already been
stated, becomes at an early period completely enclosed in a membrane
formed of flattened cells, which constitutes a definite yolk-sack. It
is, in the more typical forms of Cephalopoda, divided into an external
and an internal section, of which the former is probably a special
differentiation of the median part of the foot of other cephalophorous
Mollusca (_vide_ p. 272). At no period does the yolk-sack communicate
with the alimentary tract. The two sections of the yolk-sack are at
first not separated by a constriction. In the second half of embryonic
life the condition of the yolk-sack undergoes considerable changes.
The internal part grows greatly in size at the expense of the
external, and the latter diminishes very rapidly and becomes
constricted off from the internal part of the sack, with which it
remains connected by a narrow vitelline duct.
The internal yolk-sack becomes divided into three sections: a dilated
section in the head, a narrow section in the neck, and an enormously
developed portion in the mantle region. It is the latter part which
mainly grows at the expense of the external yolk-sack. It gives off at
its dorsal end two lobes, which pass round and embrace the lower part
of the oesophagus. The passage of the yolk from the external to the
internal yolk-sack is probably largely due to the contractions of the
former.
The external yolk-sack is not vascular, and probably the absorption of
the yolk for the nutrition of the embryo can only take place in the
internal yolk-sack. The most remarkable feature of the Cephalopod
yolk-sack is the fact that it lies on the opposite side of the
alimentary tract to the yolk cells, which form a rudimentary yolk-sack
in such Gasteropoda as Nassa and Fusus. In these forms, the yolk-sack
is at first dorsal, but subsequently is carried by the growth of the
liver to the right side. In Cephalopoda on the contrary, the yolk-sack
is placed on the ventral side of the body.
What is known of the development of the alimentary tract in the
Polyplacophora has already been mentioned.
In the Lamellibranchiata (Lankester, No. 239), the mesenteron early
grows out into two lateral lobes which form the liver, while the part
between them forms the stomach.
In Pisidium the intestine is formed from the original pedicle of
invagination, which remains permanently attached to the epiblast. The
stomodæum is formed by the usual epiblastic invagination, and becomes
the mouth and oesophagus. The development of the crystalline rod and
its sack do not appear to be known. In the adult the sack of the
crystalline rod opens into a part of the alimentary tract which
appears to belong to the mesenteron. Were however the development to
shew them to be really derived from the stomodæum they might be
interpreted as rudiments of the organ which constitutes the
odontophore and its sack in cephalophorous Mollusca--an interpretation
which would be of considerable phylogenetic interest.
BIBLIOGRAPHY.
_General._
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(249) H. Fol. "Sur le développement des Ptéropodes." _Archiv. de Zool.
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(250) H. Fol. "Sur le développement des Gastéropodes pulmonés."
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(251) H. Fol. "Sur le développement des Hétéropodes." _Archiv. de
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(252) C. Gegenbaur. "Beit. z. Entwicklungsgesch. der Landgasteropoden."
_Zeitschr. f. w. Zool._, Vol. III. 1851.
(253) C. Gegenbaur. _Untersuch. üb. Pteropoden u. Heteropoden_,
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(254) H. von Jhering. "Entwicklungsgeschichte von Helix." _Jenaische
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(255) W. Keferstein and E. Ehlers. "Beob. üb. d. Entwick. v. Æolis
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(256) J. Koren and D. C. Danielssen. "Bemärk. til Mollusk Udvikling."
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(258) A. Krohn. "Beobacht. aus d. Entwickl. der Pteropoden u.
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(260) H. de Lacaze-Duthiers. "Mém. sur l'anat. et l'embryog. des
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_Zeitschr. f. w. Zool._, Vol. XXIII. 1873.
(262) E. R. Lankester. "On the development of the Pond-Snail." _Quart.
J. of Micr. Scie._, Vol. XIV. 1874.
(263) E. R. Lankester. "On the coincidence of the blastopore and anus
in Paludina vivipara." _Quart. J. of Micr. Scie._, Vol. XVI. 1876.
(264) F. Leydig. "Ueber Paludina vivipara." _Zeitschr. f. w. Zool._,
Vol. II. 1850.
(265) J. Müller. _Ueber Synapta dig. u. üb. d. Erzeug. v. Schnecken in
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(266) J. Müller. "Bemerk. aus d. Entwickl. der Pteropoden."
_Monatsber. Berl. Akad._, 1857.
(267) C. Rabl. "Die Ontogenie d. Süsswasser-Pulmonaten." _Jenaische
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(268) C. Rabl. "Ueb. d. Entwick. d. Tellerschnecke (Planorbis)."
_Morph. Jahrbuch_, Vol. V. 1879.
(269) W. Salensky. "Beitr. zur Entwickl. d. Prosobr." _Zeitschr. f. w.
Zool._, Vol. XXII. 1872.
(270) O. Schmidt. "Ueb. Entwick. von Limax agrestis." Müller's
_Archiv_, 1851.
(271) Max S. Schultze. "Ueber d. Entwick. des Tergipes lacinulatus."
_Arch. f. Naturg._, Jahrg. XV. 1849.
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Zool._, Vol. I. 1871.
(273) E. Selenka. "Die Anlage d. Keimbl. bei Purpura lapillus."
_Niederl. Arch. f. Zool._, Vol. I. 1872.
(274) C. Semper. "Entwickl. der Ampullaria polita, etc." _Natuurk.
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(275) An. Stecker. "Furchung u. Keimblatterbildung bei Calyptræa."
_Morphol. Jahrbuch_, Vol. II. 1876.
(276) A. Stuart. "Ueb. d. Entwickl. einiger Opisthobr." _Zeitschr. f.
w. Zool._, Vol. XV. 1865.
(277) N. A. Warneck. "Ueber d. Bild. u. Entwick. d. Embryos bei
Gasterop." _Bullet. Soc. natural. de Moscou_, T. XXIII. 1850.
_Cephalopoda._
(278) P. J. van Beneden. "Recherches sur l'Embryogénie des Sépioles."
_Nouv. Mém. Acad. Roy. de Bruxelles_, Vol. XIV. 1841.
(279) N. Bobretzky. Observation on the development of the Cephalopoda
(Russian). _Nachrichten d. kaiserlichen Gesell. d. Freunde der
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(280) H. Grenacher. "Zur Entwicklungsgeschichte d. Cephalopoden."
_Zeit. f. wiss. Zool._, Bd. XXIV. 1874.
(281) A. Kölliker. _Entwicklungsgeschichte d. Cephalopoden._ Zurich,
1844.
(282) E. R. Lankester. "Observations on the development of the
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(283) E. Metschnikoff. "Le développement des Sépioles." _Archiv d. Sc.
phys. et nat._, Vol. XXX. Genève, 1867.
_Polyplacophora._
(284) A. Kowalevsky. "Ueb. d. Entwick. d. Chitonen." _Zoologischer
Anzeiger_, No. 37. 1879.
(285) S. L. Lovén. "Om utvecklingen hos slägtet Chiton." _Stockholm
öfversigt_, XII. 1855. [Vide also _Ann. and Mag. of Nat. Hist._, Vol.
XVII. 1856, and _Archiv f. Naturgeschichte_, 1856.]
_Scaphopoda._
(286) H. Lacaze-Duthiers. "Développement du Dentale." _Ann. d. Sci.
Nat._, Series IV. Vol. VII. 1857.
_Lamellibranchiata._
(287) M. Braun. "Postembryonale Entwicklung d. Süsswasser-Muscheln."
_Zoologischer Garten_.
(288) C. G. Carus. "Neue Untersuch. üb. d. Entwickl. unserer
Flussmuschel." _Verh. Leop.-Car. Akad._, Vol. XVI. 1832.
(289) W. Flemming. "Studien in d. Entwicklungsgeschichte der Najaden."
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(290) F. Leydig. "Ueber Cyclas Cornea." Müller's _Archiv_, 1855.
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Lamellibr." _Vetensk. Akad. Handl._, 1848. [Vide also _Arch. f.
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(292) C. Rabl. "Ueber d. Entwicklungsgeschichte d. Malermuschel."
_Jenaische Zeitschrift_, Vol. X. 1876.
(293) W. Salensky. "Bemerkungen über Haeckels Gastræa-Theorie
(Ostrea)." _Arch. f. Naturg._, 1874.
(294) O. Schmidt. "Ueb. d. Entwick. von Cyclas calyculata." Müller's
_Arch._, 1854.
(295) O. Schmidt. "Zur Entwickl. der Najaden." _Wien, Sitzungsber.
math.-nat. Cl._, Vol. XIX. 1856.
(296) P. Stepanoff. "Ueber die Geschlechtsorgane u. die Entwicklung
von Cyclas." _Archiv f. Naturgeschichte_, 1865.
(297) H. Lacaze-Duthiers. "Développement d. branchies d. Mollusques
Acéphales." _An. Sc. Nat._, Ser. IV. Vol. V. 1856.
CHAPTER X.
POLYZOA[118].
[118] The classification of the Polyzoa adopted in this chapter
is shewn in the subjoined table:
I. Entoprocta.
II. Ectoprocta.
{_a._ Chilostomata.
1. GYMNOLÆMATA {_b._ Ctenostomata.
{_c._ Cyclostomata.
2. PHYLACTOLÆMATA.
3. PODOSTOMATA (_Rhabdopleura_).
ENTOPROCTA.
The development of the larvæ of Pedicellina is known from the
researches of Hatschek (No. 299) far more completely than that of
Loxosoma, though it does not apparently differ from it except in
certain details. In both the known Entoproctous genera the
segmentation is regular or nearly so, though Hatschek believes that he
has detected in Pedicellina a slight difference between the two first
segmentation spheres, and regards them as constituting the animal and
vegetative poles of the embryo. The segmentation in Pedicellina, to
which genus alone the remainder of the description applies, results in
the formation of a single-layered blastosphere, with a small
segmentation cavity, in which the animal and vegetative poles can
readily be distinguished owing to the smaller size of the cells at the
animal pole.
The hypoblast cells and the vegetative[119] pole become invaginated in
the normal manner (fig. 128 A), the blastopore becomes narrowed to a
slit with an anteroposterior direction, _i.e._ parallel to the line
connecting the mouth and anus in the adult. At the hinder extremity of
the blastopore there are present two conspicuously large cells (fig.
128 B, _me_), one on each side of the middle line. These cells give
rise to the mesoblast. On the completion of the invagination the
mesoblasts become covered by the epiblast (fig. 128 C, _me_). The
blastopore then closes, but in the position it occupied the epiblast
becomes thickened to form the rudiment of the vestibule, which at this
stage constitutes a disc marked off by a shallow groove from the
remainder of the body.
[119] The succeeding statements about the gastrula are derived
from Hatschek. According to Salensky a segmentation cavity is not
present, and the hypoblast would seem to be formed by
delamination or epibole. Barrois finds a gastrula in both
Loxosoma and Pedicellina, but gives no details. Uljanin finds a
segmentation cavity in Pedicellina, and Schmidt would appear to
have observed a gastrula stage in Loxosoma. None of the accounts
we have can be compared in fulness of detail to that of Hatschek.
[FIG. 128. THREE STAGES IN THE DEVELOPMENT OF PEDICELLINA ECHINATA.
(After Hatschek.)
_s.c._ segmentation cavity; _a.e._ archenteron; _ep._ epiblast;
_me._ mesoblast; _hy._ hypoblast.
A is the commencing gastrula stage from the side in optical section.
B is a slightly later stage from above in optical section. It shews
the two primitive mesoblast cells.
C is a later stage after the closure of the blastopore, viewed from
the side in optical section.]
At the anterior extremity of this disc an invagination arises to form
the oesophagus (fig. 129 A, _oe_); and not long afterwards a posterior
invagination to form the rectum (fig. 129 B, _an.i_). The oral disc
and the oesophagus are richly ciliated. The oesophagus first, and
afterwards the rectum unite with the archenteron (fig. 130), the walls
of which soon become differentiated into a stomach and intestine, and
on the upper wall of the former the hepatic cells become especially
conspicuous (fig. 130).
During the completion of the alimentary canal a number of important
structures is formed. The disc in which the oral and anal apertures
are situated becomes converted into a true vestibule. On its floor,
between the mouth and the anus, there arises a marked prominence with
a tuft of cilia (fig. 130 B), which persists in the adult.
[FIG. 129. TWO STAGES IN THE DEVELOPMENT OF PEDICELLINA. (After
Hatschek.)
_oe._ oesophagus; _ae._ archenteron; _an.i._ anal invagination; _f._
fold of epiblast; _f.g._ ciliated disc; _x._ problematical body
derived from hypoblast (probably a bud).]
This prominence is perhaps equivalent to the epistome of the
Phylactolæmata and the disc-like organ of Rhabdopleura, which
Lankester has compared to the molluscan foot[120].
[120] Lankester. "Remarks on the Affinities of Rhabdopleura."
_Quart. J. of Micro. Science_, Vol. XIV. 1874.
Very shortly after the first formation of the vestibule there appears
at the opposite end of the larva a thickening of the epiblast, which
soon becomes invaginated, and forms an eversible pit (fig. 129 A and
B, _f.g._). Round its mouth there is formed a ring of stiff cilia
(fig. 130, _f.g._). This organ is very possibly equivalent to the
cement gland described by Kowalevsky in the adult Loxosoma. I shall
speak of it as the ciliated disc.
The epiblast cells early secrete a cuticle.
The two mesoblast cells soon increase by division, and occupy the
space between the alimentary canal and the body wall. They do not
become divided into a splanchnic and somatic layer; but give rise to
the interstitial connective tissue and muscles. From the mesoblast
there is also formed, according to Hatschek, a pair of ciliated
excretory canals, in the space between the mouth and anus (fig. 130 B,
_nph._). The development of the nervous system has not been observed.
At a comparatively late stage in the development there is formed round
the edge of the vestibule a ring of long cilia (fig. 130 B, _m._).
[FIG. 130. TWO STAGES IN THE DEVELOPMENT OF PEDICELLINA. (After
Hatschek.)
_v._ vestibule; _m._ mouth; _l._ liver; _hg._ hind-gut; _a._ anus;
_an.i._ anal invagination; _nph._ duct of kidney; _fg._ ciliated
disc; _x._ dorsal organ (probably bud).]
A remarkable organ makes its appearance on the dorsal side of the
oesophagus (the side opposite the adult ganglion) formed of an oval
mass of cells attached to the epiblast at the apex of a small ciliated
papilla (fig. 130 A and B, _x._). This organ will be spoken of as the
dorsal organ. According to Hatschek it develops as a solid outgrowth
of the hypoblastic walls of the mesenteron shortly before the
mesenteron joins the oesophagus (fig. 129 B, _x._). The cells
composing it arrange themselves as a sack, which acquires an external
opening on the dorsal surface (fig. 130 A, _x._). In a later stage the
lumen of the sack disappears, but at the junction of the organ with
the epiblast a pit is formed, lined with ciliated cells, which is
capable of being protruded as a papilla. The organ itself becomes
invested by a lining of cells, which Hatschek regards as mesoblastic.
A nearly similar organ to this is found in the embryo of Loxosoma
[Vogt (No. 302) and Barrois (No. 298)]. Here however it is double, and
forms a kind of disc connected with two eye spots.
Hatschek has made with reference to the dorsal organ the extremely
plausible suggestion that it is a rudimentary bud, and that the
hypoblastic sack it contains gives rise to the hypoblast of the young
polype developed from the bud. Although, owing to the deficiency of
our observations on the attachment of the larva, this suggestion has
not received direct confirmation, yet the relations of dorsal organs
in Pedicellina and Loxosoma respectively strongly confirm Hatschek's
view of their nature. Both of these forms increase in the adult state
by budding: in Pedicellina there is a single row of buds formed
successively on the dorsal side of the stem, corresponding with the
single dorsal organ of the embryo; while in Loxosoma a double row of
buds, right and left, is formed, in correspondence with the double
nature of the dorsal organ.
As to the mode of attachment of the embryo next to nothing is known,
the few observations we have being due to Barrois. From these
observations it would appear probable that the larva, as is usual
amongst Polyzoa, does not become directly converted into the permanent
form, but that, on becoming fixed, it undergoes a metamorphosis in the
course of which its organs atrophy. I would venture to suggest that
the whole free-swimming larva atrophies, while the dorsal organ alone
develops into the fixed form[121].
[121] My view of the metamorphosis which takes place during the
fixation of the larva involves the supposition that in Loxosoma,
about the attachment of which we know absolutely nothing, two
buds are directly formed in accordance with the double nature of
the dorsal organ.
Although the changes which take place during budding do not fall
within the province of this work, it may be well to state that
Hatschek has observed during this process the development of the
nervous system and the generative organs. The nervous system arises as
an unpaired thickening of the epiblastic floor of the vestibule,
between the mouth and the anus. On becoming constricted off from the
epiblast the nerve ganglion contains a central cavity which afterwards
vanishes.
The generative organs originate as a pair of specially large mesoblast
cells in the space between the stomach and the floor of the vestibule.
These two cells, surrounded by an investment of flattened mesoblast
cells, subsequently divide and form two masses. At a still later
period each mass divides into an anterior and a posterior part; the
former giving rise to the ovary, the latter to the testis. The
similarity of this mode of development of the generative organs to
that observed by Bütschli in Sagitta, which is described in the
sequel, is very striking.
ECTOPROCTA.
Although the embryology of the Ectoprocta has been investigated by a
very considerable number of the distinguished naturalists of the
century, many points connected with it still stand in great need of
further elucidation. The original nature of the embryo was rightly
interpreted by Grant, Dalyell and other naturalists, but it was not
till Huxley demonstrated the presence of both the ovary and testis
that the true sexual origin of the embryo in the ovicells became an
established fact in science. The recent memoir of Barrois (No. 298),
though it contains the record of a vast amount of research, and marks
a great advance in our knowledge, still leaves a great number of
points, both with reference to the early development and to the larval
metamorphosis in a very unsatisfactory condition.
Four larval forms can be distinguished, viz.
(1) A larval form which with slight modifications is common to all the
genera of the Chilostomata (except Membranipora and Flustrella) and of
the Ctenostomata.
(2) A bivalved larva of Membranipora known as _Cyphonautes_, the true
nature of which was first recognized by Schneider (No. 322), and the
closely allied larva of Flustrella.
(3) The typical Cyclostomatous larva, for the first full description
of which we are indebted to Barrois (No. 298).
(4) The larva of the Phylactolæmata.
Chilostomata and Ctenostomata. As an example of the first type of
larvæ, _Alcyonidium mytili_, one of the Ctenostomata, may be
conveniently selected for description, as having been more completely
worked out by Barrois than perhaps any other form. The segmentation
commences in the normal manner by the appearance of two vertical
furrows followed by an equatorial furrow, which divide the ovum into
eight equal spheres. The stage with eight spheres is followed,
according to Barrois, by one with sixteen, formed in a remarkable
manner by the simultaneous appearance of two vertical furrows, _both
parallel to one_ of the original vertical furrows, so that the
segmentation spheres at this stage are arranged in two layers of eight
each. In the next stage segmentation takes place along two fresh
vertical planes, similar to those of the last stage, but at right
angles to them, and therefore parallel to the second of the two
primitive vertical furrows. At the close of this stage there are
thirty-two cells arranged in two layers of sixteen each; and when
viewed from the surface each of these layers presents a regularly
symmetrical pattern. Up to the stage with sixteen cells the two poles
of the egg, separated by the primitive equatorial plane of
segmentation, remain equal, but during the stage with thirty-two cells
a peculiar change takes place in the character of the cells at the two
poles. At the one pole, which will be spoken of as the oral pole, the
four central cells become much larger than the twelve peripheral
cells.
The stages immediately following are still involved in much obscurity,
and have been described very differently by Barrois in his original
memoir (No. 298), and in a subsequent note (No. 307)[122]. In the
latter he states that the four large cells of the oral face become
enclosed by the division and growth of the twelve peripheral cells.
They are thus carried into the interior of the ovum; and there divide
into a central vitelline mass--the hypoblast--and a peripheral
mesoblastic layer.
[122] The note (No. 307) refers in the first instance to the
changes in the larvæ of the Chilostomata, but the similarity of
the larvæ of the Ctenostomata to those of the Chilostomata
renders it practically certain that the corrections, in so far as
they apply to the one group, apply also to the other.
The eight peripheral cells of the aboral pole divide vertically, and,
owing to the eight central cells at the aboral pole dividing
transversely so as to form a protuberance on the aboral surface, they
constitute a transverse ring of large cells round the ovum, which
become ciliated and constitute the main ciliated band of the embryo,
corresponding to the ciliated band at the edge of the vestibule of the
entoproctous larvæ. They divide the embryo into an aboral and an oral
region. The central part of the aboral projection forms a structure
which I shall speak of as the ciliated disc. It probably corresponds
with the ciliated disc in the Entoprocta. An invagination is next
formed on the oral surface, which gives rise to a sack opening to the
exterior (fig. 131, _st._). This was originally held by Barrois to be
the stomach; but Barrois now prefers to call it 'the internal sack.'
To my mind it is probably the stomodæum. The embryo has become in the
meantime laterally compressed, and, at what I shall call the anterior
end of the oral disc, a structure makes its appearance (fig. 131,
_m_), which is probably homologous with the dorsal organ of the larva
of Pedicellina and may go by the same name. It was originally
interpreted by Barrois as the pharynx[123].
[123] The interpretation of the larvæ given in the text must be
regarded as somewhat tentative. The opacity of the free larvæ is
very great, and almost every one of the numerous authors who have
worked on these larvæ have arrived at different conclusions, as
to the physiological significance of the various parts.
[FIG. 131. FREE-SWIMMING LARVA OF ALCYONIDIUM MYTILI. (After
Barrois.)
_m_ (?) dorsal organ; _st._ stomodæum (?); _s._ ciliated disc.]
The larva, having now acquired all the important structures it is
destined to possess, becomes free. It is shewn in fig. 131; the oral
face being turned upwards. There are two rings of cilia, one round the
edge of the ciliated disc, and a second with larger cilia on the ring
of large cells described above. This ring projects somewhat; its
projecting edge being directed towards the ciliated disc. The dorsal
organ (_m?_) is placed on the oral face at the bottom of an elongated
groove, in front of which is a bunch of long cilia or flagella. Two
long flagella are also developed at the posterior extremity of the
oral face, and two pairs (an anterior and a posterior) of eye-spots
also appear. Towards the posterior extremity of the oral face is seen
a body marked _st_, which forms the internal sack. If I am right in
regarding this as the stomodæum, it is probable that it never unites
with the invaginated hypoblast, and that the alimentary tract of the
larva remains therefore permanently in an imperfect condition.
Careful observations have been made by Repiachoff (No. 318) on the
early development of Tendra, which accord in some respects with the
results arrived at by Barrois in his second memoir. The observations
are not, unfortunately, carried down to the complete development of
the larva.
The ovum divides in the normal way into two and then four uniform
segments. These four next become divided by an equatorial furrow into
four dorsal and four ventral segments, the former constituting the
aboral pole and forming the epiblast, and the latter the oral pole.
The stages with sixteen and thirty-two cells appear to be formed in
the same manner as in Alcyonidium--but between the two layers of cells
forming the oral and aboral poles a well-marked segmentation cavity
arises at the stage with sixteen segments. At the stage with
thirty-two cells the four middle cells of the oral side, which are
larger than the others, become divided into two tiers, in such a
manner as to form a prominence projecting into the segmentation
cavity. By the appearance of a lumen in this prominence it becomes
converted into an archenteron, which communicates with the exterior by
a blastopore in the middle of the oral surface. The blastopore becomes
eventually closed.
The archenteric sack of Repiachoff is clearly the same structure as
Barrois' four invaginated cells of the oral face, their further
history has unfortunately not been followed out by Repiachoff.
The free larva swims about for some time, and then fixes itself and
undergoes a metamorphosis; but the exact course of this metamorphosis
is still very imperfectly known.
According to the latest statements of Barrois the attachment takes
place by the oral face[124]. The ciliated disc, which in the free
larva forms a kind of cup directed towards the aboral end, turns in
upon itself towards the oral face. It subsequently undergoes
degeneration and forms a nutritive or yolk-mass. The skin of the larva
after these changes gives rise to the ectocyst or cell of the future
polype. The future polype itself appears to originate, in part at any
rate, from the so-called dorsal organ[125].
[124] Barrois himself held the opposite view in his earlier
memoir, and other observers have done the same.
[125] The statements on this head are so unsatisfactory and
contradictory that it does not appear to me worth while quoting
them here; even the latest accounts of Barrois, which entirely
contradict his early statements, can hardly be regarded as
satisfactory.
The first distinct rudiment of the polype appears as a white body,
which gradually develops into the alimentary canal and lophophore.
While this is developing the ectocyst grows rapidly larger, and the
yolk in its interior separates from the walls and occupies a position
in the body cavity of the future polype, usually behind the developing
alimentary canal. According to Nitsche (No. 316) it is attached to a
protoplasmic cord (funiculus) which connects the fundus of the stomach
with the wall of the cell. It is probably (Nitsche, etc.) simply
employed as nutritive material, but, according to Barrois, becomes
converted into the muscles, especially the retractor muscles.
Adopting the hypothesis already suggested in the case of the
Entoprocta the metamorphosis just described would seem to be a case of
budding accompanied by the destruction of the original larva.
This view of the nature of the post-embryonic metamorphosis is
apparently that of Claparède and Salensky, and is supported by
Claparède's statement that the formation of the first polype
'resembles to a hair' that of the subsequent buds. The mode of budding
would, however, appear to present certain peculiarities, in that the
whole larval skin passes directly into the bud, while from the
rudimentary bud of the larva the lophophore and alimentary tract only
of the fixed polype are formed.
Flustrella and Cyphonautes. The next group of larval forms is that of
which Cyphonautes is the best known type. The larvæ composing it at
first sight appear to have but little in common with the larvæ
hitherto described. The researches of Barrois (No. 298) and
Metschnikoff (No. 314), (but especially those of the former on the
early stages of _Flustrella hispida_, the larva of which is very
similar in form to Cyphonautes, though without so great a complexity
of organisation), have given a satisfactory basis for a general
comparison of Cyphonautes with other ectoproctous larvæ.
The segmentation and early stages of the embryo of Flustrella resemble
closely those of Alcyonidium. A projecting ring of large cells is
formed, dividing the larva into oral and aboral parts. The oral part
soon however becomes very small as compared with the aboral, and
becomes vertically flattened so as to be nearly on a level with the
ring of large cells. In the next stage the flattening becomes
completed; and the ring of large cells surrounds, like the vestibule
of the Entoprocta, a flat oral disc. The aboral side is dome-shaped,
and forms the greater part of the embryo.
In the next stage a small disc--the ciliated disc--is formed in the
middle of the aboral dome. The larva becomes laterally compressed. The
ring of large cells which now constitute the edge of the vestibule is
covered, as in the larva of Pedicellina, by cilia, which are specially
long in front of the dorsal organ.
[FIG. 132. ADVANCED LARVA OF FLUSTRELLA HISPIDA. (After Barrois.)
_m_ (?) groove above dorsal organ; _Ph._ dorsal organ; _st._
stomodæum (?); _s._ ciliated disc at aboral end of body.]
In the next stage the ciliated disc (fig. 132, _s._) becomes reduced
in size, but surmounted by a ring of cilia round the edge, and a tuft
of cilia in the centre. The chief difference between this larva and
that of Alcyonidium depends on the small size of the ciliated disc,
and the oral position of the ciliated ring in the former. There are
intermediate types between these forms of larvaæ.
This stage immediately precedes the liberation of the larva. The free
larva differs from that in the ovicell mainly in the possession of a
shell formed as a cuticular structure, composed of two valves placed
on the two sides of the embryo. The aboral ciliated disc, still more
reduced in size, loses its cilia, and becomes enclosed between the two
valves of the shell.
The post-embryonic metamorphosis follows, so far as is known, the
course already described for the larva of Alcyonidium.
Cyphonautes (fig. 133) forms at certain seasons of the year one of the
commonest captures in the surface net. It was originally described by
Ehrenberg, but the important discovery of its true nature as the larva
of Membranipora (the common species _C. compressus_ is the larva of
_Mem. pilosa_), a genus of the chilostomatous Polyzoa, was made by
Schneider (No. 322). The younger stages of the larva have not been
worked out, but from a comparison with the last described larva it is
easy to make out the general relationship of the parts. The larva has
a triangular form with an aboral apex, corresponding with the summit
of the dome of the Flustrella larva, and an oral base. It is enclosed
in a bivalve shell, the two valves of which meet along the two sides,
but are separate along the base. At the apex an opening is left
between the two valves, through which a ciliated disc (_f.g_) of the
same character and nature as that of previous larvæ can be protruded.
[FIG. 133. CYPHONAUTES (LARVA OF MEMBRANIPORA). (After Hatschek.)
_m._ mouth; _a´_ anus; _f.g._ ciliated disc; _x._ problematical body
(probably a bud).]
The oral side or base is girthed by a somewhat sinuous ciliated edge,
which is continued round the anterior and posterior extremities of the
oral disc. It is no doubt equivalent to the ciliated ring of other
larvæ. Two openings are present on the oral face, both enclosed in a
special lobe of the ciliated ring. The larger of these leads into a
depression, which may be called the vestibule; and is situated on the
posterior side of the oral surface. The smaller of the two, on the
anterior side, leads into a cavity which is apparently (Hatschek)
equivalent to the rudimentary bud or dorsal organ of other larvæ. The
deeper part of the vestibule leads into the mouth (_m_) and
oesophagus; the latter is continued till close to the apex of the
larva, there bends upon itself, dilates into a stomach, and is
continued parallel to the oesophagus as the rectum which opens by an
anus (_a´_) at the posterior end of the vestibule. A peculiar paired
organ is situated on each side nearly above the stomach. Its nature is
somewhat doubtful. It was regarded as muscular by Claparède (No. 309),
though this, as shewn by Schneider, is no doubt a mistake. Allman (No.
305) regards it as hepatic, and Hatschek as a thickening of the
epidermis. Close to each of these organs is a small body regarded by
Claparède as an accessory muscle. It is placed in the normal position
for a Polyzoon ganglion, and may perhaps be therefore regarded as
nervous in nature. Allman points out its similarity to a bilobed
ganglion, but is not inclined to take this view of it. The
constitution of the parts contained in the anterior cavity (_x_) is
somewhat obscure. The most elaborate descriptions of them are given by
Schneider and Allman. Lining the cavity is apparently a mass of
spherical bodies, connected with which is a tongue-like process
provided with long cilia, which can be protruded from the orifice.
Internal to this is a striated body. A good figure of the whole
structure is given by Schneider.
The general similarity of Cyphonautes to the other larvæ is quite
obvious from the above description and figure. In the presence of an
anus, a vestibule, and possibly a nervous system, it clearly exhibits
a far more complicated organisation than any other Polyzoon larvæ
except those of the Entoprocta.
The post-embryonic metamorphosis of Cyphonautes, admirably
investigated by Schneider, takes place in the same manner as that of
other larvæ, and is accompanied by the degeneration of the larval
organs, and the formation of a clear body, which gives rise to the
alimentary cavity and lophophore of the fixed polype. The larval shell
takes part in the formation of the ectocyst of the polype.
Cyclostomata. We owe to Barrois by far the fullest account of the
development of the Cyclostomata, but how far his interpretations are
to be trusted is very doubtful. The larvæ differ very considerably
from the normal larvæ of the Chilostomata and Ctenostomata; the
difference being mainly due to the enormous development of the
ciliated disc. Barrois has investigated the larvæ of three genera,
Phalangella, Crisia, and Diastopora, and states that they very closely
resemble each other. The ovum is extremely minute.
The segmentation, so far as it has been made out, is regular. During
the segmentation growth is very rapid, and eventually there is formed
a blastosphere many times larger than the original ovum. The
blastosphere becomes flattened, and is converted into a gastrula by
bending up into a cup-like form. The gastrula opening is stated to
remain as the permanent mouth, which has a terminal and central
position. A transverse ring-like thickening is formed round the larva,
which probably corresponds with the ciliated ring of previous larvæ;
and the body of the larva in front of this ring becomes ciliated. The
aboral end of the larva becomes thickened, and grows out into an
elongated prominence, which probably corresponds to the ciliated disc.
The ring before mentioned becomes at the same time more prominent, and
forms a cylindrical sheath for the ciliated disc. At the time when the
larva becomes liberated from the maternal cell it has the form of a
barrel with a slight constriction in the middle separating the oral
from the aboral end. At the centre of the oral face is situated the
mouth, leading into a wide stomach, while the aboral end is formed of
the ciliated disc enclosed in its sheath. The whole surface is now
ciliated. No structure equivalent to the dorsal organ or bud is
described by Barrois, but in other respects, if the ciliated disc is
really equivalent in the two forms, a general comparison on the line
indicated above between this larva and the normal larvæ of the
Ctenostomata and Chilostomata seems quite possible. The fixation and
subsequent development of the larva take place in the normal manner.
Phylactolæmata. The development of the phylactolæmatous Polyzoa has
been studied by Metschnikoff (No. 315), who describes the eggs as
undergoing a complete segmentation within a peculiar brood-pouch
developed from the walls of the body of the parent. After segmentation
the cells of the embryo arrange themselves in two layers round a
central cavity. The embryo then forms the well-known cyst, from which
a colony is formed by a process of budding.
_General considerations on the Larvæ of the Polyzoa._
The different forms of embryo amongst the Polyzoa are represented in
figs. 130 B, 131, 132, and 133 in what I regard as identical
positions, and fig. 133 A is a figure of what may be regarded as an
idealized larval Polyzoon. In all the larvæ there is present a
ciliated ring, which separates an oral from an aboral face, and is
apparently homologous throughout the series. In the adult it is
probably represented by the lophophore. On the oral face is situated
in all cases the mouth, and in the entoproctous larvæ and Cyphonautes
also the anus. It thus appears that Cyphonautes, though the larva of
an ectoproctous form, is itself entoproctous--a fact which tends to
shew that the Entoprocta are the more primitive forms. In all the
larvæ, except possibly those of the Cyclostomata, there is present on
the anterior side of the mouth, in the Ectoprocta on the oral, and in
the Entoprocta on the aboral side of the ciliated ring, an organ, to
which is attached externally a plume of long cilia. This organ has
been identified throughout the series in accordance with Hatschek's
view as the dorsal organ or rudimentary bud; but it is well to bear in
mind that this identification is of a purely hypothetical character.
[FIG. 133 A. DIAGRAM OF AN IDEAL LARVA OF A POLYZOON.
_m._ mouth; _an._ anus; _st._ stomach; _s._ ciliated disc.]
On the aboral side of the ciliated ring there is present in all the
larvæ an organ, which has been called the ciliated disc, which is
probably homologous throughout the series. It perhaps remains in the
adult of Loxosoma as the cement gland, but not in other forms.
The Polyzoa present a simple and almost certainly degraded
organisation in the adult state; it is therefore more than usually
necessary to turn to their larvæ for the elucidation of their
affinities, and various plausible suggestions have been made as to the
interpretation of the characters of the larvæ.
Lankester[126] has suggested that the larvæ are essentially similar to
those of Molluscs. He compares the main ciliated ring to the velum,
but has ingeniously suggested that it represents not the simple velar
ring of most molluscan larvæ, but a more extended longitudinal ring,
of which the gills of Lamellibranchiata are supposed by him to be
remnants, and to which the Echinoderm larvæ with one continuous
ciliated band furnish a parallel.
[126] Lankester. "Remarks on the affinities of Rhabdopleura."
_Quart. J. of Micro. Science_, Vol. XIV. 1874.
The foot he finds in the epistome of the Phylactolæmata, and the disc
of Rhabdopleura--both situated between the mouth and anus, and
therefore in the situation of the molluscan foot. The peculiar
prominence between the mouth and the anus in Pedicellina (_vide_ fig.
130 B) and Loxosoma is probably the same structure.
Finally he identifies my ciliated disc, which as mentioned above is
perhaps equivalent to the cement gland in the adult Loxosoma, as the
molluscan shell-gland. Lankester's interpretations are very plausible,
but at the same time they appear to me to involve considerable
difficulties.
There is absolutely no evidence amongst the Mollusca of the existence
of a primitive longitudinal ciliated ring, such as he supposes to have
existed, and Lankester is debarred from regarding the ciliated ring of
the Polyzoa as equivalent to the simple velar ring of the Mollusca,
because his shell-gland lies in the centre and not as it should do on
the posterior side of the ciliated ring.
Another difficulty which I find is the invariable ciliation of
Lankester's shell-gland--a ciliation which never occurs amongst
Mollusca.
It appears to me that a more satisfactory comparison of the larvæ of
the Polyzoa with those of the Mollusca is obtained by dropping the
view that the ciliated disc is the shell-gland, and by regarding the
ciliated ring as equivalent to the velum. This mode of comparison has
been adopted by Hatschek.
The larva ceases however on this view to have any special molluscan
characters (except possibly the organ which Lankester has identified
as the foot), and only resembles a molluscan larva to the same extent
as it does a larva of the Polychæta. The ciliated disc lies according
to this view in the centre of the velar area or præ-oral lobe, and
therefore in the situation in which a tuft of cilia is often present
in lamellibranchiate and other molluscan larvæ, and also in the larvæ
of most Chætopoda. It is moreover at this point that the
supra-oesophageal ganglion is always formed in the Mollusca and
Chætopoda as a thickening of the epiblast (fig. 134, _sg._), so that
the thickening of the epiblast in the ciliated disc of the Polyzoa may
perhaps be a rudiment of the supra-oesophageal ganglion, which
entirely atrophies in the adult after the attachment has been effected
in the region of this disc.
The comparison between the Polyzoon larva and that of a Chætopod
becomes very much strengthened by taking as types Mitraria[127] (fig.
134) and Cyphonautes (fig. 133). The similarity between these two
forms is so striking that I am certainly inclined to view the larvæ of
the Polyzoa as trochospheres similar to those of Chætopods, Rotifera,
etc., _which become fixed in the adult by the extremity of their
præ-oral lobe_.
[127] The larva of Mitraria is figured with the aboral surface
turned upwards, instead of downwards, as in the figure of
Cyphonautes. The ciliated band is also diagrammatically put in
black for greater distinctness.
[FIG. 134. TWO STAGES IN THE DEVELOPMENT OF MITRARIA. (After
Metschnikoff.)
_m._ mouth; _an._ anus; _sg._ supra-oesophageal ganglion; _br._ and
_b._ provisional bristles; _pr.b._ præ-oral ciliated band.]
The attachment of the larva by the præ-oral lobe is not more
extraordinary than the attachment of a Barnacle by its head, and after
such a mode of attachment the atrophy of the supra-oesophageal
ganglion would be only natural.
There is one important fact which deserves to be noted in the
development of the Polyzoa, viz. that if the suggestion in the text as
to the mode of development of the adult from the so-called larva is
accepted, the Polyzoa exhibit universally _the phenomenon of
alternations of generations_. The ovum gives rise to a free form which
never becomes sexual, but produces by budding the sexual attached form.
BIBLIOGRAPHY.
_General._
(298) J. Barrois. _Recherches sur l'embryologie des Bryozoaires._
Lille, 1877.
_Entoprocta._
(299) B. Hatschek. "Embryonalentwicklung u. Knospung d. Pedicellina
echinata." _Zeitschrift für wiss. Zool._, Bd. XXIX. 1877.
(300) M. Salensky. "Etudes sur les Bryozoaires entoproctes." _Ann.
Scien. Nat._, 6th Ser. Tom. V. 1877.
(301) O. Schmidt. "Die Gattung Loxosoma." _Archiv f. mik. Anat._, Bd.
XII. 1876.
(302) C. Vogt. "Sur le Loxosome des Phascolosomes." _Archives de Zool.
expér. et génér._, Tom. V. 1876.
(303) C. Vogt. "Bemerkungen zu Dr Hatschek's Aufsatz üb.
Embryonalentwicklung u. Knospung von Pedicellina echinata." _Zeit. f.
wiss. Zool._, Bd. XXX. 1878.
_Ectoprocta._
(304) G. J. Allman. _Monograph of fresh-water Polyzoa._ Ray Society.
(305) G. J. Allman. "On the structure of Cyphonautes." _Quart. J. of
Micr. Scie._, Vol. XII. 1872.
(306) G. J. Allman. "On the structure and development of the
Phylactolæmatous Polyzoa." _Journal of the Linnean Society_, Vol. XIV.
No. 77. 1878.
(307) J. Barrois. "Le développement d. Bryozoaires Chilostomes."
_Comptes rendus_, Sept. 23, 1878.
(308) E. Claparède. "Beiträge zur Anatomie u. Entwicklungsgeschichte
d. Seebryozoen." _Zeit. für wiss. Zool._, Bd. XXI. 1871.
(309) E. Claparède. "Cyphonautes." _Anat. u. Entwick. wirbell. Thiere.
Leipzig, 1864.
(310) R. E. Grant. "Observations on the structure and nature of
Flustræ." _Edinburgh New Philosoph. Journal_, 1827.
(311) B. Hatschek. "Embryonalentwicklung u. Knospung d. Pedicellina
echinata." (_Description of Cyphonautes_). _Zeit. f. wiss. Zool._, Bd.
XXIX. 1877.
(312) T. H. Huxley. "Note on the reproductive organs of the
Cheilostome Polyzoa." _Quart. Jour. of Micr. Science_, Vol. IV. 1856.
(313) L. Joliet. "Contributions à l'histoire naturelle des Bryozoaires
des côtes de France." _Archives de Zoologie Expérimentale_, Vol. VI.
1877.
(314) E. Metschnikoff. "Ueber d. Metamorphose einiger Seethiere."
_Göttingische Nachrichten_, 1869.
(315) E. Metschnikoff. _Bull. de l'Acad. de St Pétersbourg_, XV. 1871,
p. 507.
(316) H. Nitsche. "Beiträge zur Kenntniss d. Bryozoen." _Zeit. f.
wiss. Zool._, Bd. XX. 1870.
(317) W. Repiachoff. "Zur Naturgeschichte d. chilostomen Seebryozoen."
_Zeit. f. wiss. Zool._, Bd. XXVI. 1876.
(318) W. Repiachoff. "Ueber die ersten Entwicklungsvorgänge bei Tendra
zostericola." _Zeit. f. wiss. Zool._, Bd. XXX. 1878. Supplement.
(319) W. Repiachoff. "Zur Kenntniss der Bryozoen." _Zoologischer
Anzeiger_, No. 10, Vol. I. 1878.
(320) W. Repiachoff. "Bemerkungen üb. Cyphonautes." _Zoologischer
Anzeiger_, Vol. II. 1879.
(321) M. Salensky. "Untersuchung an Seebryozoen." _Zeit. für wiss.
Zool._, Bd. XXIV. 1874.
(322) A. Schneider. "Die Entwicklung u. syst. Stellung d. Bryozoen u.
Gephyreen." _Archiv f. mikr. Anat._, Vol. V. 1869.
(323) Smitt. "Om Hafsbryozoernas utveckling och fettkroppar." _Aftryck
ur öfvers. af Kong. Vet. Akad. Förh._ Stockholm, 1865.
(324) T. Hincks. _British Marine and Polyzoa._ Van Voorst, 1880.
[Conf. also works by Farre, Hincks, Van Beneden, Dalyell, Nordmann.]
CHAPTER XI.
BRACHIOPODA[128].
[128] The classification of the Brachiopoda adopted in the
present chapter is shewn in the subjoined table:
I. Articulata. { _a._ Rhynchonellidæ.
{ _b._ Terebratulidæ.
{ _a._ Lingulidæ.
II. Inarticulata. { _b._ Craniadæ.
{ _c._ Discinidæ.
The observations which have been made on the developmental history of
the Brachiopoda have thrown very considerable light on the systematic
position of this somewhat isolated group.
_Development of the Layers._
For our knowledge of the early stages in the development of the
Brachiopoda we are almost entirely indebted to Kowalevsky[129] (No.
326). His researches extend to four forms, Argiope, Terebratula,
Terebratulina, and Thecidium. The early development of the first three
of these takes place on one plan, and that of Thecidium on a second
plan.
[129] Kowalevsky's Memoir is unfortunately written in Russian.
The account in the text is derived from an inspection of his
figures, and from an abstract in Hoffmann and Schwalbe's
_Jahresberichte_ for 1873.
In Argiope, which may be taken as typical of the first group, the ova
are transported into the oviducts (segmental organs) where they
undergo their early development. The segmentation leads to the
formation of a blastosphere, which then becomes a gastrula by
invagination. The blastopore gradually narrows, and finally closes,
while at the same time the archenteric cavity (fig. 135 A) becomes
divided into three lobes, a median (_me_) and two lateral (_pv_).
These lobes next become completely separated, and the middle one forms
the mesenteron, while the two lateral ones give rise to the body
cavity, their outer walls forming the somatic mesoblast, and their
inner the splanchnic (fig. 135 B). The embryo now elongates, and
becomes divided into three successive segments (fig. 135 B), which are
usually, though on insufficient grounds (_vide_ Thecidium), regarded
as equivalent to the segments of the Chætopoda. The alimentary tract
is not continued into the hindermost of them.
[FIG. 135. TWO STAGES IN THE DEVELOPMENT OF ARGIOPE. (After
Kowalevsky.)
A. Late gastrula stage.
B. Stage after the larva has become divided into three segments.
_bl._ blastopore; _me._ mesenteron; _pv._ body cavity; _b._
temporary bristles.]
In Thecidium the ova are very large, and development takes place in a
special incubatory pouch in the ventral valve. The embryos are
attached by suspenders to the two cirri of the arms which immediately
adjoin the mouth. There is a nearly regular segmentation, and a very
small segmentation cavity is developed. There is no invagination; but
cells are budded off from the walls of the blastosphere, which soon
form a solid central mass, enclosed by an external layer--the
epiblast. In this central mass three cavities are developed, which
constitute the mesenteron and the two halves of the body cavity.
Around these cavities distinct walls become differentiated. The body
(Lacaze Duthiers, No. 327) soon after becomes divided into two
segments, of which the posterior is the smaller. The hinder part of
the large anterior segment next becomes constricted off as a fresh
segment, and subsequently the remaining part becomes divided into two,
of which the anterior is the smallest. The embryo thus becomes divided
into four segments, of which the two foremost appear (?) together to
correspond to the cephalic segment of Argiope; but these segments are
formed not, as in Chætopoda and other truly segmented forms, by the
addition of fresh segments between the last-formed segment and
the unsegmented end of the body, but by the interpolation of fresh
segments at the cephalic end of the body as in Cestodes; so that the
hindermost segment is the oldest. Assuming the correctness[130] of
Lacaze Duthiers' observations, the mode of formation of these segments
appears to me to render it probable that they are not identical with
the segments of a Chætopod. A suspender is attached to the front end
of each embryo. Before the four segments are established the whole
embryo is covered with cilia[131], and two and then four rudimentary
eyes are developed on the anterior segment of the body.
[130] It should be stated that it is by no means clear from
Kowalevsky's figures that he agrees with Lacaze Duthiers as to
the succession of the segments.
[131] Kowalevsky in his figures leaves the penultimate lobe
unciliated.
_The history of the Larva and the development of the organs of
the Adult._
[FIG. 136. LARVA OF ARGIOPE. (From Gegenbaur, after Kowalevsky.)
_m._ mantle; _b._ setæ; _d._ archenteron.]
Articulata. The observations of Kowalevsky and Morse have given us a
fairly complete history of the larval metamorphosis of some of the
Articulata, while some of the later larval stages in the history of
the Inarticulata have been made known to us from the researches of
Fritz Müller, Brooks, etc. The embryo of Argiope, which may be taken
as the type for the Articulata, was left (fig. 135 B) as a three lobed
organism with a closed mesenteron and a body cavity divided into two
lateral compartments. On the middle segment of the body dorsal and
ventral folds, destined to form the mantle lobes, make their
appearance, and on the latter two pairs of bundles of setæ are present
(fig. 135 B). The setæ together with the mantle folds grow greatly,
and the setæ resemble in appearance the provisional setæ of many
Chætopods (fig. 152). On the hinder border of the mantle cilia make
their appearance. The anterior or cephalic segment assumes a somewhat
umbrella-like form, and round its edge is a circlet of long cilia,
while elsewhere it is provided with a coating of short cilia. Two
pairs of eyes also arise on its anterior surface (fig. 136).
[FIG. 137. TWO STAGES IN THE DEVELOPMENT OF ARGIOPE, SHEWING THE
FOLDS OF THE MANTLE GROWING OVER THE CEPHALIC LOBE. (After
Kowalevsky.)
_m._ mantle fold; _me._ mesenteron; _pd._ peduncle; _b._ provisional
setæ.]
After swimming about for some time the larva becomes fixed by its hind
lobe, and becomes gradually transformed into the adult. The hind lobe
itself becomes the peduncle. After attachment the mantle lobes bend
forward (fig. 137 A, _m_), and enclose the cephalic lobe. The valves
of the shell are formed on their outer surface as two delicate
chitinous plates (fig. 137 B). At a somewhat later stage the
provisional bristles are thrown off, and are eventually replaced by
permanent setæ round the edge of the mantle. The cephalic lobe becomes
located in the dorsal valve of the shell, and the mouth is formed near
the apex of the cephalic lobe immediately ventral to the eye-spots, by
an epiblastic invagination. The permanent muscles are formed out of
the muscles already present in the embryo.
Around the mouth there arises a ring of tentacles, very possibly
derived from the ciliated ring visible in fig. 136[132]. The ring of
tentacles is placed obliquely, and the mouth is situated near its
ventral side. The tentacles appear to form a post-oral circlet, like
that of Phoronis (Actinotrocha): they gradually increase in number as
the larva grows older.
[132] In the abstract in Hoffman and Schwalbe Kowalevsky is made
to state that the tentacles spring from the border of the mantle.
This can hardly be a correct account of what he states, since it
does not fit in with the adult anatomy of the parts. The figures
he gives might lead to the supposition that they sprang from the
edge of the cephalic lobe, or perhaps from the dorsal lobe of the
mantle.
[FIG. 138. DIAGRAM OF A LONGITUDINAL VERTICAL SECTION OF AN ADVANCED
EMBRYO OF LINGULA. (After Brooks.)
_a._ end of valves; _b._ thickened margin of mantle; _c._ mantle;
_d._ dorsal median tentacle; _e._ lophophore; _f._ lip; _g._ mouth;
_h._ mantle cavity; _i._ body cavity; _k._ wall of oesophagus; _l._
oesophagus; _m._ hepatic chamber of stomach; _n._ intestinal chamber
of stomach; _o._ intestine; _q._ ventral ganglion; _r._ posterior
muscle; _s._ dorsal valve of shell; _t._ ventral valve of shell.]
Some of the later stages in the development of the Terebratulidæ have
been made known to us by the observations of Morse (No. 328-9) on
Terebratulina septentrionalis.
The most interesting point in Morse's observations on the later stages
is the description of the gradual conversion of the disc bearing the
circlet of tentacles into the arms of the adult. The tentacles, six in
number, first form a ring round the edge of a disc springing from the
dorsal lobe of the mantle; in their centre is the mouth. In the later
stages calcareous spicula become developed on the tentacles. When the
embryo is far advanced the tentacles begin to assume a horse-shoe
arrangement, which bears a striking, though probably accidental,
resemblance to that of the tentacles on the lophophore of the
fresh-water Polyzoa. The disc bearing the tentacles is prolonged
anteriorly into two processes, the free ends of the future arms. By
this change of shape in the disc the tentacles form two rows, one on
the anterior and one on the posterior border of the disc, and
eventually become the cirri of the arms. The mouth is placed between
the two rows of tentacles, where the two arms of the lophophore meet
behind. The position of the mouth was the original centre of the ring
of tentacles before they became pulled out into a horse-shoe form. In
front of the mouth is a lip. The arms grow greatly in length in the
adult Terebratulina. In Thecidium the oral disc retains the horse-shoe
form, while in Argiope the embryonic circular arrangement of the
tentacles is only interfered with by the appearance of marginal
sinuations.
The shell is deposited as to chitinous plates, which subsequently
become calcified. It undergoes in the different genera great changes
of form during its growth.
With reference to the larval stages of other Articulata, a few points
may be noted.
The three-lobed larva of Terebratulina septentrionalis is provided
with a special tuft of cilia at the apex of the front lobe. The arms
appear to originate, in Terebratulina caput serpentis, as two
processes at the sides of the mouth, on which the tentacles are
formed.
Provisional setæ do not appear to be formed in the lobed embryos of
Thecidium and Terebratulina, but they appear at a later stage at the
edge of the mantle in the latter form. The third lobe of Thecidium
gives rise to the dorsal and ventral mantle lobes.
Inarticulata. The youngest stages in the development of the
Inarticulata are not known, and in the earliest stages observed the
shell is already developed. The young larvæ with shells differ however
from those of the Articulata in the fact that they are free-swimming,
and that the peduncle is not developed.
The larva of Discina radiata has been described by Fritz Müller (No.
331). It resembles generally a larva of the Articulata shortly after
the tentacles have become developed. Five pairs of long provisional
setæ are present, of which all but the hindermost are seated on the
ventral lobe of the mantle. Shorter setæ are also lodged on the edge
of the dorsal lobe. The mouth is placed on the ventral side of a
protrusible oral lobe. It is imperfectly surrounded by four pairs of
tentacles, which form a swimming apparatus.
A fuller history of the development of Lingula has been recently
supplied by Brooks (No. 325). The youngest larva is enveloped in two
nearly similar plate-like valves, covering the two mantle lobes. The
mouth is placed at the centre of a disc, attached to the dorsal valve,
on the margin of which is a ring of ciliated tentacles. The general
position of the disc and its relations may be gathered from fig. 138,
which represents a diagrammatic longitudinal vertical section of the
embryo.
With the growth of the embryo the tentacles increase in number, the
new pairs being always added between the odd dorsal tentacle and the
next pair. There is an axial cavity in the tentacles which, unlike the
cavity in the tentacles of the Polyzoa, does not communicate with the
perivisceral cavity. As the tentacles increase in number, the lateral
parts of the tentacular disc grow out into the two lateral arms of the
adult, while the dorsal margin forms the median coiled arm. These
changes are not effected till the larva has become fixed.
The attachment of the larva was not observed; but the peduncle, of
which there is no trace in the young stages, grows out as a simple
prolongation of the hinder end of the body while the larva is still
free. It had already reached a very great length in the youngest fixed
larva observed.
_Development of Organs._
The alimentary tract after the obliteration of the blastopore forms a
closed sack, which becomes subsequently placed in communication with
the exterior by the stomodæal invagination. The liver is formed as a
pair of dorsal outgrowths of the mesenteron. From Brooks' observations
on Lingula it would appear that the primitive mesenteron forms the
stomach of the adult only, and that the intestine grows out from this
as a solid process: this eventually meets the skin, and here the anus
is formed. In the Articulata the mesenteron is aproctous.
The origin of the body cavity as paired archenteric diverticula has
already been described. Its somatic wall becomes in Lingula ciliated,
and its cavity filled with a corpusculated fluid, as in many
Chætopods. It is eventually prolonged into the dorsal and ventral
mantle lobes as a pair of horn-like prolongations into each lobe,
which communicate with the body cavity by large ciliated openings.
Some incomplete observations of Brooks on the development of the
nervous system in Lingula shew that it arises in the embryo as a ring
round the oesophagus with a ventral sub-oesophageal (fig. 138 _q_),
and two lateral ganglia, and two dorsal otocysts. The ventral ganglion
is formed as a thickening of the epiblast, with which it remains in
continuity for life. The remainder of the ring grows out from the
ventral ganglion as two cords, which gradually meet on the dorsal side
of the oesophagus.
_General observations on the Affinity of the Brachiopoda._
The larva of Argiope, as has been noticed by many observers, has
undoubtedly very close affinities with the Chætopoda. It resembles, in
fact, a mesotrochal larval Chætopod with provisional setæ (_vide_
Chapter on Chætopoda). Lacaze Duthiers' observations point to the
lobes of the larva not being true segments, and certainly the
mesoblast does not in the embryo become segmented as it ought to do
were these lobes true segments. If this view is correct the larva is
to be compared to an unsegmented Chætopod larva. In Rhynchonella,
however, indications of two segments are afforded in the adult in the
two pairs of segmental organs.
Though the larval Brachiopod resembles a mesotrochal Chætopod larva,
it does not appear to resemble the trochosphere larvæ so far
described, or the more typical larvæ of the Chætopoda, in that the
ring of tentacles, which is probably, as already mentioned, derived
from the ciliated ring shewn in fig. 137, is _post-oral_, and not
_præ-oral_. The ring of tentacles is like the ring in Actinotrocha
(the larva of Phoronis) amongst the Gephyrea. Although there is no
doubt a striking resemblance between the tentacular disc of a larval
Brachiopod and the lophophore of a Polyzoon, which has been pointed
out by Lankester, Morse, Brooks, etc., their homology is rendered, to
my mind, very doubtful (1) by the fact that the lophophore is præ-oral
in Polyzoa[133] and post-oral in Brachiopoda; and (2) by the fact that
the concave side of the lophophore is turned in nearly opposite
directions in the two forms. In Brachiopods it is turned dorsalwards,
and in phylactolæmatous Polyzoa ventralwards.
[133] For the ectoproctous Polyzoa it might be held that the
ciliated ring of tentacles is post-oral, but the facts of
development recorded in the previous chapter appear to me to shew
that this view is untenable.
The view of Morse, that the Brachiopoda are degraded tubicolous
Chætopods, is not so far supported by any definite embryological
facts. The development of the tentacular ring as well as its
innervation from the sub-oesophageal ganglion prohibit us, as has been
pointed out by Gegenbaur, from comparing it with the tentacles of
tubicolous Chætopoda.
BIBLIOGRAPHY.
(325) W. K. Brooks. "Development of Lingula." _Chesapeake Zoological
Laboratory, Scientific Results of the Session of 1878._ Baltimore, J.
Murphy and Co.
(326) A. Kowalevsky. "Development of the Brachiopoda." _Protocol of
the First Session of the United Sections of Anatomy, Physiology, and
Comparative Anatomy at the Meeting of Russian Naturalists in Kasan_,
1873. (Russian.)
(327) H. Lacaze Duthiers. "Histoire de la Thécidie." _Ann. Scien. Nat.
etc._ Ser. 4, Vol. XV. 1861.
(328) Morse. "On the Early Stages of Terebratulina septentrionalis."
_Mem. Boston Soc. Nat. History_, Vol. II. 1869, also _Ann. & Mag. of
Nat. Hist._, Series 4, Vol. VIII. 1871.
(329) ---- "On the Embryology of Terebratulina." _Mem. Boston Soc.
Nat. History_, Vol. III., 1873.
(330) ---- "On the Systematic Position of the Brachiopoda."
_Proceedings of the Boston Soc. of Nat. Hist._, 1873.
(331) Fritz Müller. "Beschreibung einer Brachiopoden Larve." Müller's
_Archiv_, 1860.
CHAPTER XII.
CHÆTOPODA[134].
[134] The following classification of the Chætopoda is adopted in
the present section:
I. Achæta. (_Polygordius_).
II. Polychæta. { Sedentaria.
{ Errantia.
III. Oligochæta.
_Formation of the Germinal Layers._
Most Chætopoda deposit their eggs before development. The Oligochæta
lay them in peculiar cocoons or sacks formed by a secretion of the
integument. Some marine Polychæta carry them about during their
development. Autolytus cornutus has a special sack on the ventral
surface in which they are hatched. In Spirorbis Pagenstecheri they
develop inside the opercular tentacle, and in Spirorbis spirillum
inside the tube of the parent.
A few forms (_e.g._ Eunice sanguinea, Syllis vivipara, Nereis
diversicolor) are viviparous.
Perhaps the most primitive type of Chætopod development so far
observed is that of Serpula (Stossich, No. 357)[135]. There is a
regular segmentation resulting in the formation of a blastosphere with
a central segmentation cavity. An invagination of the normal type now
ensues. The blastopore soon narrows to become the permanent anus,
while the invaginated hypoblast forms a small prominence with an
imperfectly developed lumen, which _does not nearly fill up the
segmentation cavity_ (fig. 139 A). The embryo, which has in the
meantime become completely covered with cilia, now assumes more or
less the form of a cone, at the apex of which is the anus, while the
base forms the rudiment of a large præ-oral lobe. The alimentary sack
grows forwards and then bends upon itself nearly at right angles, and
meets a stomodæal invagination from the ventral side some way from the
front end of the body.
[135] The observations of Stossich are not thoroughly
satisfactory.
[FIG. 139. TWO STAGES IN THE DEVELOPMENT OF SERPULA. (After
Stossich.)
_m._ mouth; _an._ anus; _al._ archenteron.]
The alimentary canal soon differentiates itself into three regions (1)
oesophagus, (2) stomach, and (3) intestine. With these changes the
larva, which in the meantime becomes hatched, assumes the characters
of a typical Annelid larva (fig. 139 B). In front is a large præ-oral
lobe, at the sides of which the eye-spots soon appear. The primitive
segmentation cavity remains as a wide space between the curved
alimentary tract and the body walls, and becomes traversed by muscular
fibres passing between the two. The original chorion appears to serve
as cuticle, and is perforated by the cilia.
The further changes in this larval form do not present features of
general importance. A peculiar vesicle, which in anomalous cases is
double, is formed near the anus. If it were shewn to occur widely
amongst Chætopoda, it might be perhaps regarded as homologous with the
anal vesicles of the Gephyrea.
Serpula is one of the few Chætopoda at present known in which the
segmentation is quite regular[136]. In other forms it is more or less
unequal. The formation of the germinal layers has been far more fully
studied in the Oligochæta than in the Polychæta, and though
unfortunately the development is much abbreviated in the former group,
they nevertheless have to serve as our type; and unless the contrary
is indicated the statements in the remainder of the section apply to
the Oligochæta. The segmentation is nearly regular in Lumbricus
agricola (Kowalevsky) and results in the formation of a flattened
blastosphere, one of the sides of which is hypoblastic and the other
epiblastic, the hypoblast cells being easily distinguished from the
epiblast cells by their clearer aspect. An invagination takes place,
in the course of which the hypoblast becomes enclosed by the epiblast,
and a somewhat cylindrical two-layered gastrula is formed. The opening
of this gastrula at first extends over the whole of what becomes the
ventral surface of the future worm, but gradually narrows to a small
pore--the permanent mouth--near the front end. The central cavity of
the gastrula is lined by hypoblast cells, but the oral opening, which
leads by a narrow passage into the gastric cavity, is lined by
epiblast cells.
[136] According to Willemoes-Suhm, Terebellides stroemii is also
characterised by a regular segmentation.
The segmentation of Lumbricus trapezoides (Kleinenberg, No. 341), and
of Criodrilus (Hatschek, No. 339), is more unequal and more irregular
than that of Lumbricus agricola, and there is an invagination which is
intermediate between the embolic and epibolic types.
The segmentation of Lumbricus trapezoides is especially remarkable. It
is strangely irregular and at one period the segmentation cavity
communicates by a pore with the exterior. Before the completion of the
gastrula stage the ovum becomes partially divided into two halves,
each of which gives rise to a complete embryo. The two embryos are at
first united by an epiblast cord which connects their necks (fig. 141
A), but this cord is very early ruptured, and the two embryos then
become quite independent. Some of the peculiarities of the
segmentation may no doubt be explained by this remarkable embryonic
fission.
The gastrula opening in both Lumbricus trapezoides and Criodrilus is
placed on the ventral surface, and eventually narrows to form the
mouth or possibly (Criodrilus) closes at the position of the mouth. In
Lumbricus trapezoides the oral opening is at first lined by hypoblast,
and in Criodrilus is bounded anteriorly by three large peculiar
epiblast cells, which are believed by Hatschek to assist in absorbing
the albuminous fluid in which the eggs are suspended. These large
cells are eventually covered by the normal epiblast cells and
subsequently disappear. In both these types the hypoblast cells
undergo, during their invagination, peculiar changes connected with
their nutritive function.
In Euaxes (Kowalevsky) the segmentation is far more unequal than in
the other types; a typical epibolic invagination takes place (fig.
140), and the blastopore closes completely along the ventral surface.
[FIG. 140. TRANSVERSE SECTION THROUGH THE OVUM OF EUAXES DURING AN
EARLY STAGE OF DEVELOPMENT. (After Kowalevsky.)
_ep._ epiblast; _ms._ mesoblastic band; _hy._ hypoblast.]
In all the oligochætous types, with the exception of Euaxes, where the
blastopore closes completely, the blastopore becomes, or coincides
with the mouth. In Serpula it is stated (Stossich), as we have seen,
to coincide with the anus: a statement which receives confirmation
from the similar statements of Willemoes-Suhm (No. 358). It is
necessary either to suppose a mistake on the part of Stossich, or that
we have in Chætopods a case like that of Gasteropods in which a
slit-like blastopore originally extending along the ventral surface
may in some forms become reduced to a pore at the oral, or in other
forms at the anal extremity.
So far only two germinal layers--the epiblast and the hypoblast--have
been spoken of. Before the invagination of the hypoblast is completed
the mesoblast makes its appearance in the form of two bands or
streaks, extending longitudinally for the whole length of the embryo.
These are usually spoken of as germinal streaks, but to avoid the
ambiguity of this term they will be spoken of as mesoblastic bands.
Their origin and growth has been most fully studied by Kleinenberg
(No. 341) in Lum. trapezoides. They commence in this species shortly
before the gastrula stage as two large cells on the surface of the
blastoderm, which may be called mesoblasts. These cells lie one on
each side of the median line at the hind end of the embryo. They soon
travel inwards and become covered by the epiblast (fig. 141 A, _m´_),
while on their inner and anterior side a row of small cells appears
(_ms_). These rows of cells form the commencement of the mesoblastic
bands, and in the succeeding stages they extend one on each side of
the body (fig. 141 B, _ms_) till they reach the sides of the mouth.
Their forward growth takes place mainly at the expense of the
superjacent epiblast cells, but the two mesoblasts at their hinder
extremities probably assist in their growth. Each mesoblastic band is
at first composed of only a single row of cells, but soon becomes
thicker, first of all in front, and becomes composed of two, three or
more rows of cells abreast. From the above it is clear that the
mesoblastic bands have, in L. trapezoides at any rate, in a large
measure an epiblastic origin.
[FIG. 141. THREE SECTIONS ILLUSTRATING THE DEVELOPMENT OF LUMBRICUS
TRAPEZOIDES. (After Kleinenberg.)
_ms._ mesoblastic band; _m´._ mesoblast; _al._ archenteron; _pp._
body cavity.
A. Horizontal and longitudinal section of an embryo which is
dividing into two embryos at the gastrula stage. It shews the
mesoblasts and the mesoblastic bands proceeding from them.
B. Transverse section shewing the two widely separated mesoblastic
bands.
C. Transverse section at a later stage shewing the mesoblastic bands
which have approached the ventral line and developed a body cavity
_pp._]
At first the two bands end _in front_ at the sides of the mouth, but
subsequently their front ends grow dorsalwards at the expense of the
adjoining epiblast cells, and meet above the mouth, forming in this
way a mesoblastic dorsal commissure.
The mesoblastic bands soon travel from the lateral position, which
they at first occupy, towards the ventral surface. They do not however
meet ventrally for some time, but form two bands, one on each side of
the median ventral line (fig. 141 C).
The usual accounts of the origin and growth of the bands differ
somewhat from the above. By Kowalevsky (No. 342) and Hatschek (No.
339) they are believed to increase in Lumbricus rubellus and
Criodrilus entirely at the expense of the mesoblasts. Kowalevsky
moreover holds that in L. rubellus the original mesoblasts spring from
the hypoblast. In some forms, _e.g._ Lumbricus agricola, the
mesoblasts are not present.
In Euaxes the origin of the mesoblast bands is somewhat interesting as
illustrating the relation of the Chætopod mesoblastic bands to the
mesoblast of other forms. To render intelligible the origin of the
mesoblast in this form, it is necessary to say a few words about the
segmentation.
By a somewhat abnormal process of segmentation the ovum divides into
four spheres, of which one is larger than the others, and occupies a
position corresponding with the future hind end of the embryo. The
three smaller spheres give rise _on their dorsal side_ by a kind of
budding to small cells, which become the epiblast; and the epiblast is
also partly formed from the hinder large cell in that this cell
produces by budding a small cell, which again divides into two. The
anterior of the two cells so formed divides still further and becomes
incorporated in the epiblast; the posterior only divides into two
_which form the two mesoblasts_. The remainder of the mesoblast is
formed by further division of the three smaller of the primitive large
spheres, and at first forms a continuous layer between the dorsal cap
of epiblast and the four largest cells which, after giving rise to the
epiblast and mesoblast, constitute the hypoblast. As the epiblast
spreads over the hypoblast the mesoblastic sheet gives way in the
middle, and the mesoblast remains as a ridge of cells at the edge of
the epiblastic cup. It forms in fact a thickening of the lips of the
blastopore. Behind the thickening is completed by the two mesoblasts.
The appearance of the mesoblast in section is shewn in fig. 140. As
the epiblast accompanied by the mesoblast grows round the hypoblast,
the blastopore assumes an oval form, and the mesoblast appears as two
bands forming the sides of the oval. The epiblast travels over the
hypoblast more rapidly than the mesoblast, so that when the blastopore
becomes closed ventrally the mesoblastic bands are still some little
way apart on the ventral side.
In Euaxes the mesoblast originates in a manner which is very similar
to that in some of the Gasteropoda, _e.g._ _Nassa, vide_ p. 234, and
Vermes, _e.g._ _Bonellia_, etc. As mentioned in the chapter on the
Mollusca the origin of the mesoblast in Planorbis, p. 227, is very
similar to that in Lumbricus.
Hatschek has shewn that in Polygordius the mesoblast arises in
fundamentally the same way as in the Oligochæta.
Besides the mesoblast which arises from the mesoblastic bands, there
is evidence of the existence of further mesoblast in the larvæ of many
Polychæta in the form of muscular fibres which traverse the space
between the body wall and the wall of the enteric cavity prior to the
formation of the permanent body cavity. These fibres have already been
described in the embryo of Serpula, and are probably represented by
stellate cells in the cephalic region (præ-oral lobe) of the
Oligochæta. These cells are probably of the same nature as the
amoeboid cells in the larvæ of Echinodermata, some Mollusca and other
types.
_The Larval form._
True larval forms are not found in the Oligochæta where the
development is abbreviated. They occur however in the majority of the
marine Polychæta.
They present a great variety of characters with variously arranged
ciliated bands. Most of these forms can be more or less satisfactorily
derived from a larval form, like that of Serpula (fig. 139 B) or
Polygordius (fig. 142); and the constant recurrence of this form
amongst the Chætopoda, combined with the fact that it presents many
points of resemblance to the larval forms of many Rotifers, Molluscs,
and Gephyreans, seems to point to its being a primitive ancestral form
for all these groups.
The important characters of this larval form are (1) the division of
the body into a large præ-oral lobe and a relatively small post-oral
region containing the greater part of the alimentary tract; (2) the
presence of a curved alimentary canal divided into stomodæum
(oesophagus), stomach and intestine, and opening by a ventrally placed
mouth, and an anus near the hind end of the body. To these may be
added the frequent presence of (1) a ganglion at the apex of the
præ-oral lobe, (2) a large cavity between the wall of the gut and the
skin, which is the remnant of the segmentation cavity, and is usually
traversed by muscular strands, of which one connecting the apex of the
præ-oral lobe and the stomach or oesophagus is very commonly present
(fig. 142).
The arrangement of the ciliated bands presents great variations,
though in some instances it is constant through large groups. In
Chætopods there is a widely distributed præ-oral ciliated band, which
is similarly placed to the ring constantly found in the larvæ of
Molluscs, Rotifers, etc. In many of these forms the band is
practically double, the opening of the mouth being placed between its
two component rings (_vide_ fig. 142). The best introduction to the
study of the Chætopod larval forms will be the history of the changes
of a typical larval form in becoming converted into the adult.
[FIG. 142. POLYGORDIUS LARVA. (After Hatschek.)
_m._ mouth; _sg_. supra-oesophageal ganglion; _nph._ nephridion;
_me.p._ mesoblastic band; _an._ anus; _ol._ stomach.]
For this purpose no better form can be selected than the interesting
larva of Polygordius (_vide_ Agassiz, No. 332, Schneider, No. 352, and
Hatschek, No. 339), which was first discovered by Lovén, and believed
by him to be the larva of an ordinary Chætopod. Its true nature was
determined by Schneider.
At a very young stage the larva has the form (fig. 142) of a flattened
sphere, with a small conical knob at the posterior extremity.
At the equator are situated two parallel ciliated bands[137], between
which lies the ventrally placed mouth (_m_). The more conspicuous
ciliated band is formed of a double row of cilia, and is situated in
front of the mouth. The thinner ciliated band behind the mouth appears
to be absent in the American species.
[137] These two rings are at first (Hatschek) not quite closed
dorsally, calling to mind the early condition of the Echinoderm
larvæ with a præ-oral and post-oral ciliated area.
The mouth leads into an oesophagus, and this into a globular stomach
(_ol_), which is continuous with a rectum terminating by an anus
(_an_) placed at the hind end of the posterior conical knob. The whole
alimentary tract is ciliated. In the American form of larva there is a
ring of cilia round the anus, which is developed at a somewhat later
stage in the form observed by Hatschek.
[FIG. 143 POLYGORDIUS LARVA. (From Alex. Agassiz.)]
The position of the ciliated bands and the alimentary tract enables us
to divide the embryo into three regions: a præ-oral region bounded by
the anterior ciliated band, a gastric region in which the embryonic
stomach is situated, and an abdominal region formed of the posterior
conical portion, which by its subsequent elongation gives rise to the
whole segmented portion of the future Polygordius.
At the front end of the præ-oral lobe is situated the early formed
supra-oesophageal ganglion (_sg_) (first noticed by Agassiz) in
connection with which is a pair of eyes, and a ramified system of
nerves. The ganglion is marked externally by a crown of cilia.
[FIG. 144. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
The larval epidermis bears a delicate cuticula, and is separated by a
considerable interval from the walls of the alimentary tract. The
space between the two represents a provisional body cavity, which is
eventually replaced by the permanent body cavity formed between the
two layers of the mesoblast. It is doubtful when the replacement takes
place in the head. It probably does so very early. The mesoblast is
present in the usual form of two bands (_me.p_) (germinal streaks),
which are anteriorly continued into two muscular bands which pass
through the embryonic body cavity to the front end of the præ-oral
lobe. Another pair of contractile bands passes from the same region of
the præ-oral lobe to the oesophagus.
There is no trace of the ventral nerve cord. The most remarkable organ
of the larva is a paired excretory organ (_nph_) discovered by
Hatschek. This is a ciliated canal with at first one and subsequently
several funnel-shaped openings into the body cavity in front and an
external opening behind. It is situated immediately anterior to the
lateral band of mesoblast, and is parallel with, and dorsal to, the
contractile band which passes off from this. It occupies therefore a
position in front of the segmented region of the adult Polygordius.
[FIG. 145. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
The changes by which this peculiar larval form reaches the adult
condition will be easily gathered from an inspection of figs. 143-148.
They consist essentially in the elongation of what has been termed the
abdominal region of the body, and the appearance of a segmentation in
the mesoblast; the segments being formed from before backwards, and
each fresh segment being interpolated between the anus-bearing end of
the body and the last segment.
As the hind portion of the body becomes elongated the stomach extends
into it, and gives rise to the mesenteron of the adult (figs. 143,
144, and 145). For a long time the anterior spherical dilated portion
of the larva remains very large, consisting of a præ-oral lobe and a
post-oral section. The two together may be regarded as constituting
the head.
At a comparatively late stage a pair of tentacles arises from the
front end of the præ-oral lobe (fig. 146), and finally the head
becomes relatively reduced as compared with the body, and gives rise
to the simple head of the fully formed worm (fig. 148). The two
ciliated bands disappear, the posterior vanishing first. The ciliated
band at the hind end of the body also atrophies; and just in front of
it the ring of wart-like prominences used by the adult to attach
itself becomes developed.
[FIG. 146. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
At the sides of the head there is formed a pair of ciliated pits, also
found by Hatschek in the embryo of Criodrilus, and characteristic of
many Chætopod larvæ, but persistent in the adult Polygordius,
Saccocirrus, Polyophthalmus, etc. They are perhaps the same structures
as the ciliated pits in Nemertines.
[FIG. 147. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
During the external changes above described, by which the adult form
of Polygordius is reached, a series of internal changes also takes
place which are for the most part the same as in other Chætopoda; and
do not require a detailed description. The nervous[138] and muscular
systems have precisely the normal development. The division of the
mesoblast into somites is not externally indicated. The organs most
worthy of notice are the excretory organs.
[138] The structure of the ventral cord in the adult requires
further elucidation.
The essential points in the above development of Polygordius are (1)
the gradual elongation and corresponding segmentation of the
post-cephalic part of the body; and (2) the relative reduction in size
of the præ-oral lobe and its conversion together with the oral region
into the head; (3) the atrophy of the ciliated bands. The conversion
of the larva into the adult takes place in fact by the intercalation
of a segmented region between a large mouth-bearing portion of the
primitive body and a small anus-bearing portion[139].
[139] For Semper's view as to the intercalation of segments in
the cephalic region, _vide_ note on p. 333.
[FIG. 148. POLYGORDIUS LARVA. (From Alex. Agassiz.)]
The general mode of development of Chætopod larvæ is similar to the
above except in details, which are however no doubt often of great
importance. The history of the larvæ may be conveniently treated under
three heads. (1) The form of the primitive unsegmented larva; (2) the
arrangement of the cilia on the unsegmented larva, and on the larva at
later stages; (3) the character of the metamorphosis and the
development of the permanent external organs.
A larva similar to the Polygordius larva with a greatly developed
præ-oral lobe is widely distributed amongst the Annelids.
An almost identical form is that of Nepthys scolopendroides (Claparède
and Metschnikoff, No. 336); that of Phyllodoce (fig. 149) is also very
similar, and that of Saccocirrus (Metsch. and Clap. No. 336, Pl. XIII.
fig. 1), a very primitive form most nearly related to Polygordius,
clearly belongs to the same type. Many other larval forms, such as
that of Spio fuliginosus (Metsch. and Clap. No. 336), Terebella,
Nerine, etc., also closely approach this form.
[FIG. 149. LARVA OF PHYLLODOCE. (From Alex. Agassiz.)]
Other really similar forms at first sight appear very different, but
this is mainly owing to the fact that their præ-oral lobe never
attains a considerable development. Its smallness, though obviously of
no deep morphological significance, at once produces a very different
appearance in a larva.
A good example of a larval form with a small præ-oral lobe is afforded
by Capitella, which is figured by Clap. and Metsch. (No. 336, Pl.
XVII. fig. 2). The imperfect development of the præ-oral lobe is also
generally characteristic of the Oligochæta. The persistence of a
relatively large præ-oral lobe for so long a time as in Polygordius is
very unusual.
The arrangement of the cilia in Chætopod larvæ has been employed as a
means of classifying them. Although a classification so framed has no
morphological value, yet the terms themselves which have been invented
are convenient. The terms most usually employed are Atrochæ,
Monotrochæ, Telotrochæ, Polytrochæ, Mesotrochæ. The polytrochæ may
again be subdivided into Polytrochæ proper, Nototrochæ, Gasterotrochæ,
and Amphitrochæ.
The atrochæ contain forms (fig. 139) in which the larva is at first
coated by an uniform covering of cilia, which, though it may
subsequently disappear from certain areas, does not break up into a
series of definite bands.
The monotrochæ or cephalotrochæ are larvæ in which only a single
præ-oral ring is developed (fig. 150 B).
[FIG. 150. TWO CHÆTOPOD LARVÆ. (From Gegenbaur.)
_o._ mouth; _i._ intestine; _a._ anus; _v._ præ-oral ciliated band;
_w._ peri-anal ciliated band.]
In the telotrochæ there is present a præ-oral and a post-oral, _i.e._
peri-anal ring (fig. 150 A); the latter sometimes having the form of a
peri-anal patch.
The polytrochæ are segmented larvæ with perfect or imperfect rings of
cilia on the segments of the body--usually one ring to each
segment--between the two characteristic telotrochal rings. When these
rings are complete the larvæ are polytrochæ proper, when they are only
half rings they are either nototrochæ or gasterotrochæ. Sometimes
there are both dorsal and ventral half rings which do not however
correspond, such forms constitute the amphitrochæ.
In the mesotrochæ one or two rings are present in the middle of the
body, and the characteristic telotrochal rings are absent. Larvæ do
not necessarily continue to belong to the same group at all ages. A
larva may commence as a monotrochal form and then become telotrochal
and from this pass into a polytrochal condition, etc.
The atrochal forms are to be regarded as larvæ which never pass beyond
the primitive stage of uniform ciliation, which in other instances may
precede that of definite rings. They usually lose their cilia early,
as in the cases of Serpula and other larvæ described below.
The atrochal larvæ are not common. The following history of an
Eunicidan larva (probably Lumbriconereis) from Claparède and
Metschnikoff (No. 336) will illustrate their general history.