peritoneal vesicle; _rp._ right peritoneal vesicle; _rp´._
continuation of right vesicle into the stalk; _st._ stalk.]
The third outgrowth of the archenteron gives rise to the
water-vascular vesicle. It first grows round the region of the future
oesophagus and so forms the water-vascular ring. The wall of the ring
then grows towards the body wall so as to divide the oral (left)
peritoneal vesicle into two distinct vesicles, an anterior and a
posterior, shewn in fig. 253, _lp´_ and _lp_. Before this division is
completed, the water-vascular ring is produced in front into five
processes--the future tentacles (fig. 252, _wv_)--which project into
the cavity of the oral vesicle (_lp_). After the oral peritoneal space
has become completely divided into two parts, the anterior dilates
(fig. 253, _lp´_) greatly, and forms a large vestibule at the anterior
end of the body. This vestibule (_lp´_) next acquires a communication
with the mesenteron, shewn in fig. 253 at m. The anterior wall of this
vestibule is finally broken through. By this rupture the mesenteron is
placed in communication with the exterior by the opening at _m_, while
at the same time the tentacles of the water-vascular ring (_t_)
project freely to the exterior. Such is Götte's account of the
præ-oral body space, but, as he himself points out, it involves our
believing that the lining of the diverticulum derived from the
primitive alimentary vesicle becomes part of the external skin. This
occurrence is so remarkable, that more evidence appears to me
requisite before accepting it.
The formation of the anus occurs late. Its position appears to be the
same as that of the blastopore, and is indicated by a papilla of the
mesenteron attaching itself to the skin on the ventral side (fig. 253,
_an_). It eventually becomes placed in an interradial space within the
oral disc of the adult. The water-vascular ring has no direct
communication with the exterior, but the place of the madreporic canal
of other types appears to be taken in the larva by a single tube
leading from the exterior into the body cavity, the external opening
of which is placed on one of the oral plates (_vide_ p. 571) in the
next interradial space to the right of the anus, and a corresponding
diverticulum of the water-vascular ring opening into the body cavity.
The line of junction between the left and right peritoneal vesicles
forms in the larva a ring-like mesentery dividing the oral from the
aboral part of the body cavity. In the adult[220] the oral section of
the larval body cavity becomes the ventral part of the circumvisceral
division of the body cavity, and the subtentacular canals of the arms
and disc; while the aboral section becomes the dorsal part of the
circumvisceral division of the body cavity, the coeliac canals of the
arms, and the cavity of the centro-dorsal piece. The primitive
distinction between the sections of the larval body cavity becomes to
a large extent obliterated, while the axial and intervisceral sections
of the body cavity of the adult are late developments.
[220] _Vide_ P. H. Carpenter, "On the genus Actinometra."
_Linnean Trans._, 2nd Series, Zoology, Vol. II., Part I., 1879.
[FIG. 253. LONGITUDINAL SECTION THROUGH THE CALYX OF AN ADVANCED
PENTACRINOID ANTEDON LARVA WITH CLOSED VESTIBULE. (From Carpenter;
after Götte.)
_ae._ epithelium of oral vestibule; _m._ mouth; _al._ mesenteron;
_an._ rudiment of permanent anus; _lp._ posterior part of left
(oral) peritoneal sack; lp´. anterior part of left (oral) peritoneal
sack; _wr._ water-vascular ring; _t._ tentacle; _mt._ mesentery;
_rp._ right peritoneal sack; _rp´._ continuation of right peritoneal
sack into the stalk; _r._ roof of tentacular vestibule.]
The more important points in the development indicated in the
preceding pages are as follows:
(1) The blastosphere is usually elongated in the direction of the axis
of invagination, but in Comatula it is elongated transversely to this
axis.
(2) The blastopore usually becomes the permanent anus, but it closes
at the end of larval life (there being no anus in the adult) in
Ophiuroids and some Asteroids, while in Comatula it closes very early,
and a fresh anus is formed at the point where the blastopore was
placed.
(3) The larval mouth always becomes the mouth of the adult.
(4) The archenteron always gives rise to outgrowths which form the
peritoneal membrane and water-vascular systems. In Comatula there are
three such outgrowths, two paired, which form the peritoneal vesicles,
and one unpaired, which forms the water-vascular vesicle. In Asteroids
and Ophiuroids there are two outgrowths. In Ophiuroids both of these
are divided into a peritoneal and a water-vascular vesicle, but the
right water-vascular vesicle atrophies. In Asteroids only one
water-vascular vesicle is formed, which is derived from the left
peritoneal vesicle. In Echinoids and Holothuroids there is a single
vaso-peritoneal vesicle.
(5) The water-vascular vesicle grows round the larval oesophagus in
Holothuroids, Ophiuroids, and Comatula; in these cases the larval
oesophagus is carried on into the adult. In other forms the
water-vascular vesicle forms a ring which does not enclose the
oesophagus (Asteroids and Echinoids); in such cases a new oesophagus
is formed, which perforates this ring.
_Development of the larval appendages and metamorphosis._
Holothuroidea. The young larva of Synapta, to which J. Müller gave the
name Auricularia (fig. 255), is in many respects the simplest form of
Echinoderm larva. With a few exceptions the Auricularia type of larva
is common to the Holothuria.
[FIG. 254. A. THE LARVA OF A HOLOTHUROID. B. THE LARVA OF AN
ASTEROID.
_m._ mouth; _st._ stomach; _a._ anus; _l.c._ primitive longitudinal
ciliated band; _pr.c._ præ-oral ciliated band.]
It is (fig. 254 A and fig. 255) bilaterally symmetrical, presenting a
flattened ventral surface, and a convex dorsal one. The anus (_an_) is
situated nearly at the hinder pole, and the mouth (_m_) about the
middle of the ventral surface. In front of the mouth is a considerable
process, the præ-oral lobe. Between the mouth and anus is a space,
more or less concave according to the age of the embryo, interrupted
by a ciliated ridge a little in front of the anus. A similar ciliated
ridge is present on the ventral surface of the præ-oral lobe
immediately in front of the mouth. The anal and oral ridges are
connected by two lateral ciliated bands, the whole forming a
continuous band, which, since the mouth lies in the centre of it (fig.
255), may be regarded as a ring completely surrounding the body behind
the mouth, or more naturally as a longitudinal ring.
[FIG. 255. DIAGRAMMATIC FIGURES REPRESENTING THE EVOLUTION OF AN
AURICULARIA FROM THE SIMPLEST ECHINODERM LARVAL FORM. (Copied from
Müller.)
The black line represents the ciliated ridge. The shaded part is the
oral side of the ring, the clear part the aboral side.
_m._ mouth; _an._ anus.]
The bilateral Auricularia is developed from a slightly elongated
gastrula with an uniform covering of cilia. The gastrula becomes
flattened on the oral side. At the same time the cilia become
specially developed on the oral and anal ridges, and then on the
remainder of the ciliated ring, while they are simultaneously
obliterated elsewhere; and so a complete Auricularia is developed. The
water-vascular ring in the fully developed larva has already
considerably advanced in the growth round the oesophagus (fig. 256
_w.v.r_).
Most Holothurian larvæ, in their transformation from the bilateral
Auricularia form to the radial form of the adult, pass through a stage
in which the cilia form a number of transverse rings, usually five in
number, surrounding the body. The stages in this metamorphosis are
shewn in figs. 256, 257, and 258.
[FIG. 256. FULL-GROWN LARVA OF SYNAPTA. (After Metschnikoff.)
_m._ mouth; _st._ stomach; _a._ anus; _p.v._ left division of
perivisceral cavity, which is still connected with the
water-vascular system; _w.v.r._ water-vascular ring which has not
yet completely encircled the oesophagus; _l.c._ longitudinal part of
ciliated band; _pr.c._ præ-oral part of ciliated band.]
The primitive ciliated band, at a certain stage of the metamorphosis,
breaks up into a number of separate portions (fig. 256), the whole of
which are placed on the ventral surface. Four of these (fig. 257 A and
B) arrange themselves in the form of an angular ring round the mouth,
which at this period projects considerably. The remaining portions of
the primitive band change their direction from a longitudinal one to a
transverse (fig. 257 B), and eventually grow into complete rings (fig.
257 C). Of these there are five. The middle one (257 B) is the first
to develop, and is formed from the dorsal parts of the primitive ring.
The two hinder rings develop next, and last of all the two anterior
ones, one of which appears to be in front of the mouth (fig. 257 C).
[FIG. 257. THREE STAGES IN THE DEVELOPMENT OF SYNAPTA. A and B are
viewed from the ventral surface, and C from the side. (After
Metschnikoff.)
_m._ mouth; _oe._ oesophagus; _pv._ walls of the perivisceral
cavity; _wv._ longitudinal vessel of the water-vascular system; _p._
dorsal pore of water-vascular system; _cr._ ciliated ring formed
round the mouth from parts of the primitive ciliated band.]
The later development of the mouth, and of the ciliated ridge
surrounding it, is involved in some obscurity. It appears from
Metschnikoff (No. 560) that an invagination of the oesophagus takes
place, carrying with it the ciliated ridge around the mouth. This
ridge becomes eventually converted into the covering for the five
tentacular outgrowths of the water-vascular ring (fig. 258), and
possibly also forms the nervous system.
The opening of the oesophageal invagination is at first behind the
foremost ciliated ring, but eventually comes to lie in front of it,
and assumes a nearly terminal though slightly ventral position (fig.
258). No account has been given of the process by which this takes
place, but the mouth is stated by Metschnikoff (though Müller differs
from him on this point) to remain open throughout. The further changes
in the metamorphosis are not considerable. The ciliated bands
disappear, and a calcareous ring of ten pieces, five ambulacral and
five interambulacral, is formed round the oesophagus. A provisional
calcareous skeleton is also developed.
[FIG. 258. A LATE STAGE IN THE DEVELOPMENT OF SYNAPTA. (After
Metschnikoff.)
The figure shews the vestibular cavity with retracted tentacles; the
ciliated bands; the water-vascular system, etc.
_p._ dorsal pore of water-vascular system; _pv._ walls of
perivisceral cavity; _ms._ amoeboid cells.]
All the embryonic systems of organs pass in this case directly into
those of the adult.
The metamorphosis of most Holothuroidea is similar to that just
described. In Cucumaria (Selenka) there is however no Auricularia
stage, and the uniformly ciliated stage is succeeded by one with five
transverse bands of cilia, and a præ-oral and an anal ciliated cap.
The mouth is at first situated ventrally behind the præ-oral cap of
cilia, but the præ-oral cap becomes gradually absorbed, and the mouth
assumes a terminal position.
In Psolinus (Kowalevsky) there is no embryonic ciliated stage, and the
adult condition is attained without even a metamorphosis. There appear
to be five plates surrounding the mouth, which are developed before
any other part of the skeleton, and are regarded by P. H. Carpenter
(No. 548) as equivalent to the five oral plates of the Crinoidea. The
larval condition with ciliated bands is often spoken of as the pupa
stage, and during it the larvæ of Holothurians proper use their
embryonic tube feet to creep about.
Asteroidea. The commonest and most thoroughly investigated form of
Asteroid larva is a free-swimming form known as Bipinnaria.
This form in passing from the spherical to the bilateral condition
passes through at first almost identical changes to the Auricularian
larva. The cilia become at an early period confined to an oral and
anal ridge.
The anal ridge gradually extends dorsalwards, and finally forms a
complete longitudinal post-oral ring (fig. 259 A); the oral ridge also
extends dorsalwards, and forms a closed præ-oral ring (fig. 259 A),
the space within which is left unshaded in my figure.
The presence of two rings instead of one distinguishes the Bipinnaria
from the Auricularia. The two larvæ are shewn side by side in fig.
254, and it is obvious that the two bands of the Bipinnaria are (as
pointed out by Gegenbaur) equivalent to the single band of the
Auricularia divided into two. Ontologically, however, the two bands of
Bipinnaria do not appear to arise from the division of a single band.
As the Bipinnaria grows older, a series of arms grows out along lines
of the two ciliated bands (fig. 259 C), and, in many cases, three
special arms are formed, not connected with the ciliated bands, and
covered with warts. These latter arms are known as brachiolar arms,
and the larvæ provided with them as Brachiolaria (fig. 259 D).
[FIG. 259. DIAGRAMMATIC REPRESENTATION OF VARIOUS FORMS OF ASTEROID
LARVÆ. A, B, C, BIPINNARIA; D, BRACHIOLARIA. (Copied from Müller.)
The black lines represent the ciliated bands; and the shading the
space between the præ-oral and the post-oral bands.
_m._ mouth; _an._ anus.]
As a rule the following arms can be distinguished (fig. 259 C and D),
on the hinder ring (Agassiz' nomenclature) a median anal pair, a
dorsal anal pair, and a ventral anal pair, a dorsal oral pair, and an
unpaired anterior dorsal arm; on the præ-oral ring a ventral oral
pair, and sometimes (Müller) an unpaired anterior ventral arm.
The three brachiolar arms arise as processes from the base of the
unpaired dorsal arm, and the two ventral oral arms. The extent of the
development of the arms varies with the species.
The changes by which the Bipinnaria or Brachiolaria becomes converted
into the adult starfish are very much more complicated than those
which take place in Holothurians. For an accurate knowledge of them we
are largely indebted to Alex. Agassiz (No. 543). The development of
the starfish takes place entirely at the posterior end of the larva
close to the stomach.
On the right and dorsal side of the stomach, and externally to the
_right peritoneal space_, are formed five radially situated calcareous
rods arranged in the form of a somewhat irregular pentagon. The
surface on which they are deposited has a spiral form, and constitutes
together with its calcareous rods, the abactinal or dorsal surface of
the future starfish. Close to its dorsal, _i.e._ embryonic dorsal,
edge lies the dorsal pore of the water-vascular system (madreporic
canal), and close to its ventral edge the anus. On the left and
ventral side of the stomach is placed the water-vascular rosette, the
development of which was described on p. 549. It is situated on the
actinal or ventral surface of the future starfish, and is related to
the left peritoneal vesicle.
Metschnikoff (No. 560) and Agassiz (No. 543) differ slightly as to the
constitution of the water-vascular rosette. The former describes and
figures it as a completely closed rosette, the latter states that 'it
does not form a completely closed curve but is always open, forming a
sort of twisted crescent-shaped arc.'
The water-vascular rosette is provided with five lobes, corresponding
to which are folds in the larval skin, and each lobe corresponds to
one of the calcareous plates developed on the abactinal disc. The
plane of the actinal surface at first meets that of the abactinal at
an acute or nearly right angle. The two surfaces are separated by the
whole width of the stomach. The general appearance of the larva from
the ventral surface after the development of the water-vascular
rosette (_i_) and abactinal disc (_A_) is shewn in fig. 260.
[FIG. 260. BIPINNARIA LARVA OF AN ASTEROID. (From Gegenbaur; after
Müller.)
_b._ mouth; _a._ anus; _h._ madreporic canal; _i._ ambulacral
rosette; _c._ stomach; _d. g. e._ etc. arms of Bipinnaria; _A._
abactinal disc of young Asteroid.]
As development proceeds the abactinal surface becomes a firm and
definite disc, owing to the growth of the original calcareous spicules
into more or less definite plates, and to the development of five
fresh plates nearer the centre of the disc and _interradial in
position_. Still later a central calcareous plate appears on the
abactinal surface, which is thus formed of a central plate, surrounded
by a ring of five interradial plates, and then again by a ring of five
radial plates. The abactinal disc now also grows out into five short
processes, separated by five shallow notches. These processes are the
rudiments of the five arms, and each of them corresponds to one of the
lobes of the water-vascular rosette. A calcareous deposit is formed
round the opening of the water-vascular canal, which becomes the
madreporic tubercle[221]. At about this stage the absorption of the
larval appendages takes place. The whole anterior part of the larva
with the great præ-oral lobe has hitherto remained unchanged, but now
it contracts and undergoes absorption, and becomes completely
withdrawn into the disc of the future starfish. The larval mouth is
transported into the centre of the actinal disc. In the larvæ observed
by Agassiz and Metschnikoff nothing was cast off, but the whole
absorbed.
[221] The exact position of the madreporic tubercle in relation
to the abactinal plates does not seem to have been made out. It
might have been anticipated that it would be placed in one of the
primary interradial plates, but this does not seem to be the
case. The position of the anus is also obscure.
According to Müller and Koren and Danielssen this is not the case in
the larva observed by them, but part of the larva is thrown off, and
lives for some time independently.
After the absorption of the larval appendages the actinal and
abactinal surfaces of the young starfish approach each other, owing to
the flattening of the stomach; at the same time they lose their spiral
form, and become flat discs, which fit each other. Each of the lobes
of the rosette of the water-vascular system becomes one of the radial
water-vascular canals. It first becomes five-lobed, each lobe forming
a rudimentary tube foot, and on each side of the middle lobe two fresh
ones next spring out, and so on in succession. The terminal median
lobe forms the tentacle at the end of the arm, and the eye is
developed at its base. The growth of the water-vascular canals keeps
pace with that of the arms, and the tube feet become supported at
their base by an ingrowth of calcareous matter. The whole of the
calcareous skeleton of the larva passes directly into that of the
adult, and spines are very soon formed on the plates of the abactinal
surface. The original radial plates, together with the spines which
they have, are gradually pushed outwards with the growth of the arms
by the continual addition of fresh rows of spines between the terminal
plate and the plate next to it. It thus comes about that the original
radial plates persist at the end of the arms, in connection with the
unpaired tentacles which form the apex of the radial water-vascular
tubes.
It has already been mentioned that according to Metschnikoff (No. 560)
a new oesophagus is formed which perforates the water-vascular ring,
and connects the original stomach with the original mouth. Agassiz
(No. 543) maintains that the water-vascular ring grows round the
primitive oesophagus. He says--"During the shrinking of the larva the
long oesophagus becomes shortened and contracted, bringing the opening
of the mouth of the larva to the level of the opening of the
oesophagus, which eventually becomes the true mouth of the starfish."
The primitive anus is believed by Metschnikoff to disappear, but by
Agassiz to remain. This discrepancy very possibly depends upon these
investigators having worked at different species.
There is no doubt that the whole of the larval organs, with the
possible exception of the oesophagus, and anus (where absent in the
adult), pass directly into the corresponding organs of the
starfish--and that the præ-oral part of the body and arms of the larva
are absorbed and not cast off.
In addition to the Bipinnarian type of Asteroid larva a series of
other forms has been described by Müller (No. 561), Sars, Koren, and
Danielssen (No. 554) and other investigators, which are however very
imperfectly known. The best known form is one first of all discovered
by Sars in Echinaster Sarsii, and the more or less similar larvæ
subsequently investigated by Agassiz, Busch, Müller, Wyville Thomson,
etc. of another species of Echinaster and of Asteracanthion. These
larvæ on leaving the egg have an oval form, and are uniformly covered
by cilia. Four processes (or in Agassiz' type one process) grow out
from the body; by these the larvæ fix themselves. In the case of
Echinaster the larvæ are fixed in the ventral concavity of the disc of
the mother, between the five arms, where a temporary brood-pouch is
established. The main part of the body is converted directly into the
disc of the young starfish, while the four processes come to spring
from the ventral surface, and are attached to the water-vascular ring.
Eventually they atrophy completely. Of the internal structure but
little is known; till the permanent mouth is formed, after the
development of the young starfish is pretty well advanced, the stomach
has no communication with the exterior.
A second abnormal type of development is presented by the embryo of
Pteraster miliaris, as described by Koren and Danielssen[222]. The
larvæ to the number of eight to twenty develop in a peculiar pouch on
the dorsal surface of the body. The early stages are not known, but in
the later ones the whole body assumes a pentagonal appearance with a
mouth at one edge of the disc. At a later stage the anus is formed on
the dorsal side of an arm opposite the mouth. The stomach is
surrounded by a water-vascular ring, from which the madreporic canal
passes to the dorsal surface, but does not open. At a later stage the
embryonic mouth and anus vanish, to be replaced by a permanent mouth
and anus in the normal positions.
[222] The following statements are taken from the abstract in
Bronn's _Thierreichs_.
A third, and in some respects very curious, form is a worm like larva
of Müller, which is without bands of cilia. The dorsal surface of the
youngest larva is divided by transverse constrictions into five
segments. On the under side of the first of these is a five-lobed
disc, each lobe being provided with a pair of tube feet.
At a later period only three segments are visible on the dorsal
surface, but the ventral surface has assumed a pentagonal aspect. The
later stages are not known.
[FIG. 261. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OF AN
OPHIUROID PLUTEUS FROM A SIMPLE ECHINODERM LARVA. (Copied from
Müller.) The calcareous skeleton is not represented.
_m._ mouth; _an._ anus; _d._ anterior arms; _d´._ lateral arms;
_e´._ posterior arms; _g´._ anterolateral arms.]
Ophiuroidea. The full-grown larva of the Ophiuroids is known as a
Pluteus. It commences with the usual more or less spherical form; from
this it passes to a form closely resembling that of Auricularia with a
rounded dorsal surface, and a flattened ventral one. Soon however it
becomes distinguished by the growth of a post-anal lobe and the
absence of a præ-oral lobe (fig. 261 B). The post-anal lobe forms the
somewhat rounded apex of the body. In front of the mouth, and between
the mouth and anus, arise the anal and oral ciliated ridges, which
soon become continued into a single longitudinal ciliated ring. At the
same time the body becomes prolonged into a series of processes along
the ciliated band, which is continued to the extremity of each. The
primitive ciliated ring never becomes broken up into two or more
rings. A ciliated crown is usually developed at the extremity of the
post-anal lobe. The arms are arranged in the form of a ring
surrounding the mouth, and are all directed forwards.
The first arms to appear are two lateral ones, which usually remain
the most conspicuous (fig. 261 B and C, _d´_). Next arises a pair on
the sides of the mouth, which may be called the mouth or anterior arms
(C, _d_). A pair ventral to and behind the lateral arms is then
formed, constituting the posterior arms (D, _e´_), and finally a pair
between the lateral arms and the anterior, constituting the
anterolateral arms (D, _g´_).
The concave area between the arms forms the greater part of the
ventral surface of the body. Even before the appearance of any of the
arms, and before the formation of the mouth, two calcareous rods are
formed, which meet behind at the apex of the post-anal lobe, and are
continued as a central support into each of the arms as they are
successively formed. These rods are shewn at their full development in
fig. 262. The important points which distinguish a Pluteus larva from
the Auricularia or Bipinnaria are the following:
(1) The presence of the post-anal lobe at the hind end of the body.
(2) The slight development of a præ-oral lobe. (3) The provisional
calcareous skeleton in the larval arms.
[FIG. 262. PLUTEUS LARVA OF AN OPHIUROID. (From Gegenbaur; after
Müller.)
_A._ rudiment of young Ophiuroid; _d´._ lateral arms; _d._ anterior
arms; _e´._ posterior arms.]
Great variations are presented in the development of the arms and
provisional skeleton. The presence of lateral arms is however a
distinctive characteristic of the Ophiuroid Pluteus. The other arms
may be quite absent, but the lateral arms never.
The formation of the permanent Ophiuroid takes place in much the same
way as in the Asteroidea.
There is formed (fig. 262) on the right and dorsal side of stomach the
abactinal disc supported by calcareous plates, at first only five in
number and radial in position[223]. The disc is at first not
symmetrical, but becomes so at the time of the resorption of the
larval arms. It grows out into five processes--the five future rays.
The original five radial plates remain as the terminal segments of the
adult rays, and new plates are always added between the ultimate and
penultimate plate (Müller), though it is probable that in the later
stages fresh plates are added in the disc.
[223] Whether interradial plates are developed as in Asterias is
not clear. They seem to be found in Ophiopholis bellis, Agassiz,
but have not been recognised in other forms (_vide_ Carpenter,
No. 548, p. 369).
[FIG. 263. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OF ECHINOID
PLUTEI. (Copied from Müller.) The calcareous skeleton is not
represented. E. Pluteus of Spatangus.
_m._ mouth; _an._ anus; _d._ anterior arms; _d´._ point where
lateral arms arise in the Ophiuroid Pluteus; _e._ anterointernal
arms; _e´._ posterior arms; _g´._ anterolateral arms; _g._
anteroexternal arms.]
The ventral surface of the permanent Ophiuroid is formed by the
concave surface between the mouth and anus. Between this and the
stomach is situated the water-vascular ring. It is at first not
closed, but is horseshoe-shaped, with five blind appendages (fig.
262). It eventually grows round the oesophagus, which, together with
the larval mouth, is retained in the adult. The five blind appendages
become themselves lobed in the same way as in Asterias, and grow out
along the five arms of the disc and become the radial canals and
tentacles. All these parts of the water-vascular system are of course
covered by skin, and probably also surrounded by mesoblast cells, in
which at a later period the calcareous plates which lie ventral to the
radial canal are formed. The larval anus disappears. As long as the
larval appendages are not absorbed the ventral and dorsal discs of the
permanent Ophiuroid fit as little as in the case of the Brachiolaria,
but at a certain period the appendages are absorbed. The calcareous
rods of the larval arms break up, the arms and anal lobe become
absorbed, and the dorsal and ventral discs, with the intervening
stomach and other organs, are alone left. After this the discs fit
together, and there is thus formed a complete young Ophiuroid.
The whole of the internal organs of the larva (except the anus),
including the mouth, oesophagus, the body cavity, etc. are carried on
directly into the adult.
The larval skeleton is, as above stated, absorbed.
The viviparous larva of Amphiura squamata does not differ very greatly
from the larvæ with very imperfect arms. It does not develop a
distinct ciliated band, and the provisional skeleton is very
imperfect. The absence of these parts, as well as of the anus,
mentioned on p. 549, may probably be correlated with the viviparous
habits of the larva. With reference to the passage of this larva into
the adult there is practically nothing to add to what has just been
stated. When the development of the adult is fairly advanced the part
of the body with the provisional skeleton forms an elongated rod-like
process attached to the developing disc. It becomes eventually
absorbed.
[FIG. 264. TWO LARVÆ OF STRONGYLOCENTRUS. (From Agassiz.)
_m._ mouth; _a._ anus; _o._ oesophagus; _d._ stomach; _c._
intestine; _v´._ and _v._ ciliated ridges; _w._ water-vascular tube;
_r._ calcareous rods.]
Echinoidea. The Echinus larva (fig. 263) has a Pluteus form like that
of the Ophiuroids, and in most points, such as the presence of the
anal lobe, the ciliated band, the provisional skeleton, etc., develops
in the same manner. The chief difference between the two Pluteus forms
concerns the development of the lateral arms. These, which form the
most prominent arms in the Ophiuroid Pluteus, are entirely absent in
the Echinoid Pluteus, which accordingly has, as a rule, a much
narrower form than the Ophiuroid Pluteus.
A pair of ciliated epaulettes on each side of and behind the ciliated
ring is very characteristic of some Echinoid larvæ. They are
originally developed from the ciliated ring (fig. 266 A and B, _v´´_).
The presence of three processes from the anal lobe supported by
calcareous rods is characteristic of the Spatangoid Pluteus (fig. 263
E).
[FIG. 265. LATERAL AND VENTRAL VIEW OF A LARVA OF STRONGYLOCENTRUS.
(From Agassiz.) General references as in fig. 264.
_b._ dorsal opening of madreporic canal; _e´._ posterior arms;
_e´´´._ anterior arms; _eIV._ anterointernal arms.]
The first two pairs of arms to develop, employing the same names as in
Ophiuroids, are the anterior attached to the oral process (fig. 263 C,
_d_) and the posterior pair (_e´_). A pair of anterolateral arms next
becomes developed (_g´_). A fourth pair (not represented in
Ophiuroids) appears on the inner side of the anterior pair forming an
anterointernal pair (_e_), and in the Spatangoid Pluteus a fifth pair
may be added on the external side of the anterior pair forming an
anteroexternal pair (_g_).
Each of the first-formed paired calcareous rods is composed of three
processes, two of which extend into the anterior and posterior arms;
and the third and strongest passes into the anal lobe, and there meets
its fellow (fig. 265). A transverse bar in front of the arms joins the
rods of the two sides meeting them at the point where the three
processes diverge. The process in the anterolateral arm (fig. 266 B)
is at first independent of this system of rods, but eventually unites
with it. Although our knowledge of the Pluteus types in the different
groups is not sufficient to generalise with great confidence, a few
points seem to have been fairly determined[224]. The Plutei of
Strongylocentrus (figs. 266 and 267) and Echinus have eight arms and
four ciliated epaulettes. The only Cidaris-like form, the Pluteus of
which is known, is Arbacia: it presents certain peculiarities. The
anal lobe develops a pair of posterior (auricular) appendages, and the
ciliated ring, besides growing out into the normal eight appendages,
has a pair of short blunt anterior and posterior lobes. An extra pair
of non-ciliated accessory mouth arms appears also to be developed.
Ciliated epaulettes are not present. So far as is known the
Clypeastroid larva is chiefly characterized by the round form of the
anal lobe. The calcareous rods are latticed. In the Pluteus of
Spatangoids there are (fig. 263) five pairs of arms around the mouth
pointing forwards, and three arms developed from the anal lobe
pointing backwards. One of these is unpaired, and starts from the apex
of the anal lobe. All the arms have calcareous rods which, in the case
of the posterior pair, the anterolateral pair, and the unpaired arm of
the anal lobe, are latticed. Ciliated epaulettes are not developed.
[224] _Vide_ especially Müller, Agassiz, and Metschnikoff.
Viviparous larvæ of Echinoids have been described by Agassiz[225].
[225] For viviparous Echini _vide_ Agassiz, _Proc. Amer. Acad.
1876_.
The development of the permanent Echinus has been chiefly worked out
by Agassiz and Metschnikoff.
In the Pluteus of Echinus lividus the first indication of the adult
arises, when three pairs of arms are already developed, as an
invagination of the skin on the left side, between the posterior and
anterolateral arms, the bottom of which is placed close to the
water-vascular vesicle (fig. 266 B, _w´_). The base of this
invagination becomes very thick, and forms the ventral disc of the
future Echinus. The parts connecting this disc with the external skin
become however thin, and, on the narrowing of the external aperture of
invagination and the growth of the thickened disc, constitute a
covering for the disc, called by Metschnikoff the amnion. The
water-vascular vesicle adjoining this disc grows out into five
processes, forming as many tube feet, which cause the surface of the
involuted disc to be produced into the same number of processes. The
external opening of the invagination of the disc never closes, and
after the development of the tube feet begins to widen again, and the
amnion to atrophy. Through the opening of the invagination the tube
feet now project. The dorsal and right surface of the Pluteus, which
extends so as to embrace the opening of the madreporic canal and the
anus, forms the abactinal or dorsal surface of the future Echinus
(fig. 267, _a_). This disc fits on to the actinal invaginated surface
which arises on the left side of the Pluteus. On the right surface of
the larva (dorsal of permanent Echinus) two pedicellariæ appear, and
at a later period spines are formed, which are at first arranged in a
ring-like form round the edge of the primitively flat test. While
these changes are taking place, and the two surfaces of the future
Echinus are gradually moulding themselves so as to form what is
obviously a young Echinus, the arms of the Pluteus with their
contained skeleton have been gradually undergoing atrophy. They become
irregular in form, their contained skeleton breaks up into small
pieces, and they are gradually absorbed.
[FIG. 266. SIDE AND DORSAL VIEW OF A LARVA OF STRONGYLOCENTRUS.
(From Agassiz.) General reference letters as in figs. 264 and 265.
_e´´._ anterolateral arms; _v´´_. ciliated epaulettes; _w´_.
invagination to form the disc of Echinus.]
The water-vascular ring is from the first complete, so that, as in
Asterias, it is perforated through the centre by a new oesophagus.
According to Agassiz the first five tentacles or tube feet grow into
the radial canals, and form the odd terminal tentacles exactly as in
Asterias[226]. Spatangus only differs in development from Echinus in
the fact that the opening of the invagination to form the ventral disc
becomes completely closed, and that the tube feet have eventually to
force their way through the larval epidermis of the amnion, which is
ruptured in the process and eventually thrown off.
[226] Götte (No. 549) supported by Müller's and Krohn's older,
and in some points extremely erroneous observations, has
enunciated the view that the radial canals in Echinoids and
Holothuroids have a different nature from those in Asteroids and
Ophiuroids.
[FIG. 267. FULL-GROWN LARVA OF STRONGYLOCENTRUS. (From Agassiz.)
The figure shews the largely-developed abactinal disc of the young
Echinus enclosing the larval stomach. Reference letters as in
previous figs.]
Crinoidea. The larva of Antedon, while still within the egg-shell,
assumes an oval form and uniform ciliation. Before it becomes hatched
the uniform layer of cilia is replaced by four transverse bands of
cilia, and a tuft of cilia at the posterior extremity. In this
condition it escapes from the egg-shell (fig. 268 A), and becomes
bilateral, owing to a flattening of the ventral surface. On the
flattened surface appears a ciliated depression corresponding in
position with the now closed blastopore (_vide_ p. 550). The third
ciliated band bends forward to pass in front of this (fig. 269).
Behind the last ciliated band there is present a small depression of
unknown function, also situated on the ventral surface. The posterior
extremity of the embryo elongates to form the rudiment of the future
stem, and a fresh depression, marking the position of the future
mouth, makes its appearance on the anterior and ventral part.
[FIG. 268. THREE STAGES IN THE DEVELOPMENT OF ANTEDON (COMATULA.)
(From Lubbock; after Thomson.)
A. larva just hatched; B. larva with rudiment of the calcareous
plates; C. Pentacrinoid larva.]
While the ciliated bands are still at their full development, the
calcareous skeleton of the future calyx makes its appearance in the
form of two rows, each of five plates, formed of a network of spicula
(figs. 268 B and 269). The plates of the anterior ring are known as
the orals, those of the posterior as the basals. The former surround
the left, _i.e_. anterior peritoneal sack; the latter the right,
_i.e._ posterior peritoneal sack. The two rows of plates are at first
not quite transverse, but form two oblique circles, the dorsal end
being in advance of the ventral. The rows soon become transverse,
while the originally somewhat ventral oral surface is carried into the
centre of the area enclosed by the oral plates.
[FIG. 269. LARVA OF ANTEDON WITH RUDIMENTS OF CALCAREOUS SKELETON.
(From Carpenter; after Thomson.)
1. Terminal plate at the end of the stem; 3. basals; _or._ orals;
_bl._ position of blastopore.]
By the change in position of the original ventral surface relatively
to the axis of the body, the bilateral symmetry of the larva passes
into a radial symmetry. While the first skeletal elements of the calyx
are being formed, the skeleton of the stem is also established. The
terminal plate is first of all established, then the joints, eight at
first, of the stem. The centro-dorsal plate is stated by Thomson to be
formed as the uppermost joint of the stem[227]. The larva, after the
completion of the above changes, is shewn in fig. 268 B, and somewhat
more diagrammatically in fig. 269.
[227] Götte (No. 549) on the other hand holds that the
centro-dorsal plate is developed by the coalescence of a series
of at first independent rods, which originate simultaneously
with, and close to, the lower edges of the basals, and that it is
therefore similar in its origin to the basals.
[FIG. 270. YOUNG PENTACRINOID LARVA OF ANTEDON. (From Carpenter;
after Wyville Thomson.)
1. terminal plate of stem; _cd._ centro-dorsal plate; 3. basals; 4.
radials; _or._ orals.]
After the above elements of the skeleton have become established the
ciliated bands undergo atrophy, and shortly afterwards the larva
becomes attached by the terminal plate of its stem. It then passes
into the Pentacrinoid stage. The larva in this stage is shewn in fig.
268 C and fig. 270. New joints are added at the upper end of the stem
next the calyx, and a new element--the radials--makes its appearance
as a ring of five small plates, placed in the space between the basals
and orals, and in the intervals alternating with them (fig. 270, 4).
The roof of the oral vestibule (_vide_ fig. 253 and p. 551) has in the
meantime become ruptured; and the external opening of the mouth thus
becomes established. Surrounding the mouth are five petal-like lobes,
each of them supported by an oral plate (fig. 268 C). In the intervals
between them five branched and highly contractile tentacles, which
were previously enclosed within the vestibule, now sprout out: they
mark the position of the future radial canals, and are outgrowths of
the water-vascular ring. At the base of each of them a pair of
additional tentacles is soon formed. Each primary tentacle corresponds
to one of the radials. These latter are therefore, as their name
implies, radial in position; while the basals and orals are
interradial. In addition to the contractile radial tentacles ten
non-contractile tentacles, also diverticula of the water-vascular
ring, are soon formed, two for each interradius.
In the course of the further development the equatorial space between
the orals and the basals enlarges, and gives rise to a wide oral disc,
the sides of which are formed by the radials resting on the basals;
while in the centre of it are placed the five orals, each with its
special lobe.
The anus, which is formed on the ventral side in the position of the
blastopore (p. 551), becomes surrounded by an anal plate, which is
interradial in position, and lies on the surface of the oral disc
between the orals and radials. On the oral plate in the next
interradius is placed the opening of a single funnel leading into the
body cavity, which Ludwig regards as equivalent to the opening of the
madreporic canal (_vide_ p. 551)[228].
[228] I have made no attempt to discuss the homologies of the
plates of the larval Echinodermata because the criteria for such
a discussion are still in dispute. The suggestive memoirs of P.
H. Carpenter (No. 548) on this subject may be consulted by the
reader. Carpenter attempts to found his homologies on the
relation of the plates to the primitive peritoneal vesicles, and
I am inclined to believe that this method of dealing with these
homologies is the right one. Ludwig (No. 559) by regarding the
opening of the madreporic canal as a fixed point has arrived at
very different results.
From the edge of the vestibule the arms grow out, carrying with them
the tentacular prolongation of the water-vascular ring. Two additional
rows of radials are soon added.
The stalked Pentacrinoid larva becomes converted, on the absorption of
the stalk, into the adult Antedon. The stalk is functionally replaced
by a number of short cirri springing from the centro-dorsal plate. The
five basals coalesce into a single plate, known as the rosette, and
the five orals disappear, though the lobes on which they were placed
persist. In some stalked forms, _e.g._ Rhizocrinus Hyocrinus, the
orals are permanently retained. The arms bifurcate at the end of the
third radial, and the first radial becomes in Antedon rosacea (though
not in all species of Antedon) concealed from the surface by the
growth of the centro-dorsal plate. An immense number of funnels,
leading into the body cavity, are formed in addition to the single one
present in the young larva. These are regarded by Ludwig as equivalent
to so many openings of the madreporic canal; and there are developed,
in correspondence with them, diverticula of the water-vascular ring.
_Comparison of Echinoderm Larvæ and General Conclusions._
In any comparison of the various types of Echinoderm larvæ it is
necessary to distinguish between the free-swimming forms, and the
viviparous or fixed forms. A very superficial examination suffices to
shew that the free-swimming forms agree very much more closely amongst
themselves than the viviparous forms. We are therefore justified in
concluding that in the viviparous forms the development is abbreviated
and modified.
All the free forms are nearly alike in their earliest stage after the
formation of the archenteron. The surface between the anus and the
future mouth becomes flattened, and (except in Antedon, Cucumaria,
Psolinus, etc. which practically have an abbreviated development like
that of the viviparous forms) a ridge of cilia becomes established in
front of the mouth, and a second ridge between the mouth and the anus.
This larval form, which is shewn in fig. 264 A, is the type from which
the various forms of Echinoderm larvæ start.
In all cases, except in Bipinnaria, the two ciliated ridges soon
become united, and constitute a single longitudinal post-oral ciliated
ring.
The larvæ in their further growth undergo various changes, and in the
later stages they may be divided into two groups:
(1) The Pluteus larva of Echinoids and Ophiuroids.
(2) The Auricularia (Holothuroids) and Bipinnaria (Asteroids) type.
The first group is characterized by the growth of a number of arms
more or less surrounding the mouth, and supported by calcareous rods.
The ciliated band retains its primitive condition as a simple
longitudinal band throughout larval life. There is a very small
præ-oral lobe, while an anal lobe is very largely developed.
[FIG. 271. A. THE LARVA OF A HOLOTHUROID. B. THE LARVA OF AN
ASTERIAS.
_m._ mouth; _st._ stomach; _a._ anus; _l.c._ primitive longitudinal
ciliated band; _pr.c._ præ-oral ciliated band.]
The Auricularia and Bipinnaria resemble each other in shape, in the
development of a large præ-oral lobe, and in the absence of
provisional calcareous rods; but differ in the fact that the ciliated
band is single in Auricularia (fig. 271 A), and is double in
Bipinnaria (fig. 271 B).
The Bipinnaria larva shews a great tendency to develop soft arms;
while in the Auricularia the longitudinal ciliated band breaks up into
a number of transverse ciliated bands. This condition is in some
instances reached directly, and such larvæ undoubtedly approximate to
the larvæ of Antedon, in which the uniformly ciliated condition is
succeeded by one with four transverse bands, of which one is præ-oral.
All or nearly all Echinoderm larvæ are bilaterally symmetrical, and
since all Echinodermata eventually attain a radial symmetry, a change
necessarily takes place from the bilateral to the radial type.
In the case of the Holothurians and Antedon, and generally in the
viviparous types, this change is more or less completely effected in
the embryonic condition; but in the Bipinnaria and Pluteus types a
radial symmetry does not become apparent till after the absorption of
the larval appendages. It is a remarkable fact, which seems to hold
for the Asteroids, Ophiuroids, Echinoids, and Crinoids, that the
dorsal side of the larva is not directly converted into the dorsal
disc of the adult; but the dorsal and right side becomes the adult
dorsal or abactinal surface, while the ventral and left becomes the
actinal or ventral surface.
It is interesting to note with reference to the larvæ of the
Echinodermata that the various existing types of larvæ must have been
formed after the differentiation of the existing groups of the
Echinodermata; otherwise it would be necessary to adopt the impossible
position that the different groups of Echinodermata were severally
descended from the different types of larvæ. The various special
appendages, etc. of the different larvæ have therefore a purely
secondary significance; and their atrophy at the time of the passage
of the larva into the adult, which is nothing else but a complicated
metamorphosis, is easily explained.
Originally, no doubt, the transition from the larva to the adult was
very simple, as it is at present in most Holothurians; but as the
larvæ developed various provisional appendages, it became necessary
that these should be absorbed in the passage to the adult state.
It would obviously be advantageous that their absorption should be as
rapid as possible, since the larva in a state of transition to the
adult would be in a very disadvantageous position. The rapid
metamorphosis, which we find in Asteroids, Ophiuroids, and Echinoids
in the passage from the larval to the adult state, has no doubt arisen
for this reason.
In spite of the varying provisional appendages possessed by Echinoderm
larvæ it is possible, as stated above (p. 574), to recognise a type of
larva, of which all the existing Echinoderm larval forms are
modifications. This type does not appear to me to be closely related
to that of the larvæ of any group described in the preceding pages. It
has no doubt certain resemblances to the trochosphere larva of
Chætopoda, Mollusca, etc., but the differences between the two types
are more striking than the resemblances. It firstly differs from the
trochosphere larva in the character of the ciliation. Both larvæ start
from the uniformly ciliated condition, but while the præ-oral ring is
almost invariable, and a peri-anal ring very common in the
trochosphere; in the Echinoderm larva such rings are rarely found; and
even when present, _i.e._ the præ-oral ring of Bipinnaria and the
terminal though hardly peri-anal patch of Antedon, do not resemble
closely the more or less similar structures of the trochosphere. The
two ciliated ridges (fig. 264 A) common to all the Echinoderm larvæ,
and subsequently continued into a longitudinal ring, have not yet been
found in any trochosphere. The transverse ciliated rings of the
Holothurian and Crinoid larvæ are of no importance in the comparison
between the trochosphere larvæ and the larvæ of Echinodermata, since
such rings are frequently secondarily developed. Cf. Pneumodermon and
Dentalium amongst Mollusca.
In the character of the præ-oral lobe the two types again differ.
Though the præ-oral lobe is often found in Echinoderm larvæ it is
never the seat of an important (supra-oesophageal) ganglion and organs
of special sense, as it invariably is in the trochosphere.
Nothing like the vaso-peritoneal vesicles of the Echinoderm larvæ has
been found in the trochosphere; nor have the characteristic
trochosphere excretory organs been found in the Echinoderm larvæ.
The larva which most nearly approaches those of the Echinodermata is
the larva of Balanoglossus described in the next chapter.
BIBLIOGRAPHY.
(542) Alex. Agassiz. _Revision of the Echini._ Cambridge, U.S.
1872-74.
(543) Alex. Agassiz. "North American Starfishes." _Memoirs of the
Museum of Comparative Anatomy and Zoology at Harvard College_, Vol.
V., No. 1. 1877 (originally published in 1864).
(544) J. Barrois. "Embryogénie de l'Asteriscus verruculatus." _Journal
de l'Anat. et Phys._ 1879.
(545) A. Baur. _Beiträge zur Naturgeschichte d. Synapta digitata._
Dresden, 1864.
(546) H. G. Bronn. _Klassen u. Ordnungen etc. Strahlenthiere_, Vol.
II. 1860.
(547) W. B. Carpenter. "Researches on the structure, physiology and
development of Antedon." _Phil. Trans._ CLVI. 1866, and _Proceedings
of the Roy. Soc._, No. 166. 1876.
(548) P. H. Carpenter. "On the oral and apical systems of the
Echinoderms." _Quart. J. of Micr. Science_, Vol. XVIII. and XIX.
1878-9.
(549) A. Götte. "Vergleichende Entwicklungsgeschichte d. Comatula
mediterranea." _Arch. für micr. Anat._, Vol. XII. 1876.
(550) R. Greeff. "Ueber die Entwicklung des Asteracanthion rubens vom
Ei bis zur Bipinnaria u. Brachiolaria." _Schriften d. Gesellschaft zur
Beförderung d. gesammten Naturwissenschaften zu Marburg_, Bd. XII.
1876.
(551) R. Greeff. "Ueber den Bau u. die Entwicklung d. Echinodermen."
_Sitz. d. Gesell. z. Beförderung d. gesam. Naturwiss. zu Marburg_, No.
4. 1879.
(552) T. H. Huxley. "Report upon the researches of Müller into the
anat. and devel. of the Echinoderms." _Ann. and Mag. of Nat. Hist._,
2nd Ser., Vol. VIII. 1851.
(553) Koren and Danielssen. "Observations sur la Bipinnaria
asterigera." _Ann. Scien. Nat._, Ser. III., Vol. VII. 1847.
(554) Koren and Danielssen. "Observations on the development of the
Starfishes." _Ann. and Mag. of Nat. Hist._, Vol. XX. 1857.
(555) A. Kowalevsky. "Entwicklungsgeschichte d. Holothurien." _Mém.
Ac. Pétersbourg_, Ser. VII., Tom. XI., No. 6.
(556) A. Krohn. "Beobacht. a. d. Entwick. d. Holothurien u. Seeigel."
Müller's _Archiv_, 1851.
(557) A. Krohn. "Ueb. d. Entwick. d. Seesterne u. Holothurien."
Müller's _Archiv_, 1853.
(558) A. Krohn. "Beobacht. üb. Echinodermenlarven." Müller's _Archiv_,
1854.
(559) H. Ludwig. "Ueb. d. primar. Steinkanal d. Crinoideen, nebst
vergl. anat. Bemerk. üb. d. Echinodermen." _Zeit. f. wiss. Zool._,
Vol. XXXIV. 1880.
(560) E. Metschnikoff. "Studien üb. d. Entwick. d. Echinodermen u.
Nemertinen." _Mém. Ac. Pétersbourg_, Series VII., Tom. XIV., No. 8.
1869.
(561)[229] Joh. Müller. "Ueb. d. Larven u. d. Metamorphose d.
Echinodermen." _Abhandlungen d. Berlin. Akad._ (Five Memoirs), 1848,
49, 50, 52 (two Memoirs).
(562) Joh. Müller. "Allgemeiner Plan d. Entwicklung d. Echinodermen."
_Abhandl. d. Berlin. Akad._, 1853.
(563) E. Selenka. "Zur Entwicklung d. Holothurien." _Zeit. f. wiss.
Zool._, Bd. XXVII. 1876.
(564) E. Selenka. "Keimblätter u. Organanlage bei Echiniden." _Zeit.
f. wiss. Zool._, Vol. XXXIII. 1879.
(565) Sir Wyville Thomson. "On the Embryology of the Echinodermata."
_Natural History Review_, 1864.
(566) Sir Wyville Thomson. "On the Embryogeny of Antedon rosaceus."
_Phil. Trans._ 1865.
[229] The dates in this reference are the dates of publication.
CHAPTER XXI.
ENTEROPNEUSTA.
The larva of Balanoglossus is known as Tornaria. The præ-larval
development is not known, and the youngest stage (fig. 272) so far
described (Götte, No. 569) has many remarkable points of resemblance
to a young Bipinnaria.
[FIG. 272. EARLY STAGE IN THE DEVELOPMENT OF TORNARIA. (After
Götte.)
_W._ so-called water-vascular vesicle developing as an outgrowth of
the mesenteron; _m._ mouth; _an._ anus.]
A mouth (_m_), situated on the ventral surface, leads into an
alimentary canal with a terminal anus (_an_). A præ-oral lobe is well
developed, as in Bipinnaria, but there is no post-anal lobe. The bands
of cilia have the same general form as in Bipinnaria. There is a
præ-oral band, and a longitudinal post-oral band; and the two bands
nearly meet at the apex of the præ-oral lobe (fig. 273). A contractile
band passes from the oesophagus to the apex of the præ-oral lobe, and
a diverticulum (fig. 272, _W_) from the alimentary tract, directed
towards the dorsal surface, is present. Contractile cells are
scattered in the space between the body wall and the gut.
In the following stage (fig. 274 A) a conspicuous transverse post-oral
band of a single row of long cilia is formed, and the original bands
become more sinuous. The alimentary diverticulum of the last stage
becomes an independent vesicle opening by a pore on the dorsal surface
(fig. 274 A, _w_). The contractile cord is now inserted on this
vesicle. Where this cord joins the apex of the præ-oral lobe between
the two anterior bands of cilia a thickening of the epiblast (? a
ganglion) has become established, and on it are placed two eye-spots
(fig. 273 _oc_, and fig. 274 A). A deep bay is formed on the ventral
surface of the larva.
[FIG. 273. YOUNG TORNARIA. (After Müller.)
_m._ mouth; _an._ anus; _w._ water-vascular vesicle; _oc._
eye-spots; _c.c._ contractile cord.]
[FIG. 274. TWO STAGES IN THE DEVELOPMENT OF TORNARIA. (After
Metschnikoff.)
The black lines represent the ciliated bands.
_m._ mouth; _an._ anus; _br._ branchial cleft; _ht._ heart; _c._
body cavity between splanchnic and somatic mesoblast layers; _w._
water-vascular vesicle; _v._ circular blood-vessel.]
As the larva grows older the original bands of cilia become more
sinuous, and a second transverse band with small cilia is formed (in
the Mediterranean larva) between the previous transverse band and the
anus. The water-vascular vesicle is prolonged into two spurs, one on
each side of the stomach. A pulsating vesicle or heart is also formed
(fig. 274 B, _ht_), and arises, according to Spengel (No. 572), as a
thickening of the epidermis. It subsequently becomes enveloped in a
pericardium, and is placed in a depression in the water-vascular
vesicle. Two pairs of diverticula, one behind the other, grow out
(Agassiz, No. 568) from the gastric region of the alimentary canal.
The two parts of each pair form flattened compartments, which together
give rise to a complete investment of the adjoining parts of the
alimentary tract. The two parts of each coalesce, and thus form a
double-walled cylinder round the alimentary tract, but their cavities
remain separated by a dorsal and ventral septum.
Eventually (Spengel) the cavity of the anterior cylinder forms the
section of the body cavity in the collar of the adult, and that of the
posterior (fig. 274 B, _c_) the remainder of the body cavity. The
septa, separating the two halves of each, remain as dorsal and ventral
mesenteries.
The conversion of Tornaria (fig. 274 A) into Balanoglossus (fig. 274
B) is effected in a few hours, and consists mainly in certain changes
in configuration, and in the disappearance of the longitudinal
ciliated band.
The body of the young Balanoglossus (fig. 274 B) is divided into three
regions (1) the proboscidian region, (2) the collar, (3) the trunk
proper. The proboscidian region is formed by the elongation of the
præ-oral lobe into an oval body with the eye-spots at its extremity,
and provided with strong longitudinal muscles. The heart (_ht_) and
water-vascular vesicle lie near its base, but the contractile cord
connected with the latter is no longer present. The mouth is placed on
the ventral side at the base of the præ-oral lobe, and immediately
behind it is the collar. The remainder of the body is more or less
conical, and is still girt with the larval transverse ciliated band,
which lies in the middle of the gastric region in the Mediterranean
species, but in the oesophageal region in the American one.
The whole of the body, including the proboscis, becomes richly
ciliated.
One of the most important characters of the adult Balanoglossus
consists in the presence of respiratory structures comparable with the
vertebrate gill slits. The earliest traces of these structures are
distinctly formed while the larva is still in the Tornaria condition,
as one pair of pouches from the oesophagus in the Mediterranean
species, and four pairs in the American one (fig. 275, _br_).
[FIG. 275. LATE STAGE IN THE DEVELOPMENT OF BALANOGLOSSUS WITH FOUR
BRANCHIAL CLEFTS. (After Alex. Agassiz.)
_m._ mouth; _an._ anus; _br._ branchial cleft; _ht._ heart; _W._
water-vascular vesicle.]
In the Mediterranean Tornaria the two pouches meet the skin dorsally,
and in the young Balanoglossus (fig. 274 B, _br_) acquire an external
opening on the dorsal side. In the American species the first four
pouches are without external openings till additional pouches have
been formed. Fresh gill pouches continue to be formed both in the
American and probably the Mediterranean species, but the conversion of
the simple pouches into the complicated gill structure of the adult
has only been studied by Agassiz (No. 568) in the American species. It
would seem in the first place that the structure of the adult gill
slits is much less complicated in the American than in the
Mediterranean species. The simple pouches of the young become fairly
numerous. They are at first circular; they then become elliptical, and
the dorsal wall of each slit becomes folded; subsequently fresh folds
are formed which greatly increase the complexity of the gills. The
external openings are not acquired till comparatively late.
Our knowledge of the development of the internal organs, mainly
derived from Agassiz, is still imperfect. The vascular system appears
early in the form of a dorsal and a ventral vessel, both pointed, and
apparently ending blindly at their two extremities. The two spurs of
the water-vascular vesicle, which in the Tornaria stage rested upon
the stomach, now grow round the oesophagus, and form an anterior
vascular ring, which Agassiz describes as becoming connected with the
heart, though it still communicates with the exterior by the dorsal
pore and seems to become connected with the remainder of the vascular
system. According to Spengel (No. 572) the dorsal vessel becomes
connected with the heart, which remains through life in the proboscis:
the cavity of the water-vascular vesicle forms the cavity of the
proboscis in the adult, and its pore remains as a dorsal (not, as
usually stated, ventral) pore leading to the exterior.
The eye-spots disappear.
Tornaria is a very interesting larval form, since it is intermediate
in structure between the larva of an Echinoderm and trochosphere type
common to the Mollusca, Chætopoda, etc. The shape of the body
especially the form of the ventral depression, the character of the
longitudinal ciliated band, the structure and derivation of the
water-vascular vesicle, and the formation of the walls of the body
cavity as gastric diverticula, are all characters which point to a
connection with Echinoderm larvæ.
On the other hand the eye-spots at the end of the præ-oral lobe[230],
the contractile band passing from the oesophagus to the eye-spots
(fig. 273), the two posterior bands of cilia, and the terminal anus
are all trochosphere characters.
[230] It would be interesting to have further information about
the fate of the thickening of epiblast in the vicinity of the
eye-spots. The thickening should by rights be the
supra-oesophageal ganglion, and it does not seem absolutely
impossible that it may give rise to the dorso-median cord in the
region of the collar, which constitutes, according to Spengel,
the main ganglion of the adult.
The persistence of the præ-oral lobe as the proboscis is interesting,
as tending to shew that Balanoglossus is the surviving representative
of a primitive group.
BIBLIOGRAPHY.
(567) A. Agassiz. "Tornaria." _Ann. Lyceum Nat. Hist._ VIII. New York,
1866.
(568) A. Agassiz. "The History of Balanoglossus and Tornaria." _Mem.
Amer. Acad. of Arts and Scien._, Vol. IX. 1873.
(569) A. Götte. "Entwicklungsgeschichte d. Comatula Mediterranea."
_Archiv für mikr. Anat._, Bd. XII., 1876, p. 641.
(570) E. Metschnikoff. "Untersuchungen üb d. Metamorphose, etc.
(Tornaria)." _Zeit. für wiss. Zool._, Bd. XX. 1870.
(571) J. Müller. "Ueb. d. Larven u. Metamor. d. Echinodermen." _Berlin
Akad._, 1849 and 1850.
(572) J. W. Spengel. "Bau u. Entwicklung von Balanoglossus." _Tagebl.
d. Naturf. Vers. München_, 1877.
INDEX.
Abdominalia, 459, 493, 499
Acanthocephala, 379
Acanthosoma, 473, 474, 475
Acarina, 444, 454
Accipenser, 102
Achæta, 319
Achelia, 538
Achtheres percarum, 490
Acineta, 7, 8
Acraspeda, 152, 165, 167, 178, 179, 182, 185, 186
Actinia, 169, 171, 179
Actinophrys, 9
Actinotrocha, 315, 318, 363, 364
Actinozoa, 26, 102, 152, 166, 170, 171, 172, 176, 178, 179, 181,
182, 186
Actinula, 155
Aculeata, 421
Ægineta flavescens, 158
Æginidæ, 156, 158
Æginopsis Mediterranea, 158
Æquorea Mitrocoma, 182
Agalma, 163
Agelena, 436, 450
Agelena labyrinthica, 119, 438
Alciope, 74
Alcippidæ, 499
Alcyonaria, 152
Alcyonidæ, 167, 168
Alcyonidium mytili, 297, 300, 302
Alcyonium palmatum, 119, 148, 167, 182
Alima, 484, 486
Amoeba, 19, 20
Amphibia, 22, 54, 56, 59, 60, 63, 66, 74, 83, 102
Amphilina, 218
Amphioxus, 54, 56, 59, 61, 66, 93, 426
Amphipoda, 518
Amphiporus lactifloreus, 202
Amphistomum, 31
" subclavatum, 205
Amphitrochæ, 330
Amphiura squamata, 565
Anchorella, 108, 492, 520
Anelasma squalicola, 499
Anguillulidæ, 371
Annelida, 14, 25, 98, 503, 525
Anodon, 37, 38, 39, 100, 107, 259, 260, 265, 266, 268
Anopla, 189, 202
Anura, 5
Antedon, 568, 573, 574
Aphides, 15, 16, 76, 79, 116, 428, 429
Aphrodite, 42
Apis, 402, 407, 408, 412, 413
Aplysia, 99, 226, 238, 252, 253
Aplysinidæ, 146
Apoda, 459, 493
Aptera, 395, 420
Apus, 16, 79, 460, 463
Arachnida, 22, 114, 119, 413, 431, 435, 444, 454, 455, 458, 537, 539
Arachnitis, 171
Araneina, 50, 51, 436
Arbacia, 567
Arca, 38
Archigetes, 218
Archizoæa gigas, 494
Arenicola, 42
Argiope, 311, 312, 315, 317
Argonauta, 247, 248
Argulus, 492
Armata, 355
Arthropoda, 12, 16, 18, 22, 75, 77, 79, 83, 108, 110, 221, 382, 383,
434, 448, 503, 525, 534, 541, 542
Articulata, 311, 313, 316, 317
Ascaridiæ, 371
Ascaris nigrovenosa, 16, 82
" lumbricoides, 375
Ascetta, 144
Ascidia canina, 53
Ascidians, 74, 102, 208, 426
Asellus aquaticus, 112, 120, 516
Astacus, 66, 465, 477, 511, 512, 513, 525
Asteracanthion, 69, 70, 561
Asterias, 20, 68, 69, 71, 78, 80, 84, 549, 564
Asteroidea, 35, 36, 544, 549, 557, 563, 576
Astræa, 169
Astroides, 169
Atax Bonzi, 445
Atlanta, 231, 240
Atrochæ, 330
Aurelia, 167
Auricularia, 553, 554, 562, 574
Autolytus cornutus, 319, 343
Aves, 56, 59, 61, 64, 107, 109
Axolotl, 16
Balanoglossus, 576, 579, 581
Balanus balanoides, 75, 493
Belemnites, 252, 253
Bipinnaria, 557, 563, 574, 576, 579
Blatta, 374, 395
Bojanus, organ of, 264, 282
Bonellia, 20, 43, 44, 98, 324, 355, 358, 359
Bothriocephalus salmonis, 211
" proboscideus, 212
Brachiella, 492
Brachiolaria, 558, 564
Brachiopoda, 311, 317, 318
Brachyura, 466, 480, 483
Branchiobdella, 42, 43, 346
Branchiogasteropoda, 272
Branchiopoda, 79, 459, 523, 524
Branchipus, 463, 524
Branchiura, 459, 492
Branchionus urceolaris, 221
Braula, 396
Buccinum, 237, 280
Bulimus citrinas, 229
Bunodes, 169, 171
Buthus, 431
Calcispongiæ, 138, 148
Calopteryx, 402
Calycophoridæ, 152, 159
Calyptoblastic Hydroids, 184, 185
Calyptræa, 223, 280
Campanularidæ, 183, 184
Capitella, 330, 332
Carabidæ, 476
Carcinus Moenas, 481, 483
Cardium, 260, 262
" pygmæum, 262
Carinaria, 240
Caryophyllium, 168, 171
Cassiopea, 165, 167
Cecidomyia, 15, 79, 416, 417, 429
Cephalopoda, 20, 40, 41, 102, 108, 109, 135, 225, 240, 242, 244,
250, 252, 253, 270, 271, 272, 274, 279, 282, 287
Cephalothrix galatheae, 202
Ceratospongiæ, 146
Cercariae, 207, 208, 209
Cerianthus, 168, 171
Cestodes, 14, 29, 31, 32, 33, 189, 210, 212, 218, 313, 425, 541
Chætogaster, 342
Chætopoda, 5, 18, 23, 41, 43, 44, 54, 67, 209, 215, 270, 275, 307,
312, 317, 318, 319, 320, 326, 334, 338, 342, 346, 349, 350, 351,
364, 369, 383, 386, 408, 448, 457, 458, 521, 576, 582
Chætopteridæ, 333
Chætosomoidea, 371
Chelifer, 434, 436, 442, 446, 454
Chermes, 15, 429
Chilognatha, 113, 387, 389, 391, 393, 395
Chilopoda, 387, 393, 394
Chilostomata, 292, 297, 298, 304, 305
Chironomus, 15, 378, 401, 402, 415, 416, 429
Chiton, 254, 256, 257,273
Chordata, 5
Chrysaora, 165
Chthonius, 436
Cicada, 395
Cirripedia, 459, 492, 496, 503, 509, 520
Cladocera, 459, 464, 519
Clausilia, 239
Clavella, 520
Clavularia crassa, 167
Cleodora, 241
Clepsine, 73, 346, 347, 349, 351, 352, 353, 354
Clio, 242, 278
Clubione, 436
Clupeidæ, 64
Cobitis barbatula, 378
Coccidæ, 429
Coccus, 50
Coelebogyne, 79
Coelenterata, 3, 5, 13, 18, 26, 27, 28, 35, 74, 93, 94, 126, 148,
170, 178, 179, 180, 181, 191, 342
Coenurus cerebralis, 213, 214
Coleochæte, 11
Coleoptera, 396, 402, 409, 412, 420, 421, 425
Collembola, 395, 426
Comatula, 5, 552, 553
Condracanthus, 111, 120, 520
Conochilus volvox, 221
Convoluta, 32
Copepoda, 109, 120, 459, 460, 487, 489, 493, 496, 503, 509, 519
Corallium rubrum, 168, 182
Corethra, 422, 423, 424
Crangoninæ, 476
Craniadæ, 311
Craniata, 5, 6, 19, 20, 54, 56, 59, 61, 62, 64, 74, 102
Crinoidea, 35, 36, 544, 550, 568, 576
Criodilus, 321, 324, 328, 341
Crisia, 304
Crocodilia, 63
Crustacea, 5, 6, 18, 51, 66, 102, 109, 120, 458, 465, 487, 502, 521,
524, 537, 541
Cryptophialus, 499, 509
Crystalloides, 163
Ctenophora, 26, 93, 102, 152, 173, 175, 177, 178, 179, 180, 181, 182
Ctenostomata, 292, 297, 298, 304, 305
Cucullanus elegans, 46, 75, 82, 371, 376
Cucumaria, 546, 556, 574
Cumaceæ, 459, 465, 486, 506
Curculio, 421
Cyclas, 259, 260, 261, 265
Cyclops, 376, 377, 418, 489, 503
Cyclostomata, 102, 292, 304
Cymbulia, 241, 242
Cymothoa, 516, 517, 519, 520, 524, 528
Cynipidæ, 15, 421, 428
Cyphonautes, 297, 301, 304, 306, 308
Cypridina, 500, 502
Cysticercus cellulosæ, 214, 217
" fasciolaris, 216
" limacis, 213
Daphnia, 79, 464
Dasychone, 331, 336
Decapoda, 66, 248, 459, 465, 469, 504, 511
Dendrocoela, 32, 33, 189, 195, 196
Dentalium, 258, 576
Desmacidon, 147
Desor, type of, 196, 197, 201, 202, 204, 212, 424
Diastopora, 304
Dibranchiata, 225, 253
Dicyema, 9, 131, 134, 135, 136
Dimya, 225
Diphyes, 159
Diplozoon, 11, 209, 210
Diporpa, 210
Diptera, 49, 194, 204, 396, 401, 402, 407, 409, 412, 416, 420, 429
Discina radiata, 317
Discinidæ, 311
Discophora, 18, 42, 165, 346, 383
Distomeæ, 189, 205, 425
Distomum, 31
" cygnoides, 209
" globiparum, 207
" lanceolatum, 205
Dochmius duodenale, 375
" trigonocephalus, 375
Donacia, 401
Dracunculus, 376, 377
Echinaster fallax, 23
" Sarsii, 102, 561
Echinodermata, 5, 18, 24, 35, 74, 102, 325, 424, 544, 573, 574, 576,
582
Echinoidea, 35, 36, 544, 549, 565, 576
Echinorhyncus, 379, 380
Echinus lividus, 83, 84, 88
Echiurus, 44, 357, 358
Ectoprocta, 297, 306
Edriophthalmata, 459, 465
Elaphocaris, 473
Elasmobranchii, 23, 56, 59, 61, 62, 64, 67, 105, 106, 107, 108, 109
Enopla, 189, 202
Entoconcha mirabilis, 237
Entomophaga, 421
Entoprocta, 292, 298, 300, 302, 304, 306
Epeira, 436
Ephemera, 395, 409, 420, 422
Ephyra, 186
Epibulia aurantiaca, 159, 165
Erichthus, 484, 507
Errantia, 319, 336
Esperia, 147
Estheria, 463, 464
Euaxes, 101, 322, 324, 341, 346, 349
Eucharis, 178
" multicornis, 178
Eucopepoda, 459
Eucope polystyla, 23, 154
Eunice sanguinea, 319
Eupagurus prideauxii, 112, 113, 115, 511, 520
Euphausia, 465, 468, 504, 505, 518, 523
Eurostomata, 176
Eurylepta auriculata, 192
Eurynome, 483
Euspongia, 146, 147
Filaria, 377
Filaridæ, 371
Firoloidea, 240
Flagellata, 7, 8
Flustrella, 301, 303
Formica, 396
Fungia, 182, 186
Fusus, 275, 280, 284, 288
Gammarus, 122, 518
" fluviatilis, 117
" locusta, 110, 112
Ganoids, 54, 102
Gasteropoda, 39, 41, 98, 225, 226, 229, 230, 232, 233, 240, 258,
260, 261, 270, 272, 275, 279, 283, 324
Gasterosteus, 64, 210
Gastrotricha, 370
Gasterotrochæ, 330, 333
Gecarcinus, 465
Geophilus, 392, 393
Gephyrea, 5, 18, 24, 44, 54, 67, 102, 318, 320, 325, 355, 357, 361,
364
Germogen, 134
Geryonia hastata, 156
Geryonidæ, 156
Glochidia, 267, 268
Gnathobdellidæ, 346, 349
Gordiacea, 94
Gordioidea, 371, 374, 378
Gorgonia, 168
Gorgonidæ, 181
Gorgoninæ, 181
Gregarinidæ, 8
Gryllotalpa, 401, 412, 413
Gummineæ, 147, 148
Gymnoblastic Hydroids, 184, 185
Gymnolæmata, 292
Gymnosomata, 225, 240, 241, 242, 270
Gyrodactylus, 210
Halichondria, 147
Halisarca, 22, 66, 145
Halistemma, 165
Helicidæ, 238
Helioporidæ, 182
Helix, 67, 229
Hemiptera, 395, 402, 403, 409, 420, 421
Hessia, 108, 492
Heterakis vermicularis, 374
Heteronereis, 343
Heteropoda, 71, 72, 225, 226, 231, 278
Hexacoralla, 152, 179, 182
Hippopodius gleba, 27, 159
Hirudinea, 74, 84
Hirudo, 350, 351, 352, 353, 354
Holometabola, 420, 422
Holostomum, 205
Holothuria, 19, 25, 35, 549, 558, 576
Holothuroidea, 35, 544, 553, 556
Homarus, 477
Hyaleacea, 273, 275
Hyaleidæ, 241
Hydra, 21, 22, 26, 28, 29, 34, 152, 154, 155, 179, 183
Hydractinia, 539
Hydrocoralla, 152, 181, 185
Hydroidea, 152
Hydromedusæ, 152, 179, 182, 183, 184, 185, 186, 187
Hydrophilus, 374, 396, 400, 401, 402, 404, 408, 409
Hydrozoa, 14, 19, 26, 27, 67, 102, 152,155, 165, 179, 180, 181, 182,
539
Hymenoptera, 396, 401, 402, 412, 420, 421, 425
Ichneumon, 396
Inarticulata, 311, 316
Inermia, 355
Infusoria, 7, 8
Insecta, 5, 15, 18, 19, 25, 46, 395, 396, 425, 455, 458
Intoshia gigas, 136
Isidinæ, 181
Isodyctia, 147
Isopoda, 109, 515, 519, 521, 523, 527
Julus Moncletei, 387, 388, 389
Kochlorine, 499
Lacertilia, 64
Lacinularia, 221, 223
" socialis, 75
Lamellibranchiata, 23, 25, 37, 39, 98, 225, 241, 257, 258, 259, 269,
270, 271, 273, 274, 288
Lepadidæ, 498
Lepas fascicularis, 224, 493, 494, 495
Lepidoptera, 79, 396, 402, 407, 408, 412, 413, 415, 417, 420, 421,
423, 425, 426, 455
Leptodora, 16, 51
Leptoplana, 74, 189, 192, 193
Lernæopoda, 490, 492, 520
Leucifer, 507
Libellulidæ, 402, 403, 409, 420
Limax, 229, 232, 239, 278, 280
Limnadia, 79, 524
Limulus, 534
Lina, 402
Lingulidæ, 311, 316
Lithobius, 393
Lobatæ, 178
Loligo, 242, 243, 244, 247, 253
Loricata, 507, 514
Lota, 105
Loxosoma, 292, 294, 296, 306, 307
Lucernaria, 185
Lumbricus, 341, 368
" agricola, 321
" rubellus, 324
" trapezoides, 13, 321, 323
Lumbriconereis, 334
Lymnæus, 82, 98, 226, 227, 232, 238, 281
Lycosa, 436
Macrostomum, 32, 34
Macrura, 476
Malacobdella, 203
Malacodermata, 171
Malacostraca, 66, 459, 462, 465, 504, 505, 506, 511, 523
Mammalia, 56, 58, 59, 64, 66
Marsipobranchii, 59
Mastigopus, 473, 474
Medusæ, 27, 154, 157, 158, 163, 164, 176, 178, 181, 182, 183, 184,
185, 186
Megalopa, 482, 483, 484
Melolontha, 402, 421
Membranipora, 297, 303
Mermithidæ, 371
Mesotrochæ, 330
Metachætæ, 335
Metazoa, 9, 10, 12, 67, 86, 125, 135, 149, 150, 179
Millepora, 152, 181
Mitraria, 308, 337
Molgula, 102
Mollusca, 5, 18, 24, 66, 74, 84, 99, 225, 247, 248, 251, 256, 257,
262, 271, 285, 288, 307, 325, 333, 352, 576, 582
Monomya, 225
Monostomum capitellum, 205
" mutabile, 205, 206
Monotrochæ, 330
Montacuta, 260, 262
Musca, 396
Muscidæ, 420, 423
Myobia, 444, 445
Myrianida, 343
Myriapoda, 22, 111, 113, 387, 394, 395, 413, 458
Myriothela, 155
Myrmeleon, 396
Mysis, 120, 469, 472, 486, 504, 509, 525
Mytilus, 260, 261
Myxinoids, 5
Myxispongiæ, 145
Myzostomea, 369
Nais, 342
Nassa mutabilis, 101, 226, 227, 233, 262, 278, 279, 288, 324
Natantia, 487
Natica, 237, 283
Nauplius, 5, 16, 460, 461, 463, 465, 466, 469, 473, 490, 491, 493,
497
Nautilus pompilius, 253, 276
Nebaliadæ, 459, 465, 486
Nematoda, 45, 46, 50, 66, 74, 75, 371, 373, 374, 376
Nematogens, 131
Nematoidea, 18, 84, 94, 371, 374
Nematus ventricosus, 13, 427
Nemertea, 94, 189, 196, 202, 204
Nemertines, 30, 31, 33, 93, 136, 195, 202, 328, 333, 424
Nephelis, 82, 346, 349, 350, 351, 352, 354
Nereis, 343
" diversicolor, 319
" Dumerilii, 343
Neritina, 229, 237
Neuroptera, 396, 401, 420, 421
Neuroterus ventricularis, 428
Notonecta, 395
Nototrochæ, 330, 353
Nudibranchiata, 229, 241
Ocellata, 184
Octocoralla, 152, 179
Octopus, 248
Odontophora, 225, 257, 271
Odontosyllis, 333
Oedogonium, 11
Oligochæta, 42, 319, 321, 325, 330, 338, 346, 352
Olynthus, 144
Oniscus murarius, 107, 108, 109, 120, 516, 520, 528
Opercula, 31
Ophiothryx, 36, 549
Ophidia, 64
Ophiuroidea, 136, 544, 553, 562, 565, 576
Ophryotrochæ puerilis, 333
Opisthobranchiata, 225, 232, 237
Ornithodelphia, 109
Orthonectidæ, 136
Orthoptera, 395, 414, 420, 421, 425, 426
Ostracoda, 459, 500, 510
Ostrea, 259, 260, 262
Oxyuridæ, 46, 373, 374
Oxyurus ambigua, 374
" vermicularis, 375
Pæcilopoda, 534
Paguridæ, 477
Palæmon, 110
Palæmonetes, 476
Palæmoninæ, 476, 511, 512
Palinurus, 478, 480
Paludina, 66, 227, 229, 235, 270, 278, 280
" costata, 229
" vivipara, 226
Pandorina, 11
Parasita, 489
Pedalion, 221
Pedicellina, 98, 292, 296, 299, 307
Pelagia, 167, 185
Penæinæ, 476
Penæus, 110, 113, 465, 469, 473, 474, 504, 518
Pennatulidæ, 181
Pentacrinus, 5
Pentastomida, 539, 540
Pentastomum denticulatum, 540, 541
" tænoides, 539, 540, 541
Percidæ, 64
Perennichætæ, 335
Peripatus, 5, 386, 411, 412, 413, 542
Petromyzon, 61, 63, 64, 74, 83
Phalangella, 304
Phalangidæ, 436
Phallusia, 83
Phascolosoma, 44, 355, 356, 361
Pholcus, 436, 442
Phoronis, 315, 355, 363, 364
Phoxinus lævis, 378
Phryganea, 396, 401, 409
Phylactolæmata, 292, 294, 297, 305, 306
Phyllobothrium, 218
Phyllodoce, 329
Phyllopoda, 16, 459, 461, 505
Phyllosoma, 479, 480
Phylloxera, 429
Physophoridæ, 152, 162, 164
Pilidium, type of, 196, 200, 201, 202, 204, 424
Pisces, 5
Piscicola, 20, 43
Pisidium, 259, 260, 262, 264
Planaria Neapolitana, 193
Planorbis, 273, 281, 325
Platyelminthes, 18, 20, 24, 221, 424
Platygaster, 396, 416, 417, 418, 419
Pleurobrachia, 176, 177, 238
Pneumodermon, 242, 576
Podostomata, 292
Poduridæ, 401, 405
Polychæta, 42, 319, 325, 338
Polydesmus complanatus, 387, 388
Polygordius, 319, 325, 326, 327, 328, 332, 357, 386
Polynoe, 42, 331
Polyophthalmus, 328
Polyplacophora, 225, 254, 270, 271, 288
Polystomeæ, 189, 205, 209
Polystomum, 209
" integerrimum, 30, 31, 210
Polytrochæ, 330, 333
Polyxenia leucostyla, 158
Polyxenus lagurus, 387
Polyzoa, 98, 303, 305, 306, 308, 315, 316
Porcellana, 483
Porifera, 102, 138, 148
Porthesia, 115
Prorhyncus, 32, 34
Prosobranchiata, 225, 237, 281
Prostomum, 32, 34, 38, 196
Protozoa, 8, 9, 10, 11, 86, 135, 149
Protozoæa, 471
Protula Dysteri, 342
Pseudoneuroptera, 426
Pseudoscorpionidæ, 434
Psolinus, 556, 574
Psychidæ, 16
Pteraster miliaris, 561
Pteropoda, 98, 225, 226, 229, 230, 232, 240, 258, 270, 272, 279, 283