may be roughly characterised as the "embryonic" and the "larval" type
respectively, although neither description is exact. In developmental
histories of the first type so much reserve material is laid up in the egg
in the form of food-yolk that the young animal whilst in the bilateral
stage requires little or no food. In some cases, however, as in _Amphiura
squamata_, the mother pours out a nourishing exudation; but whether this is
so or not, the parent in nearly every case carries the young about with her
until they have reached the adult condition. In some Asteroidea, as for
instance in the Antarctic species _Asterias spirabilis_ (Fig. 280), the
young become fixed to the everted lips of the mother; in _Amphiura
squamata_, and some other Ophiuroidea the eggs remain in the genital
bursae, which serve as nurseries; in some Spatangoidea, {602}as for
instance in _Hemiaster philippi_ (Figs. 250, 281), the eggs are carried in
some of the deeply grooved petaloid ambulacra; whilst in Holothuroidea they
may develop in the body-cavity (_Phyllophorus urna_), or they may adhere to
the back of the mother (_Cucumaria crocea_, Fig. 259, p. 573), or they may
be protected in special brood-pouches either on the ventral side of the
parent (_Cucumaria laevigata_) or on the dorsal surface (_Psolus
ephippifer_, Fig. 261).
The majority of these cases of embryonic development have been recorded
from Arctic or Antarctic waters; it appears as if conditions there were not
favourable to the larval type of development. In Pelmatozoa the development
of _Antedon rosacea_ alone is known, and that is of the embryonic type.
[Illustration: FIG. 280.—Oral view of _Asterias spirabilis_, slightly
enlarged, showing embryos attached to the everted lips. _emb_, Embryos.
(After Perrier.)]
So far, however, as their mode of propagation is known, it may confidently
be affirmed that the development of the majority of the species of
Eleutherozoa is of the second or larval type. In this type there is little
food-yolk in the egg, and the young animal or larva is forced from a very
early period of development to seek its own living, and hence it is usually
a considerable time (from a fortnight to two months) before the adult form
is attained. When the _embryos_ of different groups of Eleutherozoa are
compared, there is no obvious agreement in structure between them; but the
_larvae_ of the four classes of Eleutherozoa exhibit with differences in
detail a most remarkable fundamental similarity in type, and we are
accordingly justified in regarding the larval development as primitive, and
the embryonic type as derived from it and differently modified in each
case.
{603}[Illustration: FIG. 281.—_Hemiaster philippi._ Enlarged view of a
single petal, showing the embryos _in situ_. (From Wyville Thomson.) The
whole animal is shown in Fig. 250, p. 555.]
In the typical larval development the eggs are fertilised after being laid,
and they then undergo segmentation into a number of equal, or nearly equal,
segments or "blastomeres." These arrange themselves in the form of a hollow
sphere or "blastula," the cavity of which is called the "blastocoel" and
afterwards becomes the primary body-cavity of the larva. This cavity
contains an albuminous fluid, at the expense of which development appears
to be carried on (Fig. 282, B). The cells forming the blastula acquire
cilia, and the embryo begins to rotate within the egg-membrane, which it
soon bursts, and, rising to the surface of the sea, begins its larval life.
The blastula is therefore the first well-marked larval stage, and it is
found in a more or less recognisable form in life-histories of members of
every large group in the animal kingdom. Only in the case of Echinodermata
and of forms still lower in the scale, however, does it appear as a larval
stage. The free-swimming blastula stage is reached in from twelve to
twenty-four hours. Soon the spherical form of the blastula is lost; one
side becomes flattened and thickened, owing to a multiplication of cells,
so that they become taller and narrower in shape. Shortly afterwards this
thickened plate becomes buckled inwards, encroaching on the cavity of the
blastocoel. The larva has now reached the second stage of its development;
it has become a "gastrula" (Fig. 282, C). The plate of thickened cells has
become {604}converted into a tube called the "archenteron" (Fig. 282, C,
_arch_), which is the rudiment of both the alimentary canal and the coelom
of the adult. This tube communicates with the exterior, in virtue of its
mode of formation, by a single opening which is called the "blastopore,"
which becomes the anus of the later larva and adult. Whilst the gastrula
stage is being acquired, the blastocoel or primary body-cavity is invaded
by wandering cells budded from the wall of the archenteron (Fig. 282, A, B,
C, _mes_). These cells, which are called "mesenchyme," are the formative
cells of the skeleton, connective tissue, and wandering cells of the adult.
When the larva has a skeleton they are formed very early, arising in the
young blastula stage (Ophiuroidea) or in the stage immediately before the
formation of the archenteron (Echinoidea, Fig. 282, A, B) and secreting the
skeleton. When the larva is devoid of a skeleton (Asteroidea and
Holothuroidea), the mesenchyme usually does not appear till the gastrula is
fully formed.
[Illustration: FIG. 282.—_Echinus esculentus._ A, optical section of living
blastula. B, section of preserved blastula. The network of strings in the
interior is the result of the coagulation of the albuminous fluid. C,
section of gastrula. _arch_, Archenteron; _mes_, mesenchyme cells, attached
by protoplasmic strands to the wall of the embryo. × 150.]
The gastrula stage is reached in twenty to thirty-six hours. Then one side
of the larva becomes concave, and the cilia become restricted to a thick
band surrounding this area. In this way is formed the rudiment of the
_longitudinal band of cilia_, which is the organ of locomotion throughout
the larval life. At the apex of the archenteron a thin-walled vesicle is
formed, which soon becomes divided off from the rest. This vesicle, which
almost immediately divides into two sacs, right and left, is the
{605}rudiment of the "coelom" or secondary body-cavity of the larva; the
remainder of the archenteron forms the definitive gut, and becomes divided
by constriction into an oesophagus, a stomach, and an intestine, and at the
same time bent into a shallower or deeper V-shape, the concavity of which
is towards the concave side of the body. Within this area of the surface a
new funnel-shaped depression makes its appearance. This is the
"stomodaeum," the rudiment of the mouth of the larva, and it soon joins the
apex of the larval oesophagus; the conjoined tubes henceforth bearing the
name oesophagus since the ectodermal and endodermal parts become
indistinguishably fused. Along the sides and floor of the oesophagus is
formed a V-shaped ridge bearing strong cilia; this is the "adoral band of
cilia" which sweeps the food (consisting of Diatoms, Infusoria, etc.) into
the mouth. The larva is now known as a Dipleurula and appears in four
modifications, each characteristic of a Class of Eleutherozoa. These differ
from one another principally in the following points:—(_a_) The folding of
the ciliated band; (_b_) the divisions of the coelomic sacs; (_c_) the
development and fate of the praeoral lobe (_i.e._ the part of the body in
front of the mouth); (_d_) the fate of the larval mouth. The types of
Dipleurula are as follows:—
[Illustration: FIG. 283.—Bipinnaria of _Luidia_. _a_, Anus; _a.b_, adoral
ciliated band; _a.c.o.b_, anterior median process; _a.d.a_, anterior dorsal
process; _a.v.a_, prae-oral process; _m_, mouth; _p.c.o.b_, median dorsal
process; _p.d.a_, posterior dorsal process; _p.l.a_, posterior lateral
process; _p.v.a_, post-oral process. (After Garstang.)]
(1) The _Bipinnaria_, the larva of Asteroidea. In this type there is a very
long prae-oral lobe. The ciliated band runs along its edges, and is
produced into a backwardly directed loop on its under surface. This loop
soon becomes separated from the rest of the band as a distinct prae-oral
loop, the rest forming a post-oral loop. Both loops are drawn {606}out into
short tag-like processes, in which we may distinguish (following
Mortensen's[517] notation) in the prae-oral loop an anterior median process
(Fig. 283, _a.c.o.b_), and a pair of prae-oral processes (_a.v.a_). In the
post-oral loop there is a median dorsal process (_p.c.o.b_) and paired
anterior dorsal (_a.d.a_), posterior dorsal (_p.d.a_), posterior lateral
(_p.l.a_), and post-oral (_p.v.a_) processes. At the apex of the prae-oral
lobe between prae-oral and post-oral ciliated rings there is an ectodermic
thickening, recalling the so-called apical plate of Annelid larvae.
[Illustration: FIG. 284.—A, Ophiopluteus of _Ophiothrix fragilis_. _hy_,
Hydrocoel; _l.p.c_, left posterior coelom; _oes_, oesophagus; _r.p.c_,
right posterior coelom; _st_, stomach. B, metamorphosis of Ophiopluteus of
_Ophiura_ sp. (After Johannes Müller.)]
(2) The _Ophiopluteus_, the larva of the Ophiuroidea. In this type the
prae-oral lobe remains small, and the primitive ciliated band is undivided.
The processes into which it is drawn out are very long, and are supported
by calcareous rods. Of these processes we may distinguish prae-oral,
postero-dorsal, postero-lateral, and post-oral. The postero-lateral are
always much longer than the rest, so that the larva when swimming appears
to the naked eye as a tiny V. In the case of _Ophiothrix fragilis_ (Fig.
284, A) the postero-lateral processes are many times longer than the rest
of the body. The Ophiopluteus was the first Echinoderm larva to be
recognised. It was discovered by Johannes Müller,[518] who also discovered
the other three types of {607}Dipleurula. He named this one _Pluteus_
(easel), from a fancied resemblance, when turned upside down, to a
painter's easel. The same name was bestowed on the next type, to which it
presents a superficial resemblance, and hence the distinguishing prefix
"Ophio-" was added to the original name by Mortensen.
(3) The _Echinopluteus_, the larva of the Echinoidea. This type is
strikingly like the preceding one in possessing a very small prae-oral lobe
and in having the processes of the ciliated ring supported by calcareous
rods, but a close inspection of these shows that they do not exactly
correspond to those of the Ophiopluteus. Thus we have prae-oral,
postero-dorsal, and post-oral processes (Fig. 285), but usually no
postero-lateral process, and when it does occur it remains short. On the
other hand, an antero-lateral process unrepresented in the Ophiopluteus is
constantly present, and in its later stage the Echinopluteus develops, out
of parts of the ciliated ring, horizontally-placed crescentic ridges of
cilia, which are termed ciliated epaulettes (Fig. 285, _a.cil.ep_). There
may even be, as in the larva of _Echinus esculentus_, a second posterior
set of these (Fig. 285, _p.cil.ep_). In the older larva at the apex of the
prae-oral lobe there is an ectodermic thickening, at the base of which are
developed nerve-cells and nerve fibres constituting a larval brain (Fig.
285, _ap_).
[Illustration: FIG. 285.—Dorsal view of larva of _Echinus esculentus._ ×
45. _a.cil.ep_, Anterior ciliated epaulette; _ap_, apical plate or larval
brain; _ech_, rudiment of Sea-urchin; _l.a.c_, left anterior coelom;
_l.oes_, larval oesophagus; _l.p.c_, _r.p.c_, as in Fig. 284; _p.cil.ep_,
posterior ciliated epaulette; _r.a.c_, right anterior coelom.]
(4) The _Auricularia_, the larva of the Holothuroidea. This type strikingly
resembles the Bipinnaria in its external features. The prae-oral lobe is
well developed, and has on its under surface a backwardly projecting loop
of the ciliated band, which is not, {608}however, as in the Bipinnaria,
separated from the rest of the band. The processes of the band are much
more faintly marked than in the Bipinnaria, the anterior median, prae-oral,
and median dorsal processes being absent; but a pair of intermediate dorsal
processes are developed in the interspace between anterior and posterior
dorsal.
[Illustration: FIG. 286.—Three views of metamorphosis of Auricularia of
_Synapta digitata_. A, fully grown _Auricularia_; B and C, stages in the
metamorphosis. _hy_, Hydrocoel; _Int_, intestine; _l.p.c_, left posterior
coelom; _O_, fragments of ciliated band which are invaginated into the
stomodaeum, and coalesce to form a ring round the mouth; _oss_, ossicle;
_pod_, rudiment of feelers which here spring directly from the hydrocoel;
_r.p.c_, right posterior coelom; _st_, stomach; _w.v.r_, rudiment of
water-vascular radial canals; 1-5, corresponding pieces in the three
figures of the longitudinal ciliated band. (After Bury.) × 40.]
In the Bipinnaria, Ophiopluteus, and Echinopluteus the coelomic vesicle,
after separation from the archenteron, divides into right and left halves.
The left then sends out a short dorsal process, which, fusing with the
ectoderm, acquires an opening to the exterior. This opening is the primary
madreporic pore, and the process of the left coelomic sac, which is
ciliated, is the pore-canal. In the Auricularia the pore and pore-canal are
formed _before_ the division of the coelom. In the Bipinnaria the right and
left sacs subsequently fuse in the front part of the prae-oral lobe. In the
first three types of larva the coelomic sac on each side then undergoes a
segmentation into anterior and posterior portions. At the hinder end of the
anterior sac on each side a swelling occurs. That on the left side is the
"hydrocoel," or rudiment of the water-vascular system (Fig. 287, A^3,
_l.hy_); it {609}quickly assumes a crescentic form, and gives off five
blunt outgrowths, which are the rudiments of the radial canals, and the
terminal tentacles. It remains in connexion with the anterior coelom by a
narrow neck, which later becomes the stone-canal. That on the right side
separates completely from the right anterior coelom; it remains small, and
forms the madreporic vesicle (Fig. 287, A^3, _r.hy_) of the adult. In the
Ophiopluteus and in the larva of _Asterina gibbosa_ (_v. infra_) it
occasionally takes on a form similar to that of the hydrocoel; from which
circumstance, as well as from the similarity in its mode of origin, it is
here regarded as a right hydrocoel, _i.e._ a rudimentary fellow of the
organ which develops into the water-vascular system.
[Illustration: FIG. 287.—Diagrams of the mode of formation and division of
the coelom in Echinodermata. _a.c_, Anterior coelom; _coe_, primitive
coelomic rudiment; _int_, intestine; _l.a.c_, left anterior coelom; _l.hy_,
left hydrocoel; _l.p.c_, left posterior coelom; _oes_, oesophagus; _p.c_,
posterior coelom; _r.a.c_, right anterior coelom; _r.hy_, right hydrocoel;
_r.p.c_, right posterior coelom; _st_, stomach; _stom_, stomodaeum.]
In Auricularia (Fig. 287, D) the coelomic vesicle, after the pore-canal is
formed, divides into an anterior and a posterior half. The posterior part
then divides into right and left halves, {610}whilst the anterior sac
divides into dorsal and ventral halves, connected by a narrow neck. The
ventral half soon assumes the familiar feature of the hydrocoel (Fig. 287,
D^3, _l.hy_), whilst the dorsal half forms an insignificant swelling on the
course of the conjoined stone- and pore-canals, which represents the left
anterior coelom of the other types; neither right anterior coelom nor right
hydrocoel being developed. The neck of communication between dorsal and
ventral halves is, of course, the stone-canal.
The Dipleurula larva leads a free-swimming life for a period varying from
two weeks to two months, and then undergoes METAMORPHOSIS into the adult
form. The details of this process have been worked out in comparatively few
cases; and the species in which they are most thoroughly known is the
Asteroid _Asterina gibbosa_. The development of this species is
intermediate in character between the embryonic and larval types. The eggs
are larger than is usual among Asteroidea, and are filled with a bright
orange yolk. The larva differs from the Bipinnaria in the absence of the
characteristic ciliated bands and in the very early occlusion of the anus.
There is, however, a band of cilia round the edge of the prae-oral lobe,
which corresponds to portions of the prae-oral and post-oral bands combined
of the Bipinnaria.[519]
The larva has a form which may be described as boot-shaped (Figs. 288,
289). The sole of the boot is the great prae-oral lobe, behind which is the
mouth. The larva takes little or no food, and completes its metamorphosis
in ten to twelve days. It does not swim at the surface, but creeps slowly
over the bottom by the aid of the ciliated band mentioned above, while it
can also attach itself, using the edges of the prae-oral lobe as a sucker.
After leading an existence of this kind for seven or eight days it fixes
itself permanently by a disc-like prominence, which appears on the anterior
surface of the prae-oral lobe within the area surrounded by the thickened
rim which, as explained above, forms a margin to the prae-oral lobe. The
larva then becomes divided by a constriction into a disc and a stalk, and
the former is gradually converted into the body of the young Starfish,
whilst {611}the latter continually diminishes in size, and eventually
entirely disappears, when the young Starfish commences to walk about on its
podia. The disc becomes bent downwards and to the left, so as to make
nearly a right angle with the stalk, and the last vestige of the latter
springs from the peristome of the Starfish inside the water-vascular ring
(Figs. 289, B, C).
[Illustration: FIG. 288.—Fully grown larval stages of _Asterina gibbosa_.
A, fully grown larva; B, left, and C, right view of a larva seven days old
in the beginning of the metamorphosis. _m_, Mouth; 1-5, the five lobes of
the hydrocoel; I.-V., the rudiments of the arms. (After Ludwig.) × 45.]
The form of the Starfish is attained principally by the preponderant growth
of the left hydrocoel and of the left posterior coelom. Both these sacs
take on the form of hoops, which, by the meeting of their ends, are
converted into rings. The hydrocoel has already grown out into five lobes,
which are the rudiments of the radial water-vascular canals, and the tips
of which become the terminal sensory tentacles (Figs. 288, 289, 1-5); but
now the left posterior coelom grows out into five lobes also, forming a
parallel but outer ring. These lobes (Figs. 288, 289, I.-V.) are the
rudiments of the arms, which are at first quite independent of those of the
radial canals, but gradually, when the larva has attained the age of nine
days (Fig. 289, B, C), they become applied to the outgrowths of the
hydrocoel. These by this time have developed each two pairs of branches,
the rudiments of the first two pairs of tube-feet in each radius. The
larval mouth vanishes, and a new mouth is formed on the left side in the
centre of the hydrocoel ring, when the metamorphosis is complete. The adult
anus is {612}formed about the same time. The primary pore-canal in
_Asterina_ as in all Dipleurulae opens into the anterior coelom; the
stone-canal is formed from a ciliated groove running along the neck of
communication between this and the hydrocoel. The constriction dividing the
body into disc and stalk divides the anterior coelom (single in _Asterina_
as in the older Bipinnaria) into two parts; the portion included in the
disc forms the axial sinus of the adult. The lower end of the axial sinus
expands and surrounds the adult mouth, forming the inner perihaemal ring;
the outer perihaemal ring is formed by the juxtaposition of four
wedge-shaped outgrowths from the left posterior coelom and one from the
anterior coelom. From these the radial perihaemal canals subsequently grow
out into the arms. The metamorphosis of Bipinnaria has been well worked out
by Goto,[520] and it agrees in essential features with that of _Asterina
gibbosa_; in fact, the differences which Goto maintains between the two
types may be reasonably explained on the supposition of some stages having
escaped the notice of this observer. The larva develops on the apex of the
prae-oral lobe three papillae for occasional attachment,[521] and in the
centre of these a cup-shaped disc for permanent fixation when the prae-oral
lobe is converted into a stalk. When these papillae (Fig. 290, _fix_) have
been developed the larva is known as a Brachiolaria.
[Illustration: FIG. 289.—Views of larvae of _Asterina gibbosa_ in the
course of metamorphosis. A, larva of eight days, from the right; B, left,
and C, right view of larva of nine days; 1-5, lobes of hydrocoel; I-V,
rudiments of arms. (After Ludwig.) × 45.]
{613}[Illustration: FIG. 290.—_Brachiolaria_ fixing itself, × 60. _Ast_,
rudiment of the body of the Starfish; _fix_, fixing processes. (After
Johannes Müller.)]
The metamorphoses of the other types of Dipleurula contain no fixed stage.
They are what might be called "cataclysmal metamorphoses." That is to say,
the outer form and habits of the larva are preserved till the last moment,
whilst the organs of the adult are being gradually perfected; then in an
hour or two all trace of larval structures disappears. The Ophiopluteus
preserves the larval mouth, round which the hydrocoel grows; the long
lateral ciliated processes are preserved till the animal has attained all
the adult characters. Before this, however, it passes through what may be
called an "Asteroid" stage in development, in which the ambulacral grooves
are open. The Echinopluteus loses both larval mouth and anus. It develops
the adult organs on the floor of a sac-like invagination of the ectoderm,
situated on the left side within a loop of the ciliated band (Fig. 291, B,
C). This invagination becomes completely closed. It is termed the "amniotic
cavity," and its roof is termed the "amnion." On its floor are developed
the primary tentacles, terminating the radial canals, as well as a number
of spines. After taking on a creeping life and losing its larval
appendages, the young Sea-urchin passes through an "Asteroid" condition, in
which the arched dorsal surface, the future periproct, is greater in extent
than the ventral, and the radial canals run horizontally out from the
water-vascular ring and terminate in free movable podia (Fig. 291, C and D,
_pod_), ending in suckers, in the centre of which are pointed sense-organs.
These {614}podia become later enclosed in grooves in the corona, and are
reduced to vestiges in the adult.
[Illustration: FIG. 291.—Four views of Echinopluteus from the left side, to
show the metamorphosis. A, B, and C are taken from the development of
_Echinus miliaris_. D is a young _Echinus esculentus_. The rudiment of the
oral disc of the _Echinus_ is seen beginning in B and larger in C.
_ad.stom_, Adult stomodaeum; _cil.ad_, adoral ciliated band; _cil.ep_,
ciliated epaulette; _coe_, coelom; _d.sp_, prismatic spine of dorsal
surface (periproct) of adult; _ech_, rudiment of _Echinus_; _int_,
intestine; _l.oes_, larval oesophagus; _mp_, madreporic pore; _nerve circ_,
nerve-ring of adult; _pod_, first paired tube-feet; _st_, stomach; _t_,
terminal tentacle of the radial band; _v.sp_, pointed ventral spine of
adult. A, B, and D magnified 45 diameters; C, 60 diameters.]
The Auricularia is the only type of Dipleurula in which larval mouth and
anus are retained. For this reason it has been supposed that its median
plane of symmetry remains the median plane of the adult. The researches of
Bury[522] have shown that this is not so. As in other types of Dipleurula
(with the possible exception of the Ophiopluteus) the adult position of the
{615}mouth is on the left side of the larva, and in the commencement of the
metamorphosis the mouth migrates into this position (Fig. 286, C). Then the
rudimentary prae-oral lobe is rapidly absorbed, so that the mouth again
acquires a terminal position. The hydrocoel (Fig. 286, A, _hy_) has by this
time completely encircled the oesophagus, and from it grow out the radial
canals which bud off the feelers[523] (buccal tentacles) into the larval
stomodaeum. This, although it later flattens out to form the adult
peristome, forms in these stages an almost closed sac, reminding us of the
amniotic cavity in the Echinopluteus. The ciliated band breaks up into a
number of pieces, which rearrange themselves so as to form a series of
transverse rings of cilia; so that the free-swimming life can be carried on
somewhat longer. The animal in this stage is called a "pupa" (Fig. 292); it
eventually loses the rings, drops to the bottom, and develops tube-feet.
From specimens which the author has seen, he has little doubt that in some
cases the young animal passes through an "Echinoid" stage, for it
possesses, besides the feelers, only median tube-feet, terminating the
radial canals, and it is covered by a cuirass of plates, which recalls the
Echinoid corona.[524]
[Illustration: FIG. 292.—"Pupa" of _Synapta digitata_. × 50. _circ.cil_,
Ciliated rings; _oss_, calcareous ossicle; _ot_, otocysts; _pod_, feelers;
_w.v.r_, radial water-vessel. (After Semon.)]
Reviewing the development of the Eleutherozoa in the light of the facts so
far presented, and using the same method of reasoning which is employed in
the case of other groups of animals, we seem to be justified in concluding
that the Echinodermata are descended from a simple free-swimming ancestor
possessing the fundamental characters of the Dipleurula. These would
include a longitudinal folded band of cilia as the principal organ of
locomotion; a thickened plate of nervous epithelium at {616}the anterior
end serving as combined sense-organ and brain; a V-shaped band of cilia
projecting into the oesophagus as the organ of nutrition; a wide, shallow
stomodaeum and an alimentary canal consisting of three well-marked
divisions, viz. oesophagus, stomach, and intestine; and finally a secondary
body-cavity or coelom, consisting of three divisions on each side, though
possibly the most anterior pair were confluent in the prae-oral lobe. On
the left side the anterior coelom opened to the exterior by a short
ciliated canal. To the hypothetical group so defined which were certainly
not Echinodermata the name PROTOCOELOMATA may be given.
[Illustration: FIG. 293.—_Tornaria_ larva. _a_, Anus; _a.c_, anterior
coelom; _a.p_, apical plate; _g.s_, rudiments of gill-sacs; _m_, mouth;
_m.c_, middle or "collar" coelom; _p_, posterior ciliated band; _p.c_,
posterior coelom; _pr_, longitudinal ciliated band. (After Morgan.)]
Now amongst the lowest types of animal in which traces of Vertebrate
structure can be detected, there is one group, the HEMICHORDATA (Vol. VII.
p. 3), in which there is a larva which strikingly recalls the Dipleurula.
This larval form belongs to Balanoglossus and is called the Tornaria. It
possesses a well-marked prae-oral lobe and a folded longitudinal ciliated
band, which resembles that of Auricularia. Its peculiarity is that in
addition there is a posterior ring of cilia (Fig. 293, _p_). The coelom is
in five divisions:—a median anterior sac (_a.c_) opening to the exterior by
a short ciliated canal on the left side; and paired middle divisions
(_m.c_) and posterior divisions (_p.c_). At the apex of the prae-oral lobe
there is a plate consisting of sensory epithelium, with nerve-fibres at its
base, which acts as a brain. Tornaria undergoes metamorphosis, assumes a
worm-like form, and takes on a burrowing life. The five divisions of
{617}the coelom are retained, and it can be proved that the pore-canal,
like the madreporite of Echinodermata, is used for taking in water.
Further, there are two aberrant sessile members of the group
(_Cephalodiscus_ and _Rhabdopleura_), in which the middle divisions of the
coelom which would correspond to the hydrocoels are produced into long
arms, each with a double row of ciliated tentacles, which strikingly recall
the radial canals and podia of the Pelmatozoa. Taking all these facts into
consideration, it seems probable that Vertebrata and Echinodermata both
arose from PROTOCOELOMATA.
When we turn to the developmental history of Echinodermata for light on the
question as to how the bilaterally symmetrical ancestor became converted
into the radially symmetrical Echinoderm, it seems probable that only in
the development of the Asteroidea can we hope to find the solution of the
problem. The abrupt changes of habits shown in the metamorphoses of the
other types are clearly secondary phenomena. No species of animal could
suddenly change its habits from swimming by means of cilia to walking with
tube-feet. In the development, however, of _Asterina gibbosa_ we get a hint
of the way in which a free-swimming life could alternate with periods of
temporary fixation, gradually passing into a condition in which the
fixation was permanent. This period in the history of the race when
ancestral Echinodermata were sessile would mark the point at which
Eleutherozoa diverged from Pelmatozoa, and the former existence of a fixed
ancestor explains the tendency first to asymmetry and later to radial
symmetry. Bilateral symmetry is characteristic of most free-swimming
animals which have to pursue a straight course through the water, but in
fixed forms no disadvantage arises from want of symmetry. A radial
disposition of organs is, however, valuable to them, since food must be
sought and danger avoided from all points of the compass; and hence we can
understand, when fixation became permanent, how one hydrocoel could grow
larger than the other, and finally assume the form of a ring.
The last question which arises is the vexed one of the mutual relationships
of the various Classes constituting the Phylum. Before attempting to seek
for light on this problem from development, it will be necessary to sketch
the life-history of _Antedon rosacea_, the only Pelmatozoon whose
development is known.
{618}[Illustration: FIG. 294.—Three views of the development of _Antedon
rosacea_. A, free-swimming larva; B, longitudinal section of free-swimming
larva; C, oral view of young fixed form. _a.c_, Anterior coelom; _amb_,
ambulacral groove; _ap_, apical plate of sensory and nervous tissue; _cil_,
ciliated ring; _hy_, hydrocoel; _l.p.c_, left posterior coelom; _mad_,
primary pore-canal; _pod_, podia; _r.p.c_, right posterior coelom; _stom_,
larval stomodaeum. (A and B after Bury; C after Perrier.)]
The eggs are comparatively large and full of food-yolk, and they adhere for
a considerable period to the pinnules. They pass through a large portion of
the development within the egg-membrane. The blastula and gastrula are
formed in the usual way, but the formation of the coelom is most remarkable
(Fig. 287, E^1, E^2). The archenteron divides into anterior and posterior
divisions. The posterior divides into right and left, posterior coelomic
sacs, but before the division is complete a dorsal and a ventral tongue
grow out from the anterior division and unite posteriorly, encircling the
band of connexion between right and left posterior coelomic sacs like a
ring. This band of connexion becomes solid and is absorbed, and _pari
passu_ the ring becomes converted, by the disappearance of its central
opening, into a sac, which is the definitive gut (Fig. 287, E). The rest of
the {619}anterior division divides into a thick-walled sac, the hydrocoel,
on the left, and a median thin-walled anterior coelom, which sends a long
extension into the anterior portion of the larva, which we may compare to
the prae-oral lobe of the Bipinnaria. The anterior coelom communicates with
the exterior by a short pore-canal, and later forms a connexion, the
stone-canal, with the hydrocoel. At the apex of the prae-oral lobe there is
formed a thickened patch of ectoderm, bearing stiff sensory hairs, and
having at their bases nerve-fibres and ganglion cells. This larval brain
corresponds to that of the Tornaria and Echinopluteus. Behind the brain
there is a glandular pit, which is used for fixation, and recalls the
similar organ in the Bipinnaria. A series of ciliated rings is then formed,
and between the second and third of them an oval depression appears. This
is the stomodaeum; but as the larva takes no food it does not communicate
with the gut (Fig. 295, A, _stom_).
The larva next escapes from the egg-membrane and swims freely for a day or
two, and then, like the Bipinnaria, fixes itself by the apex of the
prae-oral lobe, which is converted into a stalk. The larval stomodaeum
closes, and the oesophagus of the adult appears as a solid peg of cells
abutting against it; round this peg the hydrocoel grows like a ring.
The closed stomodaeum and the underlying hydrocoel are now rotated
backwards until they come to be at the end of the animal opposite the stalk
(Fig. 295, C). The left posterior coelom, which has also, as in the
Asteroid larva, assumed a hoop-like form, is carried along with them; but
the right posterior coelom becomes shifted forwards and sends out five
outgrowths into the stalk, which form the rudiments of the chambered organ,
and a central one as a continuation of the genital stolon (Fig. 295, D,
_gen.st_), the extension of the anterior coelom (Fig. 294, B) having
disappeared.
Then the outer wall of the stomodaeum splits into five valves—the future
oral valves. The radial canals appear as freely projecting tentacles, which
issue in the intervals of these valves and soon acquire two pairs of
lateral branches. The skeleton consists of five oral plates in the oral
valves, of a ring of five basals, of three small under-basals, and of a
series of "columnals," _i.e._ stem-ossicles, as rings embracing the stalk.
The area of attachment is supported by a "foot-plate." The radial
{620}plates next appear as a ring of small ossicles between the orals and
basals, and simultaneously the arms make their appearance as five
outgrowths supported by the first radials, and by the other radials when
these appear. The free radial canals now become adherent to the arms, but
these canals soon give off paired branches of unlimited growth, which are
supported by bifurcations of the primitive arms, and in this way the ten
arms of the adult are established. So far, then, as the water-vascular
system is concerned, the apparent forking is not a true dichotomy, but
results from the production of two opposite branches, whilst the main axis
ceases to grow. The appearance of cirri marks the fusion of the uppermost
stem-ossicles to form a centro-dorsal, and shortly afterwards the young
_Antedon_ snaps off its stem and swims away.
[Illustration: FIG. 295.—Four diagrams to explain the metamorphosis of the
larva of _Antedon rosacea_. _a.c_, Anterior coelom; _gen.st_, genital
stolon; _l.hy_, left hydrocoel; _l.pc_, left posterior coelom; _mp_,
madreporic pore; _r.p.c._ right posterior coelom; _stom_, stomodaeum. (A,
B, and C after Korschelt and Heider; D after Perrier.)]
Now in reviewing this life-history we cannot fail to be struck with
resemblances to the development of Asteroidea, and especially to that of
_Asterina gibbosa_. The absence of a connexion between stomodaeum and gut
is due to the embryonic mode of life. On the other hand, the presence of a
long prae-oral lobe, containing an extension of the anterior coelom and
having a fixing organ at its apex, can only be paralleled among Asteroidea.
In broad outlines, then, up to the period of fixation the two developments
are parallel, but after this point a {621}divergence takes place, which
points clearly to the splitting of the Echinoderm stem into two main
branches, corresponding with two different sets of habits. In the
Eleutherozoan stock, represented by the development of the Asteroidea, the
disc became flexed ventrally on the stalk, so that the mouth and podia were
brought within reach of material drifting along the bottom, which the podia
were employed to seize. As a consequence the base of the stalk was brought
near the mouth, and so it came about that the hydrocoel, when it became a
ring, encircled both. In the Pelmatozoan stock, on the other hand, the
podia and mouth are rotated upwards and backwards from the stalk, which
thus came to have an aboral position (Fig. 296, B). The podia are thus
placed in a favourable position for capturing free-swimming organisms,
which their cilia sweep toward them. It is worthy of note that a similar
change of position of the mouth occurs in other groups of animals which
have similar habits (Polyzoa Entoprocta, Tunicata).
[Illustration: FIG. 296—Figures to show the supposed connexion of
Eleutherozoa and Pelmatozoa. A, common fixed ancestor of the two stocks,
still bilaterally symmetrical; B, primitive Pelmatozoon; C, primitive
Eleutherozoon. _a_, Anus; _a.c_, anterior coelom; _l.p.c_, _l.p.c^1_, left
posterior coelom; _m_, mouth; _p_, primary pore-canal; _r.p.c_, right
posterior coelom.]
The division therefore of the phylum must have occurred at an extremely
remote epoch, before the hydrocoel was a closed ring, and before,
therefore, radial symmetry was completely attained.
Turning now to the question of the origin of the classes of Eleutherozoa,
we find that the study of development strongly reinforces the views gained
from the study of adult anatomy. The Asteroidea are the most primitive
group; only in their case {622}is the fixed stage retained, and both
Ophiuroidea and Echinoidea pass through an Asteroid stage in development.
The only serious competitors for the position are the Holothuroidea, which
many have imagined to have been directly derived from Cystoidea (in the old
sense; better Thecoidea). This view, though adopted by Semon,[525]
Haeckel,[526] and Bather,[527] is open to many objections. The type of
Holothuroid development referred to in these discussions is that of the
extremely aberrant _Synapta digitata_, in which the radial canals are
vestigial structures which disappear in the adult. In this species, where
the feelers are multiplied, some originate in the larva directly from the
water-vascular ring, and thus alternate with the canals. From this
circumstance Semon drew the conclusion that the radial canals of
Holothuroidea are not homologous with those of other Echinoderms, but this
conclusion is contradicted by the development of more normal species, in
which all the feelers spring from the radial canals. The meridional course
of these canals, the closure of the ambulacral grooves, coupled with the
retention of a nervous ectoderm, are all features found in Echinoidea. So
is also a reduction in the number of the genital organs, on which Bell[528]
laid such stress that he separated Holothuroidea from all other
Echinoderms. But if in Spatangoidea a reduction to four and even three can
take place (Fig. 246, p. 552), why should a reduction to two or one excite
surprise? The primitive outer appearance of the Auricularia is
counterbalanced by the development of the coelom, which is much modified,
so that the primitive bilateral arrangement is obscured. If, then,
Asteroidea are the most primitive Eleutherozoa, we may imagine that
primitive Echinoidea were derived from Asteroidea through adaptation to
life in crevices, where an upward bending of the radii was of advantage, in
order to enable the animal to attach its podia above as well as below
itself; and that Holothuroidea arose from primitive Echinoidea in which the
plates of the corona were still movable, through a further adaptation to
narrow crevices, where worm-like wriggling would be the most successful
method of adapting themselves to their environment.
{623}The final result, then, of all our inquiries leads us to a view of the
mutual affinities of the classes of Echinoderms, which may be indicated in
the following table:—
Protocoelomata
|
First fixed ancestor
/ \
Asteroidea Protopelmatozoa (Thecoidea?)
/ \ \
Ophiuroidea Protechinoidea \
/ \ Carpoidea
Echinoidea Holothuroidea / \
/ \
Cystoidea \
/ Crinoidea
Blastoidea
We shall hazard the prophecy that if ever pre-Cambrian Echinoderms are
found, there will be amongst them small stalked forms which may be
superficially classed with "Cystids," but which are in reality the fixed
ancestors of Asteroidea. They should have an irregular skeleton, and be
devoid of arms, which are secondary formations; but they should indicate,
by the proximity of the mouth to the stalk and by the relation to the stalk
of the grooves for the podia, that they have diverged from the Pelmatozoan
stock, and are the ancestors of Eleutherozoa.
INDEX
Every reference is to the page: words in italics are names of genera or
species; figures in italics indicate that the reference relates to
systematic position; figures in thick type refer to an illustration; f. =
and in following page or pages; n. = note.
Abbott, 422
Abdomen (= third chamber of Monaxonic Radiolarian shell), 84
Abdominal cirrhi, 139 f.
Aboral, blood-ring, of _Asterias rubens_, 452;
of _Ophiothrix fragilis_, 490;
of _Echinus esculentus_, 529;
coelom, of _Antedon rosacea_, 585;
nervous system, of _Antedon rosacea_, 584;
sinus, of _Asterias rubens_, 449;
of _Ophiothrix fragilis_, 490;
of _Echinus esculentus_, 528, 529
Abortive nuclei, of Myxosporidiaceae, 107
_Abyla_, _307_
Abylinae, _307_
Abyssal, Radiolaria, 76;
Sponges, 239 f.
_Acamptogorgia_, _356_
_Acanella_, _353_;
_A. simplex_, 353
Acantharia (Actipylaea), _76_, 78, _78_, 80;
myophrisks in, 80;
_Zooxanthella_ in, 80;
reproduction of, 86 n.;
skeleton of, 82
_Acanthaster_, 458, _462_
Acanthin, 71 n., 75, 76
Acanthocephala, nutrition of, 38
_Acanthoconia barrandei_, _207_, 207
_Acanthocystis_, _71_;
budding of, 73;
habitat, 75
_Acanthogorgia_, _355_
Acanthometrids (= Acantharia, _q.v._), parasitic _Amoebophrya_ in, 161
Acanthonida, _78_
_Acanthotrochus_, _577_
_Acarnus_, 223
Accumulative anabolism, growth, 9, 13, 15 f.;
in relation to brood-formation, 32 f.
Acephalinidae, _97_
_Acineta_, _159_, 160
Acinetina = Suctoria, 158 f.
Acontia, 369
Acrasieae, _90_ f.
_Acrophytum_, 333, _349_
Actine, 183
Actinelida, _78_
_Actinia_, _381_;
_A. cari_, 378;
_A. equina_, sense organ of larva of, 373;
_A. mesembryanthemum_, 379, 381;
longevity of, 375
Actiniaria, _377_ f.;
viviparity of, 373;
symbiosis of, 377;
colours of, 379
Actiniidae, _381_
Actiniina, _380_
_Actinobolus_, _137_;
tentacles and trichocysts of, 152
_Actinocrinus_, 594
_Actinodendron_, _384_;
_A. arboreum_, 384;
_A. glomeratum_, 366;
_A. plumosum_, 366, 384
_Actinoloba_ (= _Metridium_), 371, 372, _381_;
_A. dianthus_, 379, 381;
_A. marginata_, 378
_Actinolophus_, _70_;
habitat of, 75
_Actinometra_, 588, 589, _594_
_Actinomma_, _77_;
_A. asteracanthion_, skeleton of, 77
_Actinomonas_, _70_, 73, 75 n.
Actinomyxidiaceae, _98_
_Actinophrys_, _70_ f., 96 n.;
_A. sol_, 71;
habitat of, _75_
Actinopoda, _570_
_Actinosphaerium_, _70_, 72 f.;
endogamy of, 73 f., 75;
regeneration of, 35;
_A. eichornii_, 72
_Actinostola_ (usually placed in Hertwig's family Paractidae, allied to
Bunodidae, _382_), 379
_Actinotryx_, 371, _383_;
_A. mussoides_, 383
Actinozoa = Anthozoa, _326_ f.
Actipylaea (= Acantharia), _76_
_Actissa_, _77_, 83
Acystosporidae, _97_, 102 f.;
active zygote of, 34
Adambulacral, ossicle, of _Asterias rubens_, 434;
of _Ophiothrix fragilis_, 479;
of Asteroidea compared to covering-plates of Crinoidea, 589;
spine, of _Asterias rubens_, 434
_Adamsia_, _377_;
_A. palliata_, 378, 381;
_A. rondeletii_, 378
Adaptation to fresh water, 176 f.
_Adelea ovata_, syngamy of, 101
Adelocodonic, 265
Adhesion of species, 174 n.
Adinidae, 130 n.
Adoral, band of cilia in Dipleurula larva, 605;
membranellae, 145;
trichocysts, 145;
wreath, 137 f.;
wreath of _Vorticella_, 156, 157
_Aegina_, _296_
Aeginidae, _296_
_Aeginopsis_, _296_
_Aeolis_, nematocysts of, 248
Aequorea, _278_
Aequoreidae, _278_
Aerotaxy, 23
_Aethalium septicum_ (= _Fuligo varians_), 92 f.
Afferent canal of contractile vacuole, of _Stylonychia_, 139, 140;
of _Stentor_, 156
Africa, Tropical, antimalarial measures in, 106;
Trypanosomic diseases of, 120 f.
_Agalma_, 300, _307_
Agalminae, _307_
_Agaricia_, _403_
_Agaricoides_, _350_
Agassiz, 258, 312, 319;
on classification of Endocyclica, 533;
of Clypeastroidea, 548
Aggregate, formation, in _Microgromia_, 59;
social, of _Vorticella_, 158;
(rosette) of _Treponema_, 120
Aggregation of plasmodia into aethalium, 92 f.
Agitation, stimulus of, 19 f.
_Aglaophenia_, 276, _279_
_Aglaophenopsis_ (Plumulariidae, _279_), 277
_Aglaura_, _294_
Aglauridae, _294_
_Aiptasia couchii_, _382_
Air, presence of Flagellate spores in dust of, 118
Air-borne germs in relation to putrefaction, etc., 43
Alate, 185
Alcock, 268
Alcyonacea, _346_ f.
Alcyonaria, _329_ f., 340;
colour, 337;
dimorphism, 333;
food, 339;
mesenteric filaments, 333;
nematocysts, 247;
phosphorescence, 338;
reproduction, 340;
skeleton, 334;
spicules, 334;
zooids, 330
Alcyoniidae, _349_
_Alcyonium_, 330 f., 332, _349_;
nematocyst of, 247;
_A. digitatum_, 332, 338 f., 347, 349;
larva, 341;
_A. glomeratum_, 349;
_A. palmatum_, 340;
_A. purpureum_, 338
Aleurone, 37
Algae, related to holophytic colonial Flagellata, 109, 130
Algeria, dourine disease in, 119
_Alicia mirabilis_, _382_
Aliciidae, _382_
Alimentary canal, of _Asterias rubens_, 438;
of _Ophiothrix fragilis_, 485;
of _Echinus esculentus_, 516;
of _Echinarachnius parma_, 546 f.;
of _Echinocardium cordatum_, 551;
of _Antedon rosacea_, 583;
of _Hyocrinus_, 589;
of _Actinometra_, 589;
development of, in Eleutherozoa, 604, 605;
in _Antedon rosacea_, 618, 619
Allen, on food of _Echinus esculentus_ in Plymouth Sound, 516
Allman, 246, 265, 267, 273, 274;
on Cystoflagellates, 135
_Allogromia_, _59_ f., 65
Allogromidiaceae, _58_;
habitat of, 48, 59
_Allopora_, _287_;
_A. nobilis_, 287
_Alpheus_, 351
Alternating modes of brood-formation in Sporozoa, 48
Alveolar, structure (fine) of cytoplasm, 6;
system (coarse) in relation to skeleton of Radiolaria, Dreyer's scheme
of, 84
Alveolate ectoplasm, of pelagic Foraminifera, 61;
of Heliozoa, 71 f.;
of _Actinosphaerium_, 72 f.;
of Radiolaria, 79;
pedicellaria of _Leptogonaster_, 456
Alveole (= minute cavity in cytoplasm), 5 f.;
in Ciliata, 142;
in _Stylonychia_, 140;
(= large vacuole of Radiolaria, etc.), 76, 79, 84
_Alveolina_, _59_
Alveolus, of _Echinus esculentus_, 526
_Alveopora_, _397_
_Amalthea_ (Fam. Corymorphidae, _273_), 266
Ambulacral area, of _Echinarachnius parma_, 544;
of _Echinocardium cordatum_, 550
Ambulacral groove, of Asteroidea, 432;
representative in Ophiuroidea, 481;
representative in Echinoidea, 515;
of Pelmatozoa, 579;
of _Antedon rosacea_, 581, 582, 587;
of fossil Crinoidea, 595;
of Thecoidea, 596;
of Carpoidea, 596;
of young Ophiuroid, 613
Ambulacral ossicle, of Asteroidea, 432;
of _Asterias rubens_, 434;
of _Ophiothrix fragilis_, 481;
compared to auricula of _Echinus esculentus_, 526;
to inner plates of _Palaeodiscus_, 557;
to side-plates of Crinoidea, 589
Ambulacral plate, of _Echinus esculentus_, 511;
of Cidaridae, 533;
of _Sphaerechinus_, 539;
of _Strongylocentrotus_, 539;
of _Echinarachnius parma_, 544
Amines, 15
_Ammodiscus_, _59_
Amnion, 613
Amniotic cavity, 613
Amoeba, 4 f.;
reactions of, 7 f.;
devouring
a plant cell, 9;
excretion of, 14 f.;
motion of, 17 f.;
respiration of, 17 f.;
taxies of, 22
_Amoeba_, _51_;
habitat of, 57;
posterior disc or sucker, 53;
_A. binucleata_, 30, 52;
_A. coli_, habitat, 57;
_A. limax_, 5;
form of amoebulae of Acrasieae, 90;
motions of, 47 n., 52;
_A. polypodia_ in fission, 10;
_A. proteus_, 5;
brood-divisions of, 56 n.
Amoeboid, motion, 5;
stages of Acystosporidae, 97, 103 f.;
zoospores of _Trichosphaerium_, 54, 56;
gametes of Flagellata, 116 n.
_Amoebophrya_, 86, _159_, 161
Amoebula, 31, 51;
of Myxomycetes, 91, 92;
of _Didymium_, 92;
of fever parasites, 104
Amphiaster (an aster in which the actines form a whorl at each extremity
of the axis, which is straight), 222
Amphibia, hosts of Opalinidae, 111, 123
Amphiblastula, 226
Amphicaryoninae, _306_
Amphidisc, 176, 177 f., 202
Amphidiscophora, _203_ f.
_Amphihelia_, _399_
_Amphileptus_, _137_
Amphimonadidae, _111_
_Amphimonas_, _111_
_Amphinema_, _273_
_Amphiprion percula_, 378
_Amphiura_, _497_;
_A. squamata_ (= _elegans_), 485 n., 497, 601
Amphiuridae, _497_
_Amphizonella_, _51_;
test of, 53
Amphoriscidae, _192_
Ampulla, of _Millepora_, 259, 260;
of Stylasterina, 284, 285;
of tube-foot of _Asterias rubens_, 441, 443;
synonym of axial sinus of _Ophiothrix fragilis_, 487;
of stone-canal of _Echinus esculentus_, 517;
of tube-foot of _Echinus esculentus_, 517;
of respiratory trees of _Holothuria nigra_, 563;
of podia of _Holothuria nigra_, 566;
of feelers of Aspidochirota, 568, 570;
of _Pelagothuria_, 568, 570;
of Molpadiida, 568;
of Synaptida, 568;
of Elasipoda, 571
Anabolic, 12 f.
Anabolism, 12 f.;
modes of, 15 f.
Anal, cirrhi, 139 f.;
papilla (tube) of _Antedon rosacea_, 581, 583;
of _Eucalyptocrinus_, 596
Ananchytidae, _554_, 559
_Anapta_, _577_
Anatomy, of a starfish (_Asterias rubens_), 432 f.;
of _Ophiothrix fragilis_, 479 f.;
of _Echinus esculentus_, 504 f.;
of _Echinarachnius parma_, 542 f.;
of _Echinocardium cordatum_, 549 f.;
of _Holothuria nigra_, 561 f.;
of _Antedon rosacea_, 581 f.
Anatriaene (a triaene of which the cladi or branches point backwards, in
the same direction as the rhabdome or shaft), 224
Anchoring flagella, 114;
of _Dallingeria_, 112;
of _Bodo saltans_, 117
_Ancistrum_, _137_
_Anemonia_, _381_;
_A. sulcata_, 381
Animal-feeding Protista, 38
Animals, defined, 39 f.;
and plants, discussion on, 35 f.;
Higher, 31, 38
Anisochela (a chela of which the two ends are unequally developed), 222,
234
_Anisonema, 110_
Anisospores of Radiolaria, 76, 85;
of _Collozoum inerme_, 76
_Anochanus_, _554_
_Anopheles_, intermediate host of malarial parasite, 103 f.;
enemies of, 106;
precautions against, 106
_Antedon_, _594_;
_A. rosacea_, 581, 582;
external features, 581;
skeleton, 582;
alimentary canal, 583;
water-vascular system, 583;
nervous system, 583 f.;
coelom, 585;
genital organs, 586;
muscles, 587;
blood-system, 587;
development of, 617 f.;
_A. eschrichtii_, 594
_Antennularia_, _279_;
_A. antennina_, 279;
_A. ramosa_, 279
Anterior, dorsal process, of ciliated band of Bipinnaria, 606;
of Auricularia, 608;
median process, of ciliated band of Bipinnaria, 606
Antero-lateral process, of ciliated band of Echinopluteus, 607
_Anthea cereus_, _381_
Antheneidae, _471_
Anthocaulus, 389
Anthocodia, 330
Anthocyathus, 388, 389
_Anthomastus_, 333, _349_
Anthomedusae, _262_ f.
_Anthophysa_ (Flagellata), _111_, 112 f.;
(Siphonophora, Physophorinae), _308_, 302
Anthoptilidae, _362_
_Anthoptilum grandiflorum_, _362_
Anthozoa, _326_ f.;
commercial importance, 328
Antipatharia, _407_ f.
_Antipathella_, _408_;
_A. gracilis_, 408
_Antipathes_, _408_;
_A. ternatensis_, 409
Antipathidea, 367, 371, _407_ f.
Antiseptic properties of "aromatic" compounds, 36 n.
Anuncinataria, _203_
Anus, of Ciliates, 143 f.;
of _Stylonychia_, 139 f.;
of _Carchesium_, 147;
of _Vorticella_, 156;
of _Asterias rubens_, 434;
of _Echinus esculentus_, 516;
of _Echinarachnius parma_, 546;
of _Pygastrides relictus_, 548;
of Euclypeastroidea, 549;
of _Echinolampas_, 554;
of _Neolampas_, 554;
of _Holothuria nigra_, 560;
of _Antedon rosacea_, 582;
of Dipleurula larva, 604;
of _Asterina gibbosa_, 611
Aphodal, 210
Aphrothoraca, _70_
Aplanospore, 31
_Aplysilla_, 196
_Aplysina_, 225
Apocyte, 30, 32
Apocytial, condition, 32;
forms among Myxosporidiaceae, 107 f.;
Rhizopoda, 52
_Apolemia_, _308_
Apoleminae, _307_
Apopyle, 170
Aporosa, _397_
_Appendicularia_, host of _Gymnodinium pulvisculus_, 132
Aquatic organisms, minute, distribution of, 47 n.
_Arachnactis_, 373, _411_;
_A. albida_, 411;
_A. americana_, 411;
_A. lloydii_, 411
_Arachnoides_, _549_
_Arbacia_, 520, _538_
Arbaciidae, 530, 531, 532, _538_, 558
Arbacioid type of ambulacral plate, 531, 538
Arcadomyaria, _324_
_Arcella_, _51_, 53;
_A. vulgaris_, 55
Archaeocidaridae, _557_, 558
Archaeocyte, 171
_Archaster_, _467_;
_A. bifrons_, 467
Archasteridae, 456, _466_
Archenteron, definition of, 604
Archer, on Protozoa, 45;
on Heliozoa, 71
Archicoel, 450
_Arcuothrix_, _52_;
transition between pseudopodium and flagellum in, 47 n.
Arenaceous Foraminifera, _58_ f.;
Carpenter on, 63 f.;
labyrinthine structure in, 66
_Argas persica_, 121
_Aristocystis_, _599_, 599
Aristotle, 166
Aristotle's lantern, of _Echinus esculentus_, 515, 516, 524, 525;
variations of, in Endocyclica, 531;
of _Echinarachnius parma_, 546, 547;
absent in _Echinocardium cordatum_, 550
Arm, of Asteroidea, 431, 432, 453;
of _Ophiothrix fragilis_, 479;
of Ophiuroidea, 481, 491;
of Crinoidea, 580, 589;
of _Antedon rosacea_, 581;
of _Antedon_ (other species), 594;
of _Hyocrinus_, 590;
of _Rhizocrinus_, 591;
of _Bathycrinus_, 591;
of Pentacrinidae, 592;
of _Holopus_, 594;
of _Eudiocrinus_, 594;
of Inadunata, 595;
of Articulata, 595;
of Camerata, 595;
development of, in _Asterina gibbosa_, 611;
in _Antedon rosacea_, 620
Arm-spines, of _Ophiothrix fragilis_, 479;
of Ophiuroidea, 491;
of _Ophiothrix_, 492;
of _Ophiacantha_, 492;
of _Ophiopteron_ 492
Aromatic compounds in relation to nutrition and antisepsis, 36
Arthropods, hosts of Gregarines, 97 f.
Articulata, _595_
Ascitic dropsy, _Leydenia_ associated with, 91
Ascon, 185
_Asconema setubalense_, 221
Asexual reproduction of Asteroidea, 459;
of _Linckia_, 459;
of _Asterina wega_, 459
Ashworth, 331 n.
Asiphonacea, _347_
Asphyxia, its effect on contractile vacuole, 143
Aspidochirota, 568, 569, _570_ f., 577, 578
_Aspidosoma_, _476_
Aspirotrichaceae, _137_, 148, 151, 153, 154
Assimilation, assimilative anabolism, growth, 9, 13, 15 f.
Association, in Gregarines, 95, 98 f.;
in _Lankesteria ascidiae_, 95 f.
_Astasia_, _110_, 112
Aster (= centrosome of mitotically dividing cell and peripheral rays),
25, 27;
(a polyaxonid spicule), 184
_Asterias_, _473_, 475;
_A. rubens_, 432;
external features, 432;
pedicellariae, 433;
skeleton, 434;
coelom, 437;
alimentary canal, 438;
food, 439;
water-vascular system, 441;
nervous system, 444;
perihaemal spaces, 448;
blood-system, 449;
genital organs, 451;
_A. glacialis_, 473;
pedicellariae, 434;
_A. hispida_, 474;
_A. muelleri_, 473;
_A. murrayi_, 473;
_A. ochracea_, 474;
_A. polaris_, 474;
_A. spirabilis_, 601, 602;
_A. tenuispina_, 453
Asteriidae, 453, 458, _473_
_Asterina_, 454, 456, 459, 461, _463_;
_A. gibbosa_, number of arms, 453;
eggs of, 463;
development of, 609, 610 f., 617;
_A. wega_, 459
Asterinidae, 455, 458, _463_
Asternata, _554_
Asteroid stage in the development of Ophiuroidea and Echinoidea, 613, 622
Asteroidea, _430_, _431_ f.;
compared with Ophiuroidea, 477 f.;
compared with Echinoidea, 503, 558;
mesenchyme of larva of, 602;
development of, 605, 608, 609, 610 f.;
phylogeny of, 621
_Asteropsis_, _471_
_Asterosmilia_, _401_
_Asthenosoma_, _536_;
_A. hystrix_, 536, 537;
_A. urens_, _536_
Astraeidae, 387, _399_
_Astraeopora_, 390
_Astrangia_, _400_;
_A. solitaria_, 374, 400
_Astrocnida_, _501
Astrogonium_, _472_
_Astroides_, _404_
_Astronyx_, _501_
_Astropecten_, 455, 459, _467_;
fossil, 475;
_A. irregularis_, _468_;
movements, 468;
burrowing habits, 469
Astropectinidae, 454, 458, 459, _467_, 470
_Astrophyton_, 491, _501_
Astropyle, 81
_Astrorhiza_, _59_
Astrorhizidaceae, _59_
_Astroschema_, _501_;
vertebra, 481
Astroschemidae, _501_
_Astrosclera willeyana_, _194_, 194
Astroscleridae, _194_
_Astylus_, _287_
_Atelecrinus_, 588, _594_
_Athoria_ (usually placed in a subfamily Athoriinae of the Physonectidae,
_307_), 300
_Atolla_, _322_;
_A. bairdi_, 322;
_A. gigantea_, 322;
_A. valdiviae_, 322
Atollidae, _322_
Atoll, 390 f.
_Atorella_, 314, _322_
Atrophy, of oral apparatus of Ciliata during conjugation, 151
Attached, Foraminifera, 64;
Flagellata, 112 f.;
Ciliata, 152;
Suctoria, 160 f., 162
Attachment, temporary, of _Stentor_, 155;
permanent, of _Rhizocrinus_, 591;
of _Bathycrinus_, 591;
of young Pentacrinidae, 592;
of Thecoidea, 596;
of Cystoidea, 597;
— temporary, of larva of _sterina gibbosa_, 610;
of Brachiolaria larva, 612;
of larva of _Antedon rosacea_, 619
_Aulactinium_, _79_;
_A. actinastrum_, 82
_Aulena_, _220_
_Aulocystis grayi_, _208_
_Aurelia_, 310, 314, _324_;
_A. aurita_, 312
_Aureliania heterocera_, _383_
Auricula, of _Echinus esculentus_, 526;
of Cidaridae, 531;
of Arbaciidae, 531;
of _Echinarachnius parma_, 546;
represented by radial pieces of calcareous ring of Holothuroidea, 566
Auricularia, 607;
metamorphosis of, 614, 615
Auronectidae, 301, _308_
Aurophore, 308
Autodermin, 523
Automatic processes, so-called, 12
Autotrophic, 37
Autozooids, 333
Axial filament, of Heliozoan pseudopodia, 49, 71, 72, 74;
of _Actinophrys sol_, 71;
of _Actinosphaerium eichornii_, 72;
of Radiolaria Acantharia, 80
Axial sinus, of _Asterias rubens_, 448;
of _Ophiothrix fragilis_, 487;
of _Echinus esculentus_, 517, 528;
of _Echinocardium cordatum_, 552;
of Holothuroid larva, 564;
of larva of _Antedon rosacea_, 583;
development of, in _Asterina gibbosa_, 609
Axifera, _353_
_Axinella_, _216_, 222, 225;
_A. erecta_, 216
Axinellidae, _217_
Axon, 444
_Axoniderma_, _216_
_Axopora_, _262_
Azoosporeae, _89_
_Babesia_ (= _Piroplasma_), 120
Bacteria, food of Myxomycetes, 92 f.;
nutrition of, 36
_Bactronella_, _193_
Baker, H., on organisms of putrefaction, 43;
on Protozoa, 45;
on _Noctiluca_, 134 f.
Balanoglossus, larva and affinities of, 616
_Balantidium_, _137_;
habitat of, 152
Balbiani, on Protozoa, 45;
on regeneration, 35 n.;
on "Pébrine" (_Nosema bombycis_), 107
Barbados, Miocene deposit of Radiolaria, 87
Barotaxy, 20
Barrier reef, 390 f.
Bary, A. de, on methods of culture of lower organisms, 44;
on nature of Myxomycetes, 91
Basal granules, of cilia, etc., 138 n., 141
—see also Blepharoplast
Basal plates, of _Antedon rosacea_, 584;
of Crinoidea, 588;
_of Hyocrinus_, 588, 590;
of _Atelecrinus_, 588, 594;
of Rhizocrinidae, 588, 590;
of _Thaumatocrinus_, 588, 594;
of Pentacrinidae, 591;
of _Holopus_, 592;
of Cystoidea, 598;
of Blastoidea, 599
Bastian, H. Charlton, on spontaneous generation, 44 n.
Bateson, 174
Bath sponge, 221
Bather, on _Sympterura_, 502;
on classification of Crinoidea, 589;
on phylogeny of Echinodermata, 622
_Bathyactis_, _404_
_Bathyanthus_, _411_
_Bathycrinus_, 589, _591_
_Bathypathes_, _408_
_Bebryce_, _356_
Bell, F. J., 406 n.;
on classification of Ophiuroidea, 494;
of Endocyclica, 533
on relationships of Holothuroidea, 430 n., 622
Bell-animalcules, 155 f.;
feeding of, 145
—see also _Carchesium_, _Epistylis_, Peritrichaceae, _Vorticella_,
_Zoothamnium_
Beloidea, 77
Beneden, E. van, on Sporozoa, 94;
on pelagic Anthozoa, 411
Bernard, F., 241 n.
Bernard, H. M., 386 n., 397
_Beroe_, 413, _423_;
_B. cucumis_, 416, 417;
_B. ovata_, 416, 423
Beroidae, 414, _423_
Berthold, on protoplasmic movements, 16 n.
Bezzenberger, list of species of _Opalina_, 124 n.;
on parasitic Ciliata, 152 n.
_Bicoeca_, _111_
Bicoecidae, _111_
Bidder, 168, 172 n., 186 n., 235 n.
_Biemma_, 224
Bigelow, 304, 420 n.
Bile, 13
_Biloculina_, _59_, 67
Biloculine, 66
Binary sex (= syngamy with marked inequality between the pairing cells),
33 f.;
in _Centropyxis_, 57;
in _Stylorhynchus_, 99;
in _Pterocephalus_, 99;
in Coccidiaceae, 100 f.;
in _Sarcocystis tenella_, 108 n.;
in Volvocidae, 127 f.;
in _Eudorina_, 129;
in Peritrichaceae, 151 f.
_Biomyxa_, _58_
Bionomics of Protistic life, 43
Bipinnaria, 605 f., 608;
metamorphoses of, 612
Bipocillus (a microsclere consisting of a curved shaft, terminated by a
cup-shaped expansion at each end, characteristic of the genus
_Iophon_), 222
Birds, hosts of Acystosporidae, 103
Birth-pore, of brood-cavity of Suctoria, 161
Bisexual differentiation—see Binary sex
Bivium, of _Echinarachnius parma_, 543;
of _Holothuria nigra_, 561
Black Corals (= _Gerardia_, 406, and Antipatharia, 407)
Bladder, of Rotifers and Platyhelminthes, 14 n.
Blanchard, on Sporozoa, 94
Blastocoel, 603
Blastoidea, _580_, 599 f.
Blastomere, 603
Blastostyles, 265
Blasts of Coccidiaceae, morphology of, 120;
of Acystosporidae, 104 f.
Blastula, definition of, 603
Blepharoplast, 19;
of Heliozoa, 72;
of Flagellata, 109, 115;
of _Trypanosoma_, 121;
of nuclear origin in _Trypanosoma_, 109 n.;
of _T. noctuae_, 120 f.;
of Ciliata, 141
Block-musculature, of spines of _Echinus esculentus_, 506
Blood, Acystosporidae and Haemosporidae parasitic in, 97, 102 f.;
Protomastigaceae parasitic in, in fever, 119 f.
Blood-corpuscle, entered by Haemosporidae and Acystosporidae, 102 f., 104
f.;
by _Treponema_ and _Trypanosoma_, 120
Blood-system, of _Asterias rubens_, 449 f.;
of _Echinus esculentus_, 529;
of _Holothuria nigra_, 567;
of _Antedon rosacea_, 587
Bloody rain, 125
Blowflies, alleged spontaneous generation of, 42
_Bodo_, _111_, 119;
_B. caudatus_, 116 n.;
_B. saltans_, 114, 116 n., 117 f.
Bodonidae, _111_;
movements of, 114 n.
Body-cavity—see Coelom
_Bohemura_, 502
_Bolina_, _419_;
_B. infundibulum_, 416
Bolinidae, _419_
_Bolinopsis_, _419_
_Bolocera tuediae_, _381_
Borchgrevink, 310 n.
Borgert, on Dictyochidae, 87;
on fission in _Aulacantha_, 85;
on phaeodium in Radiolaria, 81 n.
Botanists, contributors to knowledge of Flagellates, 119
_Bothriocidaris_, _557_, 561
Botryoidea, _79_
_Bougainvillia_, 263, 264, 266, _269_;
_B. ramosa_, 269
Bougainvilliidae, _269_
Boulenger, 294 n.
Bourne, G. C., 246, 337, 338 n., 347
Bourrelet, 553
Boveri, on functions of chromatin, 28 n.
Bowerbank, 168, 169, 234 n.
Brachial ossicle, of _Antedon rosacea_, 582;
of fossil Crinoidea, 595
Brachiolaria, 612, 613
_Brachionus_, often found with _Euglena viridis_, 124
Brachiopoda, 428, 579
Brachyenemic, 405
Brachyeneminae, _405_
Brady, classification of Foraminifera, 58 f.
Brain-Coral, 401
Branched, endoplasm, of _Noctiluca_, _Trachelius_, and _Loxodes_, 133,
153;
meganucleus, of _Dendrosoma_, 160;
theca, of _Dinobryon_, 112;
of _Schizotricha_, 152
Branching colony, of _Carchesium_, _Epistylis_, _Zoothamnium_, 158
Brandt, Karl, on commensals and parasites of Radiolaria, 87 n.;
on Radiolaria, 88
Breeding temperature of Protozoa, 47
Breeze Flies, intermediate hosts of _Trypanosoma evansii_, 119
Briareidae, _350_
_Brisinga_, _474_
Brisingidae, 453, 454, _474_
Brissidae, _556_
_Brissopsis_, _556_;
_B. lyrifera_, 556
Brittle Star—see Ophiuroidea
Brood-cavity, in Suctoria, 160 f., 162
Brood-cell, 31
Brood-cyst, of Proteomyxa, 88 f.
Brood-division, brood-formation, 30 f.;
in Rhizopoda, 56 f.;
in Foraminifera, 67 f.;
in Radiolaria, 85 f.;
in Proteomyxa, 49, 88 f.;
in Sporozoa, 48, 94 f.;
in Flagellata, 109, 115, 117 f.;
in Polytomeae, 115;
in Chlamydomonadidae, 115;
in _Paramoeba eilhardii_, 116 n.;
in Choanoflagellates, 122;
in _Proterospongia_, 122;
in Volvocidae, 127 f.;
in Ciliata, 147;
in _Colpoda cucullus_, 147, 153;
of male Peritrichaceae, 151 f., 157;
retarded, 31 f.
Brood-mother-cell, 31
Brood-pouch, of _Pteraster_, 465, 466;
of _Hemiaster philippi_, 556, 602;
of _Anochanus_, 554;
of _Cucumaria laevigata_, 602;
of _Psolus ephippifer_, 602
Brooks, 288, 290, 308 n.
Brown colour of lakes or ponds often due to Dinoflagellates, 131
Browne, 273, 281 n., 291 n., 312
Bruce, Col. and Mrs., on Trypanosomic fever and sleeping sickness, 120
Bubbles of carbon dioxide formed in _Arcella_, etc., 53
Buccal sinus of _Holothuria nigra_, 566
Buccal tentacle—see Buccal tube-foot
Buccal tube-foot (and tentacle), of _Ophiothrix fragilis_, 486;
of _Echinus esculentus_, 518;
of _Echinocardium cordatum_, 551, 561;
of _Holothuria nigra_, 561;
of Spatangoidea, 577;
development of, in Holothuroid pupa, 615
Budding, in _Trichosphaerium_, 54;
in Rhizopoda, 56;
in _Acanthocystis_, 73;
in Acantharia, 86 n.;
in _Spirochona_, 147;
in Suctoria, 160, 162;
in Sponges, 177 f., 228, 229, 230;
in Hydrozoa, 250 f., 263, 275;
in Medusae, 270, 296;
in Scyphistoma, 317;
in Alcyonaria, 340;
in Zoantharia, 371
Bud-fission in testaceous Rhizopoda, 55;
in _Euglypha_, 29
Buffon, on organisms of putrefaction, 43
_Bulimina_, _59_
Bulk and surface of organism, ratio between, 14
_Bunodactis gemmacea_, 378
_Bunodeopsis_, _382_
_Bunodes_, 378, _382_;
_B. gemmacea_, 382
Bunodidae, _382_
Burrowing habits, of certain Asteroidea, 461;
of _Astropecten_, 469;
of Paxillosa, 469;
of Porcellanasteridae, 471;
of _Strongylocentrotus lividus_, 541;
of _Echinarachnius parma_, 546;
of _Echinocardium cordatum_, 549;
of _Spatangus purpureus_, 555;
of Molpadiida, 575;
of Synaptida, 576;
of _Synapta inhaerens_, 577
Burrowing Urchins, 529
_Bursaria_, _137_, 155
Bury, on primary axial sinus of Holothuroid larva, 564;
on change in position of mouth during metamorphosis of Auricularia, 614
Bütschli, on protoplasm, 16 n.;
on paramylum, 95 n.;
on Protozoa, 46;
on Cystoflagellates, 135;
on classification of Ciliata, 137;
on _Strombidium_ and _Torquatella_, 155 n.;
on Sponges, 168, 172 n.