in Asteroidea and Ophiuroidea. There is an oral ring of lymphoid tissue
surrounding the oesophagus below the water-vascular ring. From this are
given off two strands, the so-called "dorsal" (Fig. 231, _b.v_), and
"ventral" vessels (Fig. 234, 16), which run along the two opposite sides of
the stomach or first coil of the alimentary canal. The position of these
strands suggests that like the lacteals of the human intestine they are
channels along which the products of digestion exude from the stomach. The
dorsal strand is situated on the same side as the genital stolon, and from
it branches are given off which ramify on the surface of the stolon, on
account of which this organ, as in Asteroidea, was at one time regarded as
a "heart," but the distinction of the stolon from the strands is easily
made out. An aboral ring enclosing the genital rachis lies embedded in the
septum dividing the aboral sinus (Fig. 234, 20) from the general coelom.
CLASSIFICATION OF ECHINOIDEA.
The Echinoidea are sharply divided into three main orders, which differ
from each other profoundly in their habits and structure. These are: (1)
The ENDOCYCLICA or Regular Urchins, of which the species just described may
be taken as the type. (2) The CLYPEASTROIDEA or Cake-urchins, which are of
extremely flattened form, and in which the periproct is shifted from the
apical pole so that it is no longer surrounded by the genital plates, while
some of the tube-feet of the dorsal surface are flattened so as to serve as
gills. (3) The SPATANGOIDEA or Heart-urchins, in which the outline is oval:
the periproct is shifted, as in the Cake-urchins, and the dorsal tube-feet
are similarly modified; but the Heart-urchins have totally lost Aristotle's
lantern, whilst the Cake-urchins have retained it. This strongly-marked
cleavage of the group was primarily due, as in all such cases, to the
adoption of different habits by different members of the same group. Were
we to term the three orders Rock-urchins, Sand-urchins, and
Burrowing-urchins, it would not be entirely true, for secondary invasions
of the other's territory on {530}the part of each order have undoubtedly
taken place; but still the statement would remain roughly true, and would
give a fair idea of the differences in habitat which have led to the
differentiation of the group.
ORDER I. ENDOCYCLICA (_Regular Urchins_).
The principal variations concern (1) the peristome, (2) the periproct, (3)
the corona, (4) Aristotle's lantern and its appendages, (5) the spines, (6)
the pedicellariae, and lastly, (7) the tube-feet. We shall consider these
points in order.
PERISTOME.—In the vast majority of species this region is covered only with
flexible skin in which ten small plates are embedded, pierced by pores for
the buccal tube-feet; besides these there are irregularly arranged thin
plates. In the Cidaridae both the ambulacral and the interambulacral series
of plates are continued on it; these plates differ from those of the corona
in being movable on one another. In Echinothuriidae only the ambulacral
series of plates is continued on to the peristome. In the case of both
these families there are a considerable number of tube-feet within the
region of the peristome which may be classed as buccal.
PERIPROCT.—This area, which represents the whole dorsal surface of
Asteroidea, is very large in the Cidaridae, where, as in _Echinus_, it is
covered with leathery skin in which small plates are embedded. In the
Saleniidae it is covered with a single large sur-anal plate, in the edge of
which the anus is excavated; in the Arbaciidae it is covered with four
valve-like plates; whilst in the remaining species its condition is similar
to that described in the case of _Echinus esculentus_.
CORONA.—In Echinothuriidae all the plates are separated by slips of
membranous skin, so that the test is flexible. In all other families it is
an unyielding cuirass. In the Cidaridae the pore-plates remain separate
throughout life, and are therefore identical with the ambulacral plates.
These are small and placed in two vertical rows, and so the ambulacra are
exceedingly narrow. In Echinothuriidae there is some tendency to adhesion
amongst the pore-plates; these are of different sizes, and usually one
larger and one smaller adhere to one another. In all other species regular
ambulacral plates are formed at least {531}in the lower part of the radii
near the peristome by the adhesion of the pore-plates in groups of two,
three, or more. Sometimes as many as nine pore-plates may thus adhere.
When adhesion takes place between the pore-plates it is of course preceded
by crowding, and this interferes with their equal development. Some which
extend so far horizontally as to meet their fellows of the opposite side of
the radius are called primary plates; others which are small and wedged in
between the larger ones are called demi-plates. Systems of classification
have been built up (chiefly by palaeontologists) in which great stress has
been laid on how the primaries and secondaries enter into the constitution
of the compound plate, but it does not seem to the present author as if
this were at all a satisfactory basis for classification. All the
pore-plates are primarily equivalent, and the question as to which are
interfered with in their growth so as to become secondary is trivial. The
so-called Arbacioid type consists of one primary with a secondary on each
side; the Diadematoid type of three primaries, with occasionally a
secondary between the aboral and the middle primary; and finally the
Triplechinoid type of two primaries, with one or more secondaries between
them.
ARISTOTLE'S LANTERN.—Under this head we may consider the auriculae and
gills as well as the jaws and teeth. In Cidaridae external gills appear to
be absent, but from the lantern coelom large radial pouches project upwards
into the general coelom cavity. These pouches are supposed to be
respiratory, and are termed INTERNAL GILLS or STEWART'S ORGANS.[488] They
co-exist with external gills in Echinothuriidae and in Diadematidae, though
in the last family they are present only in a vestigial form, two being
found in each radius. The auricular arch both in Cidaridae and in
Arbaciidae is composed of two pillars which do not meet, but in the
last-named family they are based, as in Echinidae, generally on the
ambulacral plates, whereas in Cidaridae they arise from the interambulacral
plates (the ambulacral plates being here very narrow). The epiphyses are
absent in Cidaridae and Arbaciidae, and are imperfect in Diadematidae.
SPINES.—These organs are extraordinarily variable, and {532}usually differ
very much in species of the same genus. In the vast majority of species
there is a limited number of long spines called "primaries," amongst the
bases of which a large number of much shorter "secondaries" are
distributed. In Cidaridae the primaries are very long and thick and blunt
at the ends, and the secondaries form small circles around their bases. The
primaries in Cidaridae and the tips of the primaries in Arbaciidae and
Echinothuriidae are covered with a special investment of extremely close,
hard, calcareous matter very different from the loosely fenestrated
material out of which the bodies of the spines of all species are composed.
In _Colobocentrotus_ and _Heterocentrotus_ the primaries are very thick and
triangular in section, whilst the secondaries on the aboral surface have
expanded outer ends, which form a close-set pavement protecting the
ectoderm from the shocks of the breakers. In Echinothuriidae the primaries
are short and so delicate as to be termed silky.
PEDICELLARIAE.—In Cidaridae only gemmiform and tridactyle pedicellariae are
found. In the gemmiform the glands lie inside the grooved blades instead of
outside as normally, and they are covered internally by ingrowths of
calcareous matter from the edges. In Echinothuriidae only tridactyle and
trifoliate are found in most species, but rudimentary gemmiform are found
in one species and well-developed ophicephalous in another. In some species
(_Centrostephanus longispinosus_) there are found gemmiform pedicellariae
which have lost the jaws but retained the glands. These are termed
"globiferae." Mortensen[489] uses minute details in the structure of the
pedicellariae to discriminate species and even genera, but in this the
present author is not prepared to follow him.
TUBE-FEET.—The tube-feet belonging to the aboral surface are pointed and
devoid of a sucker in Diadematidae, Echinothuriidae, Arbaciidae, and
Cidaridae;[490] in the last-named family those belonging to the oral
surface have suckers, in the centre of which a pointed (sensory) prominence
is to be noted.
The CLASSIFICATION of the Endocyclica is by no means in a satisfactory
condition, and different authorities have arrived at {533}widely different
results. Agassiz,[491] for instance, places the genera _Echinus_ (the
common British form) and _Strongylocentrotus_ (the commonest American form)
in different families. Bell,[492] on the other hand, considers them to be
closely allied. Bell's system, based as it is on the development of the
peristome, seems to the present author the most justifiable, for the
peristome is undoubtedly a differentiation of the corona, which has been
brought about by the manner in which the animal breathes and masticates,
two functions of prime importance. The periproct is also of importance,
representing as it does the whole aboral surface of the Starfish, and so
are to a less extent the arrangements of the spines and of the tube-feet.
Proceeding in this way, living Endocyclica can be divided into six
families, which are briefly described below.
[Illustration: FIG. 235.—Oral view of dried and cleaned test of _Cidaris_.
_p_, Pores for tube-feet arranged in single series; _per_, peristome with
both ambulacral and interambulacral plates; _t_, tubercle of a large
interambulacral spine.]
FAM. 1. CIDARIDAE.—Endocyclica with a large peristome and a large
periproct. The peristome is covered with a regular series of both
ambulacral and interambulacral plates, the former pierced by tube-feet. No
special buccal tube-feet and no external gills. The periproct is large, and
is covered with irregular plates (Fig. 236, A). The lantern coelom is
provided with large Stewart's organs.
{534}The auriculae are incomplete and consist only of pillars arising from
the interambulacral plates. The ambulacral pore-plates remain disunited,
and the pores are arranged in a single vertical series; hence the ambulacra
are very narrow. The interambulacral plates each bear one large primary
spine surrounded by several circles of secondaries. No ophicephalous or
trifoliate pedicellariae are to be found, and the gland of the gemmiform
pedicellaria is placed inside the concavity of the blade.
[Illustration: FIG. 236.—Figure showing periprocts of A, Cidaris; B,
Echinus, × 1. _amb_, Ambulacral plate; _g.o_, genital opening; _g.p_,
genital plate; _inter_, interambulacral plate; _m.p_, madreporite; _oc_,
ocular pore.]
The Cidaridae are in many respects the most primitive of the six families
living. They are distributed all over the world, and chiefly inhabit deep
water. No two naturalists agree as to how they are to be divided into
genera. Mortensen,[493] who takes the structure of the pedicellariae as his
principal guide, recognises fourteen genera. Others (as for instance Bell)
have been inclined to attribute nearly all the living species to one
polymorphic genus, _Cidaris_, finding all attempts to divide the genera
from one another frustrated by the discovery of transitional forms.
_Goniocidaris_ (Fig. 237), however, distinguished by its comparatively
broad poriferous zones, by bare places in the middle line of both radii and
interradii, and by deep pits on the lines of suture of the plates, is by
general consent distinct. This genus is confined to the Eastern Pacific,
but from British waters three species of _Cidaris_ have been recorded, only
one of which, _C._ (_Dorocidaris_) _papillata_, is at all common. It is
found in water from 100 to 500 fathoms in depth {535}off the western coast
of Ireland and Scotland. It also occurs in the Mediterranean, and has been
carefully examined and described when living by Prouho.[494] From his
description it appears that locomotion is effected almost entirely by
spines, and that the tube-feet of the lower parts of the radii have each in
the centre of the disc a pointed sense-organ like those in the centre of
the first tube-feet of the just metamorphosed _Echinus_, whilst those of
the aboral surface have no suckers.
[Illustration: FIG. 237.—_Goniocidaris canaliculata._ × 2. (From Wyville
Thomson.)]
FAM. 2. ECHINOTHURIIDAE.—Endocyclica with a large peristome and
comparatively small periproct. The peristome has a regular series of
ambulacral plates bearing pores for tube-feet, but no interambulacral
plates. No specially modified buccal tube-feet, but external gills are
present, and internal gills (Stewart's organs) also occur. The periproct is
covered with numerous small plates. All the plates of the corona are
separated by thin slips of flexible body wall. Numerous comparatively short
primary spines on both ambulacral and interambulacral plates; these spines
are covered on the tips with a layer of hard dense material.
{536}[Illustration: FIG. 238.—Oral view of _Asthenosoma hystrix_. × ⅔.
(From Wyville Thomson.)]
This remarkable family is divided by Mortensen into ten genera, based as
usual on the pedicellariae, but taking into account also the shape of the
tip of hard material on the spines. Most authors refer the majority of the
species to two genera, _Phormosoma_ and _Asthenosoma_ (Fig. 238),
recognising also a genus _Sperosoma_ for one or two aberrant species.
_Asthenosoma_ is distinguished by having wide interspaces of membrane
between the plates, and by having ten longitudinal folds of the body-wall,
two in each radius, in which powerful longitudinal muscles are developed
projecting inwards in the radii. The organs of Stewart are very large. In
_Phormosoma_, on the contrary, the interspaces of membrane are very narrow,
and the longitudinal folds are thin and membranous and the organs of
Stewart are vestigial. _Asthenosoma hystrix_ and _Phormosoma placenta_ have
both been dredged in deep water off the Irish coast. _A. urens_, in which
there are ectodermic poison-sacs at the bases of the spines, inhabits the
Indian Ocean near Ceylon, and was thoroughly {537}described by the
Sarasins,[495] who regarded its structure as a proof that Echinoidea were
derived from Holothuroidea. Both palaeontology and embryology have,
however, yielded strong evidence that Echinoidea were derived from
Asteroidea, and hence there is ground for believing that Holothuroidea are
descended from primitive Echinoidea and not _vice versa_. The
Echinothuriidae may perhaps be regarded as showing the first steps in the
change, and though possibly not closely related to the actual ancestors of
the Holothuroidea, they at any rate show parallel modifications.
[Illustration: FIG. 239.—View of peristome of _Asthenosoma hystrix_. _amb_,
Ambulacral plates on the lower edge of the corona; _inter_, lower plates of
the interambulacral area. (From Wyville Thomson.)]
FAM. 3. SALENIIDAE.—Endocyclica with a large peristome and periproct. The
peristome is covered with thin, scattered, irregular plates. There are five
pairs of special buccal tube-feet, each supported by a special plate, and
there are external gills. The periproct is excavated in the side of a large
central pentagonal plate. It is covered with fifteen or twenty plates.[496]
The ambulacral plates are separate as in the Cidaridae, but occasionally
adhere in pairs near the peristome. The interambulacral plates also, as in
Cidaridae, each bear one large primary spine surrounded by a {538}circle of
secondaries. A few deep-water forms belong to this family, the type genus
_Salenia_ (Fig. 240) being the best known. None occur in the British area.
Superficially they resemble the Cidaridae, but in reality they are widely
separated by the essentially modern character of the peristome.
[Illustration: FIG. 240.—Dried and cleaned shell of _Salenia varispina_,
showing periproct covered by one large plate. × 4. (From Wyville Thomson.)]
FAM. 4. ARBACIIDAE.—Endocyclica with a peristome on which, as in
Saleniidae, there are only ten prominent plates perforated by the buccal
tube-feet, and besides these thin irregular plates; external gills are
present, and the auricles consist of incomplete arches springing from the
ambulacral plates. The periproct is covered by four valve-like plates. The
ambulacral pore-plates are separate near the periproct, but near the
peristome unite on the "Arbacioid" pattern (_v._ p. 531) to form secondary
plates. The interambulacral plates each carry several spines. No
representatives of this remarkable family are known in British waters, but
_Arbacia_ is found both on the east coast of North America and in the
Mediterranean. It is distinguished by its conical test. All the upper
tube-feet are devoid of a sucker; only those on the oral surface are used
for locomotion.
Uexküll has studied the Mediterranean species, and has shown that the
spines converge no matter how strong the stimulus may be, and so are
incapable of aiding in locomotion; also that the ectoderm is devoid of
ciliation, and hence the faecal matter which falls on the surface of the
animal is not, as in other genera, allowed to fall off by the divergence of
the spines nor swept off by the action of the cilia. In its natural habitat
the wash of the ripples on the shore cleanses the animal. In captivity it
is liable to suffocate itself.
FAM. 5. DIADEMATIDAE.—Endocyclica with a peristome similar to that of the
Arbaciidae and the Saleniidae. External gills present and ten buccal
tube-feet. Periproct small, covered with numerous small plates. The
auricles form complete arches arising from the ambulacral region.
Aristotle's lantern is provided with rudimentary Stewart's organs. The
ambulacral pore-plates are separated at the apex, but unite orally in
{539}"Diadematoid" fashion (p. 531) to form compound plates. The
interambulacral plates bear numerous primaries. The aboral tube-feet are
pointed, having lost their suckers.
This family is represented (according to Agassiz) at the present day by
seven genera, none of which are found in British waters, though one
(_Centrostephanus_) enters the Mediterranean. _C. longispinus_[497] was
investigated by Uexküll and found to be distinguished by its sensitiveness
to light and shade, and by the quickness of its movements, which were
mainly carried out by its long spines. The family resembles the Arbaciidae
in the pointed aboral tube-feet, but in the complete auriculae it resembles
the next family.
FAM. 6. ECHINIDAE.—Endocyclica with peristome and periproct as in the
preceding family. External gills and buccal tube-feet present, but
Stewart's organs totally absent. Ambulacral plates combined on the
"Triplechinoid" plan (p. 531) to form secondary plates. Interambulacral
plates with numerous tubercles. All the tube-feet have suckers.
This family contains by far the larger number of living genera. It is
divided into two sub-families, viz.:—
(_a_) TEMNOPLEURINAE.—Echinidae in which the plates of the corona dovetail
into each other by means of pits and knobs along the line of suture. This
sub-family does not occur in British waters; almost all the species are
confined to the Indian and Pacific Oceans, but on the east coast of America
it is represented by several genera, which however inhabit deep water,
_e.g._ _Trigonocidaris arbacina_.
(_b_) ECHININAE.—Echinidae in which the plates meet each other in straight,
simple sutures.
This sub-family is represented in British waters by three genera, viz.
_Echinus_, _Sphaerechinus_, and _Strongylocentrotus_. _Echinus_ is
distinguished by having its pores arranged in arcs of three, owing to the
fact that its pore-plates are united in threes to form secondary plates,
whilst in the other two genera the ambulacral plates are composed of four
or more pore-plates. Six species of _Echinus_ have been recorded from
British waters, viz. _E. esculentus_, _E. acutus_, _E. miliaris_, _E.
norvegicus_, _E. microstoma_, and _E. elegans_. The validity of the last
three is very doubtful. {540}Mortensen[498] regards _E. norvegicus_ and _E.
microstoma_ as mere variations of _E. acutus_, and this is probably
correct. _E. esculentus_ has already been described; its most marked
character is the forest of comparatively short, close-packed, reddish or
white primary spines with which it is covered, between the bases of which
the delicate secondaries are hard to detect. It is essentially a
shallow-water species. _E. acutus_ is distinguished by having much fewer
and longer primaries and numerous delicate secondaries. It is an inhabitant
of deeper water, being abundant at 100 fathoms, though stragglers are found
in shallower water. At the depths at which it lives wave-disturbance can
scarcely be felt, and hence the long primaries are not irritated.
_E. elegans_ has spines intermediate in character between those of _E.
esculentus_ and those of _E. acutus_. Like the latter it is an inhabitant
of the deeper water. It seems to the present author not at all improbable
that further research might show that _E. acutus_, _E. elegans_, and _E.
esculentus_ are all members of continuous series of forms; certainly the
larvae and early development of _E. acutus_ and _E. esculentus_, the
extreme members of the series, are strikingly similar.
_E. miliaris_ differs somewhat widely from the other species and is closely
allied to _E. microtuberculatus_ of the Mediterranean, from which it is
distinguished mainly by the greater thickness of the scattered plates on
the peristome of the latter species. From the other British species it
differs in its much smaller size and in the greenish hue of its primary
spines, which are short and thick and possess purple tips. Its larva is
markedly distinct from the larva of _E. esculentus_. _E. miliaris_ is a
littoral species, and is found in great numbers in some of the Scottish
sea-lochs; when the tide recedes, under every stone of the gravelly beach
several specimens will be found. It has a curious habit of "dressing"
itself, _i.e._ of covering itself with fragments of dead shell, sea-weed,
etc., which are held in position by the aboral tube-feet. This habit aids
in concealing the animal, and has probably been developed on account of the
dangers to which _E. miliaris_ is exposed owing to its littoral habit of
life.
_Sphaerechinus_ differs from _Echinus_ in the structure of the ambulacral
plates, in which it agrees with _Strongylocentrotus_, but it is
distinguished from this genus by the very deep {541}gill-clefts, or
indentations of the edge of the corona from which the gills are extruded.
Its most marked peculiarity, however, as shown by both Mortensen and
Uexküll, consists in the highly developed character of its gemmiform
pedicellariae, on the stalks of which are situated glands. When the head
with its poison-glands is torn off, the secretion of these stalk-glands can
envelop an enemy with a glutinous secretion, which impedes its movements.
The blades on a slight mechanical stimulus divaricate very widely and
become locked in this position, so that the enemy's body gets in well
within their reach. The muscles of the poison-glands contract, but their
ducts are bent by the act of opening, so that the secretion cannot escape.
The sense-organs have stiff hairs, which penetrate the surface of the enemy
and cause its juices to exude and so stimulate the blades to close, and at
the same time permit the poison to be expelled. It will be remembered that
the gemmiform pedicellariae of _Echinus_ open in response to a chemical
stimulus and close on a mechanical one being superadded; so that their
responses are the direct opposite of what occurs in _Sphaerechinus_. _S.
granularis_, a Mediterranean species with short red spines, just reaches
the Channel Islands.
_Strongylocentrotus_ has shallow gill-clefts and gemmiform pedicellariae,
like those of _Echinus_, except that they have a muscular stalk. In the
British area it is represented by two species, _S. lividus_, in which the
primary spines are markedly longer than the secondaries and are of a
brownish purple colour, and _S. droëbachiensis_, in which the primaries are
little longer than the secondaries and are of a greenish brown colour. _S.
lividus_ occurs abundantly in the Mediterranean, and reaches the English
Channel and the west coast of Ireland. In the last-named locality, where it
is exposed to the full sweep of the Atlantic, it is said to excavate holes
for itself in the limestone rocks, about ten inches in depth.[499] _S.
droëbachiensis_, which has been recorded in the British area, chiefly from
the west coast of Scotland, is one of the most abundant members of the
fauna of the east coast of America. In the Gulf of St. Lawrence and in the
branches of the Bay of Fundy it is found in thousands, and {542}is
frequently left bare at low tide. It thus takes the place of _E. miliaris_
in the British fauna. An allied if not identical species, _S. purpuratus_,
is found in Puget Sound on the Pacific coast.
Other interesting genera of the Echininae are _Echinometra_,
_Colobocentrotus_, and _Heterocentrotus_. All possess large, thick
primaries, and all are elliptical in outline. In _Echinometra_ the
primaries are pointed, and the long axis of the body makes an oblique angle
with the axis passing through mouth and madreporite. In _Colobocentrotus_
and _Heterocentrotus_ the axis passing through mouth and madreporite is the
short axis of the ellipse, and the primary spines are very thick and
triangular in section, whilst the expanded ends of the secondaries form a
closely set armour between the bases of these. In _Colobocentrotus_ the
test is markedly flattened on the under side, and this flattened area is
fringed with a circle of primaries; but in _Heterocentrotus_ there are a
few rows of primaries all over the test. These are tropical genera and are
found on the outer side of coral reefs, and they require the cuirass of
expanded secondaries to protect them against the waves.
ORDER II. CLYPEASTROIDEA (_Cake-urchins_).
The "Cake-urchins" have only one representative in the British area, and
this is unsuitable for dissection on account of its small size. We shall
therefore select as type the "Sand-dollar" _Echinarachnius parma_ (Figs.
241, 242), which occurs abundantly in shallow water on the east coast of
North America. As its popular name implies, this is an extremely flattened
Sea-urchin of nearly circular outline, so as to suggest a resemblance to
the silver dollar of North American currency. The peristome is exceedingly
small, and is placed in the centre of the lower surface (Fig. 241), whilst
the periproct is placed on one edge. The outline is not quite circular, for
the periproct lies in a slight indentation of the edge; and this side is
broader and of a lesser degree of curvature than the opposite one, so that
a secondary bilateral symmetry is superimposed on the fundamental radial
symmetry common to all Echinoderms. A line drawn so as to pass through the
anus and the centre of the disc will divide the animal into two similar
halves; the periproct of course lies in {543}an interradius and the axis of
symmetry passes through the centre of one radius. We can thus distinguish
an anterior group of three radii, or "trivium," from a posterior pair or
"bivium." The madreporite lies in the left anterior interradius. The five
genitals and five oculars surround a dorso-central plate, which covers the
spot which in Endocyclica is occupied by the periproct.
[Illustration: FIG. 241.—Oral view of "Sand-dollar" (_Echinarachnius
parma_), with spines. _amb_, Ambulacral furrow, × 1.]
The whole test is covered with extremely short delicate spines, which form
a velvety felt-work, and are all of approximately the same length; they are
of a brownish purple colour. The spines on the dorsal surface are all
ciliated, and these cilia cause a current of fresh sea-water to flow
continually over the modified tube-feet. Pedicellariae are scattered
amongst the bases of the {544}spines; they are of the tridactyle, the
gemmiform, and the ophicephalous types, but they have only two jaws.
[Illustration: FIG. 242.—Aboral view of the "Sand-dollar" (_Echinarachnius
parma_), with its spines. _m.p_, Madreporite; _pod_, small tube-foot with
sucker; _pod'_, flattened respiratory tube-foot. × 1.]
The ambulacral areas on the upper surface of the test can be distinguished
only by the flattened respiratory tube-feet (Fig. 242, _pod'_), which can
be seen protruding from between the spines. Below these areas are clearly
marked, for in the centre of each is a well-marked groove proceeding
inwards to the peristome. This groove receives lateral branches on its
course which traverse the adjacent interambulacral regions. The purpose of
these grooves will be explained later. The interambulacral regions do not
reach the peristome, which is entirely surrounded by the ambulacral areas.
The ambulacral and interambulacral areas both consist of somewhat large
hexagonal plates, except in the region of the respiratory tube-feet. Here
the pore-plates are not united with one another. This region in each radius
is termed {545}a "petal" (Fig. 243, A, _p_), for the respiratory tube-feet
are arranged in two rows which diverge from their commencement at the
"calyx" and slightly converge again towards the outer margin of the disc,
and thus in a dried specimen the two rows of double pores outline an area
having some resemblance in shape to the petal of a flower. Besides these
double pores for the larger tube-feet there are numerous small single pores
for the smaller tube-feet; these are found in all the plates, ambulacral
and interambulacral, of the dorsal surface, but in the neighbourhood of the
grooves only on the ventral side.
[Illustration: FIG. 243.—A, aboral, B, oral view of _Echinarachnius parma_
after spines have been removed, _amb_ (in A), Ambulacral plates, (in B),
ambulacral furrows; _g.p_, genital pore; _inter_, interambulacral plate;
_p_, petal; _t.t_, pore for terminal tentacle, × ½.]
The sphaeridia are only present to the number of one in each radius. Each
sphaeridium is enclosed in a pit situated near the edge of the peristome.
A remarkable feature in the skeleton of _Echinarachnius_ which is
characteristic in greater or lesser degree of all Clypeastroidea is the
presence of vertical partitions of calcareous matter traversing the coelom
and stretching from the upper to the lower surface of the test. These are
found principally in the peripheral region of the animal; and there can be
no doubt that they have originated as cellular bands traversing the coelom,
for the formation of similar structures can be followed step by step in the
Crinoidea. In the axis of these trabeculae, or folds of the coelomic wall,
jelly is secreted, and into this the lime-producing {546}amoebocytes
wander. In _Echinarachnius_ these partitions are arranged in groups, each
group radiating from a common centre.
The main peculiarities in the structure of _Echinarachnius_ are
comprehensible when the species is viewed from above in its normal
environment. It is found in comparatively shallow water on a sandy bottom,
and normally is nearly but not quite buried in the sand. It might thus be
overturned by the force of the waves and currents, and it is protected
against this fate by its flattened shape. This shape, however, necessitates
some kind of support for the upper part of the test, and this is provided
by the internal partitions.
In order to view the internal anatomy of the "Sand-dollar," it is necessary
carefully to pick away the dorsal surface of the shell piece by piece. In
this way the whole course of the alimentary canal is exposed; as in
_Echinus esculentus_ it can be seen to issue from the upper surface of
Aristotle's lantern. It then bends sharply to the left, and makes a
complete circle round the edge of the disc; this portion is the stomach,
and is considerably inflated and accompanied by a "siphon." It then bends
sharply back on itself, but only goes half way round; when it reaches the
posterior interradius it ends in the anus (Fig. 244).
Aristotle's lantern is greatly simplified as compared with its condition in
the Regular Urchins. Both rotulae and compasses are absent; the jaws are
sharply bent on themselves, and their appearance gives one the impression
that they have shared in the process of compression which the test as a
whole has undergone, and have thus become bent. The teeth are nearly
horizontal, and they actually articulate with the auriculae, which, as in
Cidaridae, consist of disconnected pillars and spring from the plates of
the interradius. Each pillar is fused with the adjacent one belonging to
the next radius, so that the system which in _Echinus_ consists of five
radial arches here consists of five interradial pillars. Aristotle's
lantern has lost its respiratory function and apparently its masticatory
function as well, for the teeth are used as spades to shovel into the mouth
the sand mixed with organic detritus and small organisms on which the
animal lives.
The water-vascular system is highly modified. There are two sharply marked
kinds of tube-feet—(_a_) the respiratory tube-feet, (_b_) the locomotor
tube-feet. Both kinds are terminated by {547}suckers, but the first variety
are much larger than the second; they possess a flattened lobed base, and
are connected with the ampulla by a double canal. They issue only from the
double pores which form the petal. The locomotor tube-feet are small and
cylindrical; they are, as already mentioned, scattered over the whole upper
surface of the test, penetrating both ambulacral and interambulacral
plates, but all are connected by transverse canals with the radial canals
of the water-vascular system. On the under surface they are confined to the
neighbourhood of the ambulacral grooves, which have nothing to do with the
ambulacral grooves of an Asteroid, but are due to secondary localisations
of the tube-feet, which are here also connected in each radius with a
single radial canal. The appearance of a living _Echinarachnius_ covered
with a veritable forest of short brown tube-feet is very striking.[500]
[Illustration: FIG. 244.—Dissection of _Echinarachnius parma_. × 1. The
oesophagus has been cut through and moved to one side so as to expose
Aristotle's lantern. The aboral part of the test has been removed. _gon_,
Genital organ; _int_, intestine; _musc_, transverse muscle connecting jaws
of adjacent interradii; _rect_, rectum; _siph_, siphon; _st_, stomach.]
The condition of the water-vascular system is to be explained entirely by
the peculiar environment of the animal. The demand for specialised
respiratory organs is brought about by the habit of living half buried in
the sand. Under these circumstances the strain of supplying the needful
oxygen is thrown on the {548}dorsal tube-feet, and they become modified in
order to fit them for this function. The locomotor tube-feet are very small
and feeble compared with those of _Echinus esculentus_, but this is
comprehensible when it is recollected how little resistance the yielding
sand would offer to the pull of a powerful tube-foot like that of the
Regular Urchins, for in order to move the creature through the sand a
multitude of feeble pulls distributed all over its surface is necessary,
and the locomotor tube-feet are exactly fitted, both as to size and number,
for this object.
The principal points in which Clypeastroidea vary amongst themselves are
(1) the nature of the internal skeleton, (2) the shape, and (3) the spines.
INTERNAL SKELETON.—In _Echinocyamus_ and its allies this consists in each
interradius of two simple partitions radiating out towards the edge of the
disc; in _Laganum_ it consists of walls parallel to the edge of the disc;
in _Clypeaster_, of isolated pillars.
SHAPE.—In _Echinocyamus_ the outline is oval and the test comparatively
high. In _Clypeaster_ and its allies the outline is pentagonal, and the
test is swollen up into a blunt elevation in the centre. In a large number
of genera, however, the test is, as in _Echinarachnius_, extremely thin and
flat, and the outline may be variously indented. A first indication of this
process is seen in _Echinarachnius_ itself, but in _Rotula_ the edge is
drawn out into finger-like processes which are all interradial. In
_Mellita_ these processes unite with one another distally so as to surround
spaces called "lunules," which appear as perforations of the test.
The CLASSIFICATION of the Clypeastroidea adopted by Agassiz is based
chiefly on the degree of development of the internal skeleton, and as this
is of great physiological importance to the animals we shall follow it
here; but since it was published the remarkable discovery has been made of
_Pygastrides_, a type previously known only from fossils. We must therefore
recognise two sub-orders:—
SUB-ORDER I. PROTOCLYPEASTROIDEA.
Anus on dorsal surface near apical pole. One species, _Pygastrides
relictus_,[501] with no "petals," from deep water in the Caribbean Sea.
{549}SUB-ORDER II. EUCLYPEASTROIDEA.
Anus on under surface.
FAM. 1. FIBULARIDAE.—The "petals" are short and imperfect, and the internal
skeleton consists of two short outwardly-directed septa in each
interradius. To this family the only British Clypeastroid, _Echinocyamus
pusillus_, belongs. This animal never exceeds an inch in length, and has an
oval outline. It inhabits shallow water, and is often found in the same
ground as _Echinus miliaris_, but like all Clypeastroids it prefers a sandy
bottom.
FAM. 2. ECHINANTHIDAE or CLYPEASTRIDAE.—"Petals" well marked, internal
skeleton consisting of isolated pillars. The largest Cake-urchins belong to
this family, which is found chiefly in tropical waters. _Clypeaster_, the
great Cake-urchin, with a deeply sunken peristome, belongs to this family.
FAM. 3. LAGANIDAE.—Closely allied to the foregoing, but distinguished by
the fact that the internal skeleton consists of walls parallel to the edge
of the test. (_Laganum_, _Arachnoides_, _Peronella_.)
FAM. 4. SCUTELLIDAE.—This family includes about half the genera, and is
sharply distinguished from all the rest by (1) the extremely flattened
shape, (2) the indentation of the outline in the anal interradius and often
elsewhere, (3) the branching of ambulacral furrows on the under surface.
_Echinarachnius_, taken as the type in describing the anatomy of the
Cake-urchins, is the best-known genus. Others are _Mellita_, with five
perforations in the edge of the test; and _Rotula_, with the edge produced
into a number of finger-like processes.
ORDER III. SPATANGOIDEA (_Heart-urchins_).
As the type we may select _Echinocardium cordatum_, which occurs abundantly
in the Clyde and on the west coast of Ireland. The animal is found buried
in sand at a depth of about 8-10 inches from the surface. At this depth it
lies in a burrow, the walls of which are kept from collapsing by the
somewhat broadened tips to the spines. This burrow communicates with the
surface by a narrow cylindrical opening similar to the opening of the
burrows made by the Clams and other bivalves. A little practice, however,
enables one to distinguish the burrow of the Heart-urchin from these.
{550}The animal is about the size of a small potato, and is of light straw
colour. Its outline is oval, and the test is about two-thirds as high as
the shorter diameter. It is thus higher in proportion to its width than is
the case with any living Cake-urchin. The highest point is behind the
centre. The narrower end of the animal terminates in a vertical edge, in
the upper part of which is a large periproct covered with a number of thin
movable plates. The mouth is situated on the under surface, considerably
nearer the front end of the test than the hinder end. It is entirely devoid
of jaws or of teeth, and also of gills or of a movable peristome.
Aristotle's lantern has entirely disappeared, leaving as the only trace of
its former presence a canal with membranous walls encircling the mouth,
which has the form of a transverse slit, the posterior lip projecting
considerably forward.
The ambulacral areas are easily distinguishable from the interambulacral
areas by being comparatively bare of spines. On the upper surface they are
distinctly grooved, the groove being especially deep in the case of the
anterior one. On the lower surface they coalesce round the mouth, shutting
out the interambulacral regions, and are here perforated by the large pores
of the buccal tube-feet. Between the two posterior radii on the oral
surface there is a space with specially arranged spines called the plastron
or sternum. The interambulacral plates composing this region are very much
lengthened, and interdigitate with one another at the sutures. To this
lengthening is due the apparent forward shift of the mouth. The spines are
very characteristic, and are very different from any which have as yet been
described. They are the sole organs of locomotion. The primaries are long
and curved, with flattened tips, admirably adapted to plough through the
sand in which the animal lives. On the upper surface, mingled with the
tube-feet, are a large number of small secondary spines. Between the two
posterior petals there is a hoop-shaped band of very small black spines.
These spines are ciliated, and draw a current of fresh sea-water over the
respiratory tube-feet. Beneath the periproct there is a similar band called
the "sub-anal fasciole"; this probably produces a current of water which
sweeps away the material ejected from the anus.
The pedicellariae are of the trifoliate and gemmiform varieties. {551}The
sphaeridia are situated in open pits, one or two in each, situated at the
bases of the tube-feet nearest the mouth.
When the upper part of the test is picked away, the course of the
alimentary canal is exposed (Fig. 247). It is very similar to the
alimentary canal of _Echinarachnius_, except that from the first coil a
large blind pouch, called the caecum, is given off.
The water-vascular system shows many characteristic features. The tube-feet
are confined to two rows in each ambulacrum, the scattered smaller feet
found in such abundance in _Echinarachnius_ being entirely absent. There
are four distinct varieties of tube-feet in _Echinocardium_, which are as
follows:—(_a_) The respiratory tube-feet of the petals. These have, as in
_Echinarachnius_, broad flat bases, but they have lost the sucker. (_b_)
The prehensile tube-feet of the anterior ambulacrum. These are enormously
long structures, measuring when expanded several times the length of the
body. They end in discs, which are frayed out into fingers, so as to look
like miniature sea-anemones. These tube-feet are comparatively few in
number and are confined to the apical portion of the anterior ambulacrum.
(_c_) The buccal tube-feet. These are short, thick, and pointed, and
covered with a multitude of club-shaped processes. They are found on all
the ambulacra in the neighbourhood of the mouth. (_d_) The degenerate
tube-feet found in the portions of the ambulacra between the "floscelle"
(see p. 553) and the petals. These are single and pointed, few in number,
and issue from single pores in the test.
[Illustration: FIG. 245.—_Echinocardium cordatum_. A, aboral view; B, oral
view, × 1.]
This extraordinary diversity in the tube-feet is fully explained {552}when
the habits of the animal are known. The function of the respiratory
tube-feet requires, of course, no special elucidation, but the peculiar
anterior ambulacrum was a mystery till the feeding habits of the animal
were observed by the late Dr. Robertson[502] of Cumbrae. He found that the
animal protruded the long prehensile tube-feet through the opening of the
burrow up to the surface of the sand. With their finger-like processes they
then collected the surface film of the sand, which was impregnated with
Diatoms and other small organisms. When a "handful," so to speak, of this
nutritive material has been collected, the long tube-foot is withdrawn down
the burrow and passed over the deeply grooved part of the ambulacrum to the
buccal tube-feet, to which the food is given up. These last then push it
into the mouth. Only one prehensile tube-foot is extended at a time.
[Illustration: FIG. 246.—Interior of test of _Hemiaster philippi_, showing
the genital organs and their ducts (only three are developed in this
species). Slightly enlarged. (From Wyville Thomson.)]
[Illustration: FIG. 247.—Dissection of _Echinocardium cordatum_. × 1. The
oral part of the test has been removed. _caec_, Blind pouch of the stomach;
_gon_, genital organ; _int_, intestine; _oe_, oesophagus; _rect_, rectum;
_siph_, siphon; _st_, stomach; _w.v.r_, water-vascular ring.]
The stone-canal is very short and soon opens into the axial sinus; it is
widely separated from the pore-canals which traverse the madreporite.
Communication between the two is effected by the long axial sinus. There
are only four genital organs.
Heart-urchins vary amongst themselves chiefly in the {553}following points,
viz.:—(1) the shape and position of the peristome, (2) the characters of
the "petals," and (3) the number and position of the fascioles.
PERISTOME.—In many genera this is pentagonal and central, and in these
cases the interradii commence at the peristome with a single plate, which
is often covered with a thick crowd of small spines and is termed a
"bourrelet." The oral ends of the radii also often consist of a crowded
series of narrow plates, looking something like a "petal," and termed a
"phyllode." The five bourrelets and five phyllodes constitute a flower-like
figure termed a "floscelle."
AMBULACRA.—In _Echinoneus_ all five are alike and are provided with similar
tube-feet, which are respiratory but possess suckers. The ambulacra are not
grooved, and the petaloid arrangement of the pores is hardly marked; but in
_Cassidulus_, _Pourtalesia_, and many other genera the five petals are well
marked, though they are all similar to one another.
FASCIOLES.—These structures are often entirely absent; the sub-anal one
alone is present in _Spatangus_. In _Eupatagus_ a peripetalous one is
added. This surrounds all the "petals," and has obviously the function of
sweeping fresh water over the respiratory tube-feet. In _Echinocardium_, as
we have seen, there is an "internal fasciole" between the two anterior
petals which has a similar function. In addition, this genus possesses an
anal fasciole which surrounds the anus and sweeps away the faeces.
[Illustration: FIG. 248.—Young _Echinoneus_ to show five equal radii
scarcely petaloid. _amb_, Ambulacral area; _g.p_, genital pore; _mad_,
madreporite. (After Agassiz.)]
The CLASSIFICATION of the _Spatangoidea_ is based mainly on the degree of
development of the petals, that is to say, on the extent to which the
burrowing habit has been developed. But weight is also laid on the shape of
the peristome, the pentagonal form being more primitive. Seven families are
recognised, which are as follows:—
FAM. 1. ECHINONIDAE.—"Petals" hardly marked at all; peristome in the centre
of the lower surface and pentagonal. Floscelle not developed.
One genus, _Echinoneus_ (Fig. 248).
{554}FAM. 2. NUCLEOLIDAE.—"Petals" distinct; peristome as in the foregoing
family. No floscelle. _Nucleolites_, with the anus in a furrow.
_Anochanus_, with a concave apical system serving as brood-pouch.
FAM. 3. CASSIDULIDAE.—"Petals" usually distinct; peristome eccentric, but
provided with a well-marked floscelle.
_Echinolampas_, with the anus on the under surface.
_Neolampas_, with the anus on a projecting papilla. One specimen of this
genus has been dredged in the British area.
The three foregoing families probably use their tube-feet to walk with, and
bury themselves only to a slight extent. They are often united as a
sub-order, the ASTERNATA, and distinguished from all the rest which possess
an eccentric mouth and well-marked plastron. These families are then
grouped together as STERNATA. They are as follows:—
FAM. 4. ANANCHYTIDAE.[503]—Spatangoidea with elongated apical system,
ambulacra all similar and not grooved. Petals feebly marked. _Pourtalesia_,
with bottle-shaped posterior prolongation of the test. _Platybrissus_, with
flattened test.
[Illustration: FIG. 249.—_Pourtalesia jeffreysi_, slightly enlarged. (From
Wyville Thomson.)]
FAM. 5. PALAEOSTOMATIDAE.—An aberrant family consisting of one genus,
_Palaeostoma_. Petals grooved, with a peripetalous fasciole, but peristome
central and pentagonal.
{555}[Illustration: FIG. 250.—_Hemiaster philippi_. × 2. (From Wyville
Thomson.)]
FAM. 6. SPATANGIDAE.—Spatangoidea of more or less flattened shape, with
well-marked petals and a sub-anal plastron as well as the ventral one. One
fascicle at least, but a peripetalous one never present. The anterior
ambulacrum grooved and different from the rest. This family is represented
in British waters by two genera, _Spatangus_ and _Echinocardium_. The
former possesses only a sub-anal fasciole, and has specially long curved
spines on the ventral plastron. It is represented by two species, _S.
purpureus_ and _S. raschi_, the latter being distinguished by a pointed
lower lip. It is a deep-water species, found in 100 fathoms and over on the
west coast. _S. purpureus_ is fairly common in rather shallow water. From
observations made on specimens kept in confinement it appears to burrow
only so far as to leave the petals uncovered; hence there is no need of a
peripetalous fasciole. _Echinocardium_ is devoid of the thicker spines on
the plastron, and has an internal fasciole and a perianal one as well as
the sub-anal. As already mentioned, it is a deep burrower. It is
represented by three species, _E. cordatum_, _E. pennatifidum_, and _E.
flavescens_. The first, described as the type of the Spatangoidea, has a
deeply grooved anterior ambulacrum. In the remaining two species this
ambulacrum is not grooved. _E. flavescens_ has only six or seven pairs of
pores in the posterior {556}petals, _E. pennatifidum_ twelve to fourteen.
Both come from deeper water than _E. cordatum_.
FAM. 7. BRISSIDAE.—Allied to the Spatangidae, but distinguished by sunken
petals and a peripetalous fasciole.
Two genera are recorded from the British area, _Schizaster_ and
_Brissopsis_, but the first has only been found once in deep water; the
second is common. _Schizaster_ has the front petals three times as long as
the hind ones, and no sub-anal fasciole. _Brissopsis_ has the front and
hind petals of about the same length, and a sub-anal fasciole. The only
British species is called _B. lyrifera_, on account of the fiddle-shaped
outline of the peripetalous fasciole.
_Hemiaster_ (Fig. 250) in general resembles _Schizaster_, but the petals
are equal in length, and the two posterior serve as brood-pouches for the
young. This genus is mainly Antarctic.
[Illustration: FIG. 251.—Dried shell of _Schizaster_, showing peripetalous
fasciole. _ant.amb_, Anterior ambulacrum; _fasc_, peripetalous fasciole;
_g.p_, genital pores, × 1. (After Agassiz.)]
FOSSIL ECHINOIDEA.—Echinoidea are well represented in the geological
record, and form a characteristic element in many fossil faunas. They
appear in the Ordovician formation, but the first representatives of an
existing family (Cidaridae) only appear in the Permian.
Space will only permit us to treat of the extinct members of the group very
briefly. Leaving out of sight the representatives of families still living,
the fossil Echinoidea may be divided into two great groups, viz.:—
(_a_) Palaeozoic forms, which in some points serve to connect the
Endocyclica with the primitive Asteroidea.
(_b_) Mesozoic forms, which serve to connect the Clypeastroidea and
Spatangoidea with the Endocyclica.
{557}[Illustration: FIG. 252.—A, _Bothriocidaris_. × 1. B, _Palaeoechinus_.
× 1. _amb_, Ambulacral plates; _inter_, interambulacral plates. (After
Zittel.)]
The Palaeozoic forms are often called Palaeoechinoidea, and they are above
all distinguished by the fact that the number of vertical bands of plates
composing the corona is variable, in a word, that the corona has not yet
acquired a fixed definite constitution. One genus (_Echinocystites_) has
the anus outside the apical system. It has four rows of pore-plates in each
radius, and numerous rows of plates each with a single spine in the
interradii. Another (_Palaeodiscus_) has been shown by Sollas[504] to be in
many respects the missing link between Asteroidea and Echinoidea. Inside
the plates of the corona there is a series of ambulacral plates like those
of Asteroidea. The tube-feet in the oral portion of the radii seem to have
issued between the (outer) ambulacral plates. No anus has been detected.
All the rest are Endocyclic. The oldest known form, _Bothriocidaris_ (Fig.
252, A), from the Ordovician, has only one row of interambulacral plates
and two of ambulacral; no peristome is distinguishable from the corona. The
ARCHAEOCIDARIDAE appear in the Devonian. They have narrow ambulacra of two
rows of pore-plates as in the Cidaridae, but the interambulacra consist of
many rows, the members of which overlap, and therefore were probably
slightly movable, as in the Echinothuriidae; the primary tubercles are
large, and there is only one on each plate. The MELONITIDAE (Fig. 252, B)
appear in the Carboniferous. Each interambulacral plate, of which there may
be five rows in each interradius, bears numerous small tubercles, and there
may be four or more vertical rows of pore-plates, though in the genus
figured, _Palaeoechinus_, there are only two. The TIARECHINIDAE are
represented by one genus, _Tiarechinus_, with an enormous apical system,
from the Triassic of the Tyrol. The interambulacra consist of one plate
bordering the mouth, three, {558}side by side, forming the interradial area
of the corona, and one large genital plate; the ambulacra, of two rows of
pore-plates. This family consists of dwarfed forms which probably inhabited
the land-locked seas and salt lagoons of the Triassic epoch.
When we recollect that some of the oldest Asteroidea known to us had very
narrow arms and interradial areas edged by large square marginals, it does
not require a very great effort to imagine how these marginals could be
converted into the vertical rows of the interambulacra, and the pointed
narrow arms becoming recurved, could have formed the ambulacra. The
physiological advantage of this will be discussed in the chapter on
development.
True Cidaridae occur in the Permian, and are abundant in all the younger
formations. One Cretaceous genus, _Tetracidaris_, has four rows of
interambulacral plates near the mouth, diminishing to two at the apex. This
circumstance renders it probable that the Cidaridae are the direct
descendants of the Archaeocidaridae. The Saleniidae, Echinothuriidae, and
Diadematidae appear in the Jurassic, the Echinidae in the Cretaceous, and
the Arbaciidae only in the Tertiary epoch.
[Illustration: FIG. 253.—_Hyboclypus gibberulus_. × 1. _a_, Anus. (After
Zittel.)]
Turning now to the Mesozoic forms with an excentric anus, there were a
number of forms which have been grouped together as HOLECTYPOIDEA which had
auricles and teeth and gills, although these were only feebly developed,
and in which the pore-plates remained separate. The periproct was a
comparatively large area, and in _Pygaster_, as in the surviving form
_Pygastrides_, it was in contact with the apical system, although outside
it. Many of the genera were of considerable height in proportion to their
length. In _Conoclypeus_ and _Discoidea_ the jaws and auricles were very
weak. The ECHINOCONIDAE have only vestigial auricles, and on this account
are often definitely grouped with the Spatangoidea, but they are closely
allied to the Holectypoidea. They are Cretaceous forms of high conical
shape (_Galerites_). In _Hyboclypus_ (Fig. 253) all trace of the teeth has
disappeared, {559}but the periproct is large and in contact with the apical
system; these forms appeared in the Jurassic. The COLLYRITIDAE, also
Jurassic, had a marginal anus. The apical system was so much elongated that
two of the ocular plates are widely separated from the other three, two
opposite interambulacra meeting between them. Unmistakable Spatangoidea
(Spatangidae and Ananchytidae) appear in the Cretaceous, true
Clypeastroidea (_Fibularites_) in the Cretaceous, the other families in the
Tertiary.
Reviewing these facts, we see that from the Holectypoidea we can pass by
insensible steps on the one hand into true Clypeastroidea, and on the other
hand into true Spatangoidea. The Holectypoidea differed from Endocyclica
only in the position of the anus, and the initial step in the backward
shift of this organ is seen in _Pygaster_. One result follows from this
conclusion, that the modification of the dorsal tube-feet into breathing
organs, and the consequent appearance of petals which accompany the taking
on of burrowing habits, were independently developed in the Clypeastroidea
and Spatangoidea, since these features were absent in the more primitive
members of both groups.
{560}CHAPTER XIX
ECHINODERMATA (_CONTINUED_): HOLOTHUROIDEA = SEA-CUCUMBERS
CLASS IV. HOLOTHUROIDEA
This class of the Eleutherozoa comprises those sausage-shaped, leathery
Echinodermata familiarly known as Sea-cucumbers. They are named
Holothuroidea from ὁλοθούριον, an animal described by Aristotle, and
believed to belong to this class.
The Holothuroidea resemble Echinoidea in the fact that the radial canals of
the water-vascular system run backwards and upwards from the ring-canal
over the surface of the body, terminating in small papillae near the anus,
which, as in the Echinoidea Endocyclica, is situated at the upper pole of
the body. There are, of course, no arms; and a further resemblance to
Echinoidea is shown by the fact that the ambulacral grooves are represented
by closed epineural canals, and that the ectoderm consists of long,
slender, flagellated cells interspersed with gland-cells, underneath which
is a plexus consisting of nerve-fibres and small bi-polar ganglion cells.
There are, however, no spines or pedicellariae; and Holothuroidea differ
not only from Echinoidea, but from all other Echinodermata, in the
vestigial character of their skeleton, which consists merely of isolated
nodules of calcium carbonate embedded in the skin. The body-wall is
provided with transverse muscles running across the interradii, and also
with powerful longitudinal muscles, running along the radii, by means of
which worm-like contractions are carried out. Similar muscles, though much
less developed, occur in the Echinothuriidae, and must have been present in
many extinct Echinoidea in which the plates of the corona overlapped; and
hence it is exceedingly probable that from some of these {561}early forms,
as, for instance, _Bothriocidaris_, Holothuroidea may have been evolved.
The MUSCULAR BODY-WALL has indeed been as important a factor in the
evolution and differentiation of the Holothuroidea as the MUSCULAR ARM in
that of Ophiuroidea, or the MOVABLE SPINE in the case of Echinoidea.
[Illustration: FIG. 254.—_Holothuria nigra_. _t_, Buccal tentacle or
"feeler." × ½.]
There are about 520 species of living Holothuroidea, and of these about
twenty-one have been recorded from British waters. One of the best-known of
the British species is _Holothuria nigra_ (Fig. 254), commonly known as the
"Cotton-spinner"; and this we shall take as a type for special description.
The animal may attain a length of a foot when fully extended, and has a
diameter of from 3 to 4 inches. It is of a very dark brown colour on one
side, which in crawling it keeps uppermost, whilst on the lower side it is
of a tawny yellow hue. Three of the radii (often termed the "trivium") are
situated on the lower surface; two (termed the "bivium") on the upper
surface. The podia are scattered fairly evenly over the whole surface
without reference to the radii; below they are regular tube-feet provided
with suckers, whilst on the upper surface they are pointed tentacles,
employed only for sensory purposes.
If the animal be observed alive and in its natural surroundings, a ring of
twenty large tentacles can be seen surrounding the mouth. These buccal
tentacles are in every respect comparable with the buccal tube-feet of
Ophiuroidea and Spatangoidea, and, like them, are employed in shovelling
the muddy substratum on which the animal lies into the mouth.
Ludwig employs the term "feeler" for these buccal tentacles, {562}in order
to distinguish them from the pointed podia scattered over the bivium. This
procedure will be adopted here. In the Cotton-spinner the feelers, when
extended, show a short smooth stem, from the apex of which springs a circle
of short branches, which are in turn beset with a double row of branchlets,
themselves branched. Such feelers are said to be shield-shaped.
A transverse section through the RADIUS of a Sea-cucumber is, in general,
like one through the radius of a Sea-urchin; the points of difference to be
noted are: (_a_) In the Sea-cucumber, beneath the ectoderm, is a thick
dermis with small plates scattered in it, instead of the whole dermis being
calcified, as is the case in the Sea-urchin; (_b_) the ampulla of each
podium is connected with the peripheral portion by one canal, not two, as
in the case of the Sea-urchin; (_c_) there is a development of coelomic
nervous tissue from the outer side of the perihaemal canal; (_d_) internal
to the radial water-vascular canal are to be seen cross-sections of two
great bands of longitudinal muscles, by the contraction of which the body
is shortened. Lengthening is brought about by the contraction of transverse
muscles, which are found on the inner side of the body-wall in each
interradius; the five sets taken together act like circular muscles, or a
rubber band, on the incompressible fluid in the body-cavity.
When the Sea-cucumber is opened by a cut along the left dorsal interradius,
the spacious coelom is laid open, and lying in it is seen the ALIMENTARY
CANAL. This tube is bent on itself, so that it has a form like ~ (Fig. 255,
B) running backwards to the posterior end of the body, then running
forwards to near the anterior end, before it finally turns to run backwards
to the anus. By taking cross-sections of the body at different levels, it
can be shown that the alimentary canal makes a half-turn round the
longitudinal axis (Fig. 255, A). It is suspended by bands of membrane,
termed "mesenteries," to the body-wall, and of these there are three, the
first of which (_i.e._ the one nearest the mouth) is attached to the
mid-dorsal interradius (Fig. 255, A, M^1), the next to the left dorsal
interradius (M^2), and the last to the right ventral interradius (M^3).
The alimentary tube shows four regions, which are distinguished as
follows:—(1) A short oesophagus with strongly-marked longitudinal folds in
its walls; this is separated by a constriction from (2) the stomach, a very
short region, {563}characterised by its strong musculature. Next follows
(3) the intestine, a thin-walled tube comprising the middle limb and most
of the descending and ascending limbs. This finally passes into (4) the
wide terminal "rectum," or "cloaca," which is connected to the body-wall by
muscular bands which traverse the coelom (Fig. 256, 10).
The cells lining the oesophagus resemble ectodermal cells; those lining the
stomach are nearly all gland-cells, and obviously secrete the digestive
juice. The powerful muscles of this portion of the gut produce a strong
peristalsis which thoroughly mixes the juice with the food, and in the
thin-walled intestine absorption of the digested material takes place. The
extreme thinness of the intestinal wall is common to many animals (e.g.
_Sipunculus_, Vol. II. p. 412) which swallow mud and sand for the sake of
the organic matter which they contain.
[Illustration: FIG. 255.—A, diagrammatic cross-section of a Holothurian; B,
diagrammatic longitudinal section. I-V, radii; _Int.1-Int.3_, the three
limbs of the alimentary canal; _M^1-M^3_, the three mesenteries attaching
the same to the body-wall. (After Ludwig.)]
The rectum, or cloaca, is one of the most characteristic features in this
and most other Sea-cucumbers. In addition to the passing of faeces, it is
used to pump water in and out, and it thus serves as a breathing organ.
This pumping is effected by alternate contractions of the radiating muscles
attaching the cloaca to the body-wall, and of the circular muscles which
immediately surround it. Two long branched tubes termed RESPIRATORY TREES
(Fig. 256, 11) open into the cloaca, and into these the inspired water
penetrates. The finer branches of these gills end in rounded thin-walled
swellings termed "ampullae"; and when water is forced into these they
become tense, and a considerable quantity diffuses through their walls,
carrying {564}oxygen into the fluid which fills the coelom. If a
Sea-cucumber be left in a limited quantity of water, it will sometimes
direct the posterior end upwards until it reaches the surface of the
liquid, and will pump air into the trees. Besides the trees, other much
shorter tubes open into the cloaca, termed the CUVIERIAN ORGANS. These
tubes are really the modified basal branches of the trees. They are
unbranched, and their peritoneum consists of cells which secrete a slime
which swells up enormously on the addition of sea water. When the
Cotton-spinner is strongly irritated, it contracts all the muscles of the
body-wall, and these, acting on the incompressible fluid in the
body-cavity, transmit the pressure to the thin rectum, which tears, and
allows a portion of the viscera to be forced out. The first parts to be
rejected are the Cuvierian organs, and the cells covering these absorb
water, and their contained mucus splits up into a tangle of white threads,
in which an enemy may be completely ensnared. A large lobster has been seen
so enveloped with this "cotton" as to be completely incapable of motion.
The origin of the name "Cotton-spinner" requires no further elucidation.
Such self-mutilation, even when it involves not only the Cuvierian organs,
but the trees and the whole of the intestine, is not necessarily fatal. If
the animal be left alone, it can regenerate the whole of these organs.
The WATER-VASCULAR SYSTEM in its general features resembles that of the
Echinoidea. We notice as its first striking peculiarity the modification of
the stone-canal. This is often multiplied, as in the species (_H.
tubulosa_) represented in Fig. 256, where there are five; but whether there
is one or many, they do not reach the body-wall, but end each in a swelling
projecting into and bathed by the coelomic fluid. These swellings are
termed "internal madreporites." They are pierced by numerous fine ciliated
canals, which lead into a space from which the stone-canal takes its
origin. Both stone-canal and madreporite (especially the latter) are
stiffened by the deposition of carbonate of lime. In the young Holothurian
there is a single ciliated pore-canal opening to the exterior and leading
into a thin-walled axial sinus, which, as Bury[505] has shown, is later
converted into the internal madreporite; the pore-canal, which represents
the external madreporite of other Echinoderms, disappearing at the same
time.
{565}[Illustration: FIG. 256.—Dissection of _Holothuria tubulosa_. × ½. 1,
Feelers; 2, feeler-ampullae; 3, ring-canal; 4, Polian vesicle; 5,
stone-canals; 6, radial canal; 7, one of a pair of longitudinal muscles; 8,
genital tubes; 9, intestine; 10, radiating muscles of cloaca; 11, base of
respiratory tree; 12, ventral blood-vessel; 13, plexus of dorsal
blood-vessel. (After Ludwig.)]
{566}This extraordinary modification is the consequence of the habit of
forcing water into the respiratory trees. The body-cavity is by this means
kept tensely filled with fluid, and the stone-canal is enabled to draw on
it for the supply to the water-vascular system, thus rendering the external
madreporite supererogatory. A large-stalked sac—the Polian vesicle (Fig.
256, 4)—multiplied in many species, hangs down from the water-vascular ring
and serves as a reservoir of fluid.
All the podia, including the feelers, have ampullae. In the feelers a
semicircular valve is situated just where the external part passes into its
long ampulla. When this valve is expanded, the feeler is moved about by the
contraction of its muscles, but when it is contracted, the contents of the
feeler can flow back into the ampulla, so that the feeler is reduced to an
insignificant papilla (as in Fig. 254). The interior of the feeler is
ciliated, and a current seems to flow up one side and down the other, so
that this organ, like the dorsal tube-foot of a Cake-urchin or
Heart-urchin, seems to assist in respiration.
The NERVOUS SYSTEM differs from that of Echinoidea in the absence of the
pigment spot (or so-called eye) on the terminal podium of the radial
water-vascular canal. Each podium receives a so-called nerve—really an
extension of the radial nerve-cord with its ganglion-cells—and this ends in
a plate of sensory epithelium in the sucker of the tube-foot or tip of the
tentacle, or of each of its branches in the case of the feeler.
There is a coelomic nervous system developed from the radial perihaemal
canals. The PERIHAEMAL RING is represented in Echinoidea by the lantern
coelom, in Holothuroidea in all probability by the "buccal sinus," a space
intervening between the water-vascular ring and the oesophagus. In the
outer wall of this are developed ossicles, which constitute the CALCAREOUS
RING found in all[506] Sea-cucumbers (Fig. 257, A and B). In this ring
(Fig. 257, B) are to be distinguished radial and interradial pieces. The
former are notched at their upper ends, and in all probability represent
the auriculae of Echinoidea, as the radial nerve-cords pass out over the
notches, whilst the interradial pieces probably represent a coalesced pair
of jaws and their included tooth, since these ossicles develop from a
single rudiment in the larval Echinoid.
{567}The so-called BLOOD SYSTEM is in its main features similar to that of
Echinoidea. It consists of a blood-ring surrounding the oesophagus inside
the water-vascular ring, and sending branches along the stone-canal, and of
dorsal and ventral strands accompanying the gut in its course. These are
best marked in the region of the intestine, where absorption principally
takes place; in the wall of the stomach they are represented by a delicate
plexus which can hardly be traced into connexion with the blood-ring. The
dorsal "vessel" is situated in a fold of peritoneum projecting from the
intestinal wall; it gives off branches to the intestine, which unite on its
surface to form a plexus. In the middle limb of the intestine these
branches are grouped into tufts, and the fold of peritoneum between
successive tufts becomes absorbed; through the holes so formed branches of
the respiratory tree penetrate, so that the trees cannot be separated from
the intestine without tearing the dorsal vessel (Fig. 256, 13).
[Illustration: FIG. 257.—Types of calcareous rings and of ossicles. A,
calcareous ring of _Phyllophorus rugosus_, × 2; B, calcareous ring of
_Holothuria cinerascens_; C, ossicle of _Holothuria atra_; D, ossicle of
_Holothuria fusco-rubra_. _r_, Radial piece. (After Ludwig.)]
The GENITAL ORGANS consist of a single group of branched tubes situated on
the left side of the dorsal mesentery, which converge to open into a short
genital duct, which leads to a pore situated in the mid-dorsal line, a
short distance behind the feelers. From the common point of origin of the
tubes, the "genital base," as it is called, a worm-shaped genital
stolon[507] extends back along the genital duct towards the body-wall.
There is no genital rachis.
CLASSIFICATION OF HOLOTHUROIDEA.
The class is in many points of structure exceedingly variable, but many
striking variations in important organs occur in allied {568}species, and
even in the same species, and hence are probably not of physiological
importance. We shall therefore confine our attention mainly to those
differences in structure which are correlated with differences in habits,
and therefore of systematic importance. We shall consider in order (1) the
feelers; (2) the method of protecting these; (3) the rest of the
water-vascular system; (4) the gills; and (5) the skeleton.
FEELERS.—These organs have been made the basis of the division of the
Holothuroidea into orders, and as they are the means by which food is
obtained, and are thus of first-class physiological importance, this
procedure is fully justified. In three orders they have the shield-shaped
ends described in the case of _Holothuria nigra_, but in another large
order (Dendrochirota) they are much branched, and end in a mass of delicate
twigs. In another order (Synaptida) they are feather-shaped, with two rows
only of branches, whilst finally in Molpadiida they are simple
finger-shaped processes with one or two lateral branches. The number of the
feelers varies from ten to thirty.
In the Dendrochirota the entire anterior portion of the body can be
introverted into the interior, so that in this way the crown of feelers can
be effectively protected. The retractor muscles are modified portions of
the longitudinal muscles of the body-wall, which traverse the body-cavity,
and are inserted into the radial pieces of the calcareous ring. Similar
muscles are found in the genus _Molpadia_ and in many of the Synaptida. In
Aspidochirota and _Pelagothuria_ they are totally wanting, and here the
feelers possess long ampullae which allow of the tentacles being
individually contracted to very small dimensions. These ampullae seem to be
present in nearly all cases in Molpadiida, and in Synaptida, although in
the last-named order they are very feebly developed, and must be looked on
as vestigial. In Dendrochirota, owing to the strongly developed retractors,
they would be useless, and so are absent.
WATER-VASCULAR SYSTEM.—In Synaptida the radial canals are totally absent in
the adult, and the only podia are the feelers, which spring directly from
the ring-canal. The radial canals are present in _Pelagothuria_, but the
feelers are still the only podia; in the Molpadiida there are only five
small terminal tentacles round the anus in addition to the feelers. In the
Elasipoda all the podia have pointed ends, but the dorsal podia {569}are
few, long, and stiff, and often coalescent in places to form grotesque or
remarkable appendages. In the remaining forms the podia of the trivium have
always suckers, whilst those of the bivium may or may not be pointed. In
_Psolus_ the two dorsal radial canals and their podia are totally absent.
RESPIRATORY TREES.—These are present in Aspidochirota, Dendrochirota, and
Molpadiida, totally absent in the Synaptida and _Pelagothuria_, and
doubtfully represented in a few Elasipoda by a single unbranched outgrowth
of the gut.
SKELETON.—This consists, as explained above, of the scattered deposits in
the skin and of the calcareous ring. As regards the first, their shape
varies immensely, and yet one or two principal types characteristic of each
of the main divisions can be defined. Thus the Synaptida are characterised
by wheels, with spokes ending in a hub, and by anchors attached to a plate.
The Elasipoda have simple St. Andrew's crosses, whilst the Aspidochirota
are mainly characterised by "stools" (Fig. 257, C) and buckles (Fig. 257,
D). The Dendrochirota have a bewildering variety of forms; the most
characteristic, however, are a right-angled cross and a grating, very
similar to the buckles of the Aspidochirota, except that in the former
there are usually four holes placed cross-wise, whilst the buckle has
generally two parallel rows of three holes. Since these ossicles are the
only records we possess of the existence of fossil Holothuroidea, they have
been studied with great care. The calcareous ring varies very much. The
radials are always five (except in individuals where there are more than
five radii), but the interradials are increased in the Synaptida, and in
the other orders are in some cases diminished or occasionally suppressed
altogether. The last is the case in nearly all Elasipoda; here the radials
consist of a central horizontal piece with two diverging arms at each side.
These arms, which can branch repeatedly, traverse the adjacent interradii,
meeting those of the next radii, so that interradials are in most cases
entirely absent. The Aspidochirota have usually a ring consisting of small
squarish ossicles (Fig. 257, B). In the Molpadiida and Dendrochirota the
radials are prolonged backwards into forked tails, which in some
Dendrochirota are broken into a number of small pieces (Fig. 257, A), the
lower parts of the interradialia being similarly divided.
The CLASSIFICATION of the Holothuroidea is comparatively easy. {570}All
authors recognise six divisions, and the only dispute is as to whether they
are to be regarded as families or orders. Ludwig[508] divides the group
into two orders, PARACTINOPODA and ACTINOPODA, but the first includes only
those forms which have lost the radial canals, and this is only one step
farther in a degeneration, intermediate stages of which can be traced in
the other divisions. There is really no ground for placing the
Paractinopoda in contrast to all the other divisions, and the only
alternative is to regard the six main divisions as orders, since a class
must be divided into orders. In the case of only one, however, is a further
division into families practicable, and therefore each of the others will
contain a single family.
ORDER I. ASPIDOCHIROTA.
Holothuroidea with shield-shaped feelers provided with ampullae; with
radial canals and numerous podia and with respiratory trees. Retractor
muscles absent. Nearly a third (158) of the species of Holothuroidea belong
to this order, but there are only six genera, and of these _Holothuria_
includes no less than 109 species. The Aspidochirota seem for the most part
to live on somewhat firm ground, the surface of which they are continually
sweeping with their shield-shaped feelers, which brush the adherent
organisms into the capacious mouth. Four species of _Holothuria_—viz. _H.
intestinalis_, _H. tremula_, _H. aspera_, and _H. nigra_ are recorded from
British waters. The first-named is a northern form, distinguished by the
fact that all its podia have suckers; it is found in the north of Scotland.
_H. tremula_ is intermediate in structure between _H. intestinalis_ and _H.
nigra_, and is found in deep water off our western coasts. _H. aspera_,
remarkable for the radiating spines growing out from its ossicles, has been
recorded only once from deep water. Of the other genera it is only
necessary to mention _Stichopus_, remarkable for the square outline of its
transverse section, and for the restriction of the ventral tube-feet to the
radii; there is also a well-marked tapering of the anterior end, so that
this genus may be said to have a neck. _Stichopus_ is almost entirely
confined to tropical waters, and some of its species, as also species of
the ubiquitous genus _Holothuria_, as well as many other undetermined
species, constitute the valuable "Trepang," which is a delicacy much prized
{571}by the Chinese. The Trepang are caught in various parts of the Malay
Archipelago. They are cooked in sea water to preserve them, dried in the
sun, and boiled in fresh water repeatedly, till all the salt is extracted.
They are then dried and sent to market, where they are used in making soup.
ORDER II. ELASIPODA.
Holothuroidea with shield-shaped feelers, destitute of retractor muscles;
all the podia have more or less pointed ends,[509] but there is a marked
contrast between dorsal and ventral podia, and the ventral surface is
flattened so as to constitute a creeping sole. No respiratory trees, at
most a simple diverticulum of the intestine; frequently the primitive
external madreporite is retained, and contains several pores.
A number of spherical sacs containing little spherical calcifications
(otocysts) are attached to the nerve-ring in some genera. Can these be
metamorphosed sphaeridia of Echinoid ancestors?
[Illustration: FIG. 258.—_Ilyodaemon maculatus._ × ⅔. (After Théel.)]
The first member of this remarkable order to be discovered was _Elpidia_,
which was dredged in 1875 by the Swedish Arctic Expedition, and described
by Théel.[510] The majority of the known members of the order were
discovered by the dredging expedition of H.M.S. "Challenger." The species
composing it are, with one exception, inhabitants of what may be termed the
abysmal depths of the sea. The exception alluded to (_Ilyodaemon
maculatus_) is confined to the belt between 100 and 150 fathoms in depth.
The well-marked sole and the absence of suckers point to a life consisting
of constant peregrinations over {572}the soft ooze forming the ocean floor.
The ooze forms their food, and as their weight must to a certain extent
immerse them in it, we can understand why the stiff, long dorsal podia have
been specialised as respiratory organs, since there are no respiratory
trees. These respiratory podia sometimes undergo extraordinary development;
thus in _Peniagone_ several very long ones cohere to form a huge vertical
sail, whilst in _Psychropotes_ one or two cohere to form a backwardly
projecting tail. On the other hand, in _Ilyodaemon_ (Fig. 258) the dorsal
podia are numerous and slender.
ORDER III. PELAGOTHURIIDA.
Holothuroidea with shield-shaped feelers provided with long ampullae which
project outwards, pushing the skin before them so as to form external
appendages, connected at the base by a web. Calcifications absent. No
retractor muscles. No respiratory trees. The external madreporite is
retained, but all podia other than the feelers have disappeared, although
the radial canals have been retained.
This order contains one species, _Pelagothuria natans_, which is the only
free-swimming Holothuroid known, the muscular web connecting the freely
projecting ampullae being the organ of locomotion.
ORDER IV. DENDROCHIROTA.
Holothuroidea with long repeatedly branched feelers terminating in fine
pointed twigs. No feeler-ampullae; but retractor muscles are present, which
can introvert the anterior end of the body. Respiratory trees well
developed. This order includes twelve genera and over 180 species, and,
like the Aspidochirota, is of worldwide distribution. So far as can be
safely generalised from the few species whose habits have been closely
observed, it seems that this order is adapted to catch swimming prey—it is
an order of fishers. The long branched tentacles are extended like the
lines of an angler. Their surface is coated with adhesive slime, and before
long becomes covered with small organisms which have come in contact with
it. When a feeler has captured in this way a large enough haul, it is
turned round and pushed into the mouth, which is closed on it. It is then
forcibly pulled out, during which process the prey is, so to speak,
stripped off it. {573}Four genera (_Cucumaria_, _Thyone_, _Phyllophorus_,
and _Psolus_) and sixteen species have been recorded from British waters.
[Illustration: FIG. 259.—_Cucumaria crocea_, carrying its young, × 1. (From
Wyville Thomson.)]
_Cucumaria_ is remarkable for being the only genus of Holothuroidea in
which the body is pentagonal in cross-section. In the majority of its
species the tube-feet are confined to two rows along each radius, but in a
few there are some scattered tube-feet in addition. There are only ten
buccal tentacles. The species figured (_C. crocea_) is an Antarctic one
which carries the young on the back. _Thyone_ differs in being circular in
cross-section and in having the tube-feet scattered evenly over the whole
surface. In _Phyllophorus_ (Fig. 260) the tentacles are more than fifteen,
and are disposed in two circles, an inner of smaller and an outer of larger
tentacles. The other podia are, as in _Thyone_, scattered.
{574}[Illustration: FIG. 260.—_Phyllophorus urna._ × 1.]
[Illustration: FIG. 261.—_Psolus ephippifer._ × 3. A, with feelers
retracted and brood-pouch closed; B, with feelers extended and some of the
plates of the brood-pouch removed. (From Wyville Thomson.)]
_Psolus_ is a most extraordinary genus. There is a well-marked sole, to
which the tube-feet are confined, whilst the dorsal radial canals, and
consequently all the dorsal tube-feet, are absent. The dorsal ossicles are
enlarged to form a complete mail of plates, recalling the corona of a
Sea-urchin. The two British species are small, and found in comparatively
deep water, but a fine large species is found in the Gulf of St. Lawrence,
and {575}extends into brackish water up the estuary. The species figured
(_P. ephippifer_) is an Antarctic one, which carries the eggs until
development is complete in a dorsal brood-pouch.
ORDER V. MOLPADIIDA.
Holothuroidea with simple, finger-shaped feelers, provided with ampullae;
retractor muscles occasionally present; respiratory trees present. Besides
the feelers, the only podia are five minute papillae terminating the radial
canals in the neighbourhood of the anus.
[Illustration: FIG. 262.—_Trochostoma violaceum._ × 1. _m_, Mouth.]
This order includes six genera and about thirty species. Its peculiarities
seem to be due to the fact that its members are burrowers, leading a life
like an earthworm. Hence the absence of the tube-feet, and the small,
almost vestigial character of the feelers. _Trochostoma_ (Fig. 262) and
_Caudina_ are remarkable for the presence of a tail. This appendage is in
reality only the narrow posterior end of the body, and is especially long
in _Caudina_; and observations on a species found off the coast of Maine,
U.S.A.,[511] have shown that the tail, like the siphon of a Mollusc,
projects up from the burrow to the surface in order to maintain the
respiratory current of water.
ORDER VI. SYNAPTIDA.
Holothuroidea with short bipinnate (_i.e._ feather-shaped) feelers,
provided with only vestigial ampullae, and with well-developed retractor
muscles. No other podia; radial canals absent in the {576}adult.
Respiratory trees absent, and transverse muscles of adjacent interradii
continuous, so as to form circular muscles. Otocysts attached to the
nerve-ring as in Elasipoda.
[Illustration: FIG. 263.—_Synapta digitata._ A, animal viewed from the
side, × 7/13; B, anterior end, with tentacles extended, × 2.]
The members of this remarkable order, like those of the preceding one, are
burrowers; but though their feelers are larger, the rest of their anatomy
has undergone much more profound modification than that experienced by the
Molpadiida. The loss of the radial canals, which must be practically
functionless in Molpadiida, is not a great step, but the change in the mode
of respiration is a greater modification. Respiration appears to be
effected by diffusion through the body-wall, which is always comparatively
thin. The circulation of the body-cavity fluid is assisted by a number of
stalked, ciliated cups placed on the mesenteries near the line of their
insertion on the body-wall. In dealing with Asteroidea it was pointed out
that the ends of the tube-feet are the only places where numerous
sense-hairs are to be found, and which, therefore, can be called
sense-organs. This is true generally throughout Echinodermata. Now in
Synaptida, where the tube-feet are lost, the surface of the body has
scattered over it little sense-organs consisting of hillocks of ectoderm
with an aggregation of sense-cells. These may be regarded as representing
the discs of the missing tube-feet. One is involuntarily reminded by the
ciliated cups and scattered sense-organs of the ciliated urns and
sense-organs of the Sipunculidae, which lead a similar life; and taking
into consideration the general superficial likeness in {577}appearance of
the two groups, the epigram is almost justified that "if the Synaptida were
not extremely careful they would become Gephyrea."
This order is represented in British waters by three species of the genus
_Synapta_, which is remarkable for possessing, as ossicles, only the
peculiar anchors attached to anchor plates. The present author has dug up
the commonest species (_S. inhaerens_) from its burrows in the sand at low
water in the Clyde. These animals seem to seek their food at the surface;
the feather-shaped feelers are used to seize small algae and zoophytes, of
which the food apparently consists. If seized, _S. inhaerens_ readily
amputates the posterior part of the body, whilst the head with its feelers
immediately buries itself. The other genera of the order (except _Anapta_)
are characterised by the possession of wheels with spokes as their
characteristic ossicle, as the names _Trochodota_, _Trochoderma_,
_Acanthotrochus_ bear witness.
The only fossil remains of Holothuroidea consist of isolated
ossicles—wheels, gratings, anchors, etc.—which first make their appearance
in the Carboniferous limestone and tell us practically nothing of the
evolution of the group. From a comparison with one another of the living
families, certain conclusions can be drawn. The Aspidochirote feeler and
the method of using it recall forcibly the shape and function of the buccal
tube-feet of Spatangoidea. It is probably safe to assume that it is the
primitive form from which the other forms of feeler have been derived.
Secondly, the anal respiration and the curious internal madreporite have
been developed in correlation with one another, and are like nothing found
elsewhere among the Eleutherozoa. Hence we may with high probability assume
a Protoholothuroid stock with shield-shaped feelers but devoid of
respiratory trees, and with an external madreporite. From this stock the
Elasipoda developed by migrating into deeper water, whilst the
Pelagothuriida sprang from the same root by taking to swimming; the
Aspidochirota constituting the main line. The Dendrochirota were developed
from a stock with respiratory trees and internal madreporite—in a word,
from Aspidochirota. From them the Synaptida and the Molpadiida have
developed as offshoots at different periods through taking to a burrowing
life. These relationships are shown by the following diagram:—
Protoholothuroidea {578}
/ | \
/ | \
Elasipoda | Pelagothuriida
|
Protaspidochirota
/ \
Aspidochirota Protodendrochirota
|\
| Synaptida
|\
| Molpadiida
|
Dendrochirota
{579}CHAPTER XX
ECHINODERMATA (_CONTINUED_): PELMATOZOA—CRINOIDEA =
SEA-LILIES—THECOIDEA—CARPOIDEA—CYSTOIDEA—BLASTOIDEA
SUB-PHYLUM II. PELMATOZOA
The PELMATOZOA differ from the ELEUTHEROZOA in several important respects.
They are fixed (at any rate in the young stage) by the centre of the aboral
surface, and this portion of the body usually takes on the form of a stem
supported by a definite series of ossicles, so that we can discriminate a
"calyx"—the main part of the body—from the "stem." Further, the podia and
the ambulacral grooves seem to be always covered with powerful cilia, which
are employed in producing a current which sweeps small organisms to the
mouth. The podia are never locomotor in function; their use is similar to
that of the tentacles on the lophophore of Polyzoa and Brachiopoda.
The living Pelmatozoa are very few in number compared with the extinct
forms. It may with justice be said that the group is nearly extinct;
indeed, out of its five classes one alone, and that the most highly
specialised class, survives till the present day. Now we have already seen
that, in the case of the Eleutherozoa, if the annectant fossil types were
taken into consideration, the definition of the classes would be difficult,
so that it is not to be wondered at if the classes of the Pelmatozoa are
also somewhat difficult to define; and it must be added that this
difficulty is not only due to the fact that intermediate types occasionally
occur, but also to our ignorance of the functions of many structures found
in fossil types, speculations regarding which are to be received with
caution. Bearing in mind, then, the provisional nature {580}of the
classification, we may give the diagnoses of the principal divisions as
follows:—
Class I. CRINOIDEA.—Pelmatozoa provided typically with a well-marked stem;
calyx consisting of an aboral "patina" of two or three circles of plates,
and a flexible "tegmen" or oral surface with small plates or none; radial
canals supported by long branched arms, which are developed as direct
prolongations of the uppermost circle of plates in the patina.
Class II. THECOIDEA (Jaekel) = EDRIOASTEROIDEA (Bather).—Pelmatozoa without
a stalk, fixed to the substratum by the whole aboral surface. The radial
canals run out over the oral surface in grooves, which are closed by
specially modified plates; but there are no arms of any kind.
Class III. CARPOIDEA (Jaekel).—Pelmatozoa with a well-developed stalk. The
radial canals and their branches are devoid of a skeleton, and either
produce no modifications at all on the skeleton of the calyx, or at most
are supported by short horn-like outgrowths of some of its plates.
Class IV. CYSTOIDEA.—Pelmatozoa which typically possess a well-developed
stalk, a sac-like calyx contracted at the mouth and covered with plates,
some of which are pierced with pores or slits; the radial canals, though
they may for part of their course run over the surface in grooves, have
their terminal portions supported by free unbranched arms ("fingers").
Class V. BLASTOIDEA.—Pelmatozoa provided with a well-developed stalk and
ovoid bud-like calyx. From the mouth the radial canals run backwards over
the calyx, as in Echinoidea, but they give rise to numerous lateral
branches, which are supported by free unbranched arms ("fingers"). Special
respiratory organs occur on the interradial areas in the form of parallel
folds called "hydrospires."
CLASS I. CRINOIDEA
This is the only class which has living representatives. There are twelve
recent genera, of which eight retain the stalk throughout life; the
remaining four lose it when adult, retaining only a stump, termed the
"centro-dorsal," covered with fixing organs ("cirri"). The stalked forms
are confined to considerable depths, and can only be obtained by deep
dredging, whereas many of the {581}stalkless forms are comparatively
common. We shall select as type for special description the common
Feather-star, _Antedon rosacea_ (_bifida_), which can be dredged in depths
of ten fathoms off the south-west coast of England.
[Illustration: FIG. 264.—Oral view of _Antedon rosacea_. × 3. _a_, Arm;
_a.g_, ambulacral groove; _an_, anus; _c_, calyx; _m_, mouth; _p_,
pinnules.]
The animal consists of a small flattened calyx, from which radiate out ten
long delicate arms, each fringed with a double series of short branches
called "pinnules." In the centre of the aboral surface can be seen the
centre-dorsal plate (Fig. 265, _c_), a knob-like stump of the broken-off
stem, covered with small whip-like outgrowths called "cirri," by means of
which the animal is anchored to the substratum (Fig. 265, _cir_). When
_Antedon_ is disturbed it relaxes its hold, and swims by graceful muscular
movements of the arms. These are arranged in five pairs, and the
corresponding members (right and left) of all the pairs are bent and
relaxed together. On coming to rest the animal reattaches itself by means
of the cirri. These are composed of cylindrical ossicles joined to one
another by muscles, and they can thus act as efficient grasping organs. In
the centre of the oral surface, which is termed the "tegmen," and is soft,
flexible, and without visible calcifications, is situated the mouth,
surrounded by five short triangular flaps called "oral valves." In the
intervals between these valves, grooves radiate from the mouth which
bifurcate at the points of origin of each pair of arms, and are continued
over their surfaces. These grooves correspond to the ambulacral grooves of
Asteroidea, and to the epineural canals of the other classes of
Eleutherozoa. At each side of each groove {582}are to be found a series of
podia in the form of delicate finger-like processes, which serve only for
respiration and for producing a current of water, their surfaces, like that
of the grooves between them, being covered with powerful cilia. The anus is
at the extremity of a little knob called the anal papilla, situated in one
of the interradii (Fig. 264, _an_).
As in Ophiuroidea, the ectoderm cells have disappeared over the whole
surface of the body, except the grooves and the podia, the only trace of
their former existence being a cuticle with adherent nuclei. Pedicellariae
are unknown in all Pelmatozoa; and spines have only been described from one
fossil species of Crinoid. Beneath the cuticle is the dermis, having the
composition described in the case of _Asterias rubens_; this on the aboral
side of the calyx gives rise to the "patina," consisting of plates, in part
movable on one another, in part immovably fused together. Those visible
from the outside are (1) the centro-dorsal ossicle, from which the cirri
spring; (2) five columns of ossicles termed radials (Fig. 266, _R^1_,
_R^2_, _R^3_); each column consists of three radials, extending from the
centro-dorsal to the origin of a pair of arms. The uppermost radial in each
column bears two facets for the articulation of these arms. Each arm is
supported by a series of "brachial ossicles" (_Br_).
[Illustration: FIG. 265.—View of _Antedon rosacea_ from aboral surface, ×
4. _c_, Centro-dorsal; _cir_, cirrus; _R^1_, _R^2_, _R^3_, the three radial
plates of one column; _syz_, syzygy.]
It is evident, both from the number of ambulacral grooves and of the
columns of radials, that _Antedon_ has only five radii, and each pair of
arms must be regarded as having arisen by the bifurcation of a primitive
arm. This is proved to be true by {583}a study of the development, and it
can further be shown that the arms fork repeatedly; but in these further
bifurcations one fork remains short, and forms a pinnule, whilst the other
continues the arm. Thus the arm, instead of being a single axis, is really
a series of axes—in a word, it is a "sympodium."
If in the case of any bifurcation the two forks were to develop equally,
the number of arms in that ray would be doubled, and this actually happens
in the case of other species of _Antedon_.
DIGESTIVE SYSTEM.—The mouth leads through a short vertical oesophagus into
an enlarged stomach, which lies horizontally curved around the axis of the
calyx. The stomach is succeeded by a short intestine, which leads into the
anal papilla. Both oesophagus and stomach are ciliated, and the food
consists of minute organisms, swept into the mouth by the current produced
by the cilia covering the ambulacral grooves and podia; the ten arms may
indeed be compared to a net spread out in the water to catch swimming prey.
The WATER-VASCULAR SYSTEM consists of a ring closely surrounding the mouth,
from which radial canals are given off which underlie the ambulacral
grooves and bifurcate with them. The podia have no ampullae, but muscular
strands traverse the cavities of the radial canals, and that of the
ring-canal, and by their action water can be forced into the podia, which
are thus extended. Numerous stone-canals hang down from the ring-canal, and
open freely into the coelom; they do not, as in Holothuroidea (where the
same arrangement occurs), end in sieve-like madreporites. The tegmen,
_i.e._ the ventral surface of the calyx, is pierced by a number of isolated
pores lined by ciliated cells, which suck in water. In the oldest
Pelmatozoa there seems to have been a regular madreporite. In the larva of
_Antedon_ there is but one pore-canal, which, as in most Eleutherozoa,
leads into a special section of the coelom, the "axial sinus," embedded in
the body-wall, with which also the single stone-canal communicates; but
later the division between the axial sinus and the rest of the coelom
breaks down, and then the pore-canals and stone-canals become multiplied
independently of each other (Fig. 266, _m.p_, _p.c_, and _st.c_).
NERVOUS SYSTEM.—In the young stalked form the nervous system, as in other
Echinoderms, consists of a ring round the {584}mouth, from which radial
cords are given off which run under the ambulacral grooves (Fig. 266,
_nerv.rad.v_).
The fibres of this nervous system are, as in Asteroidea, immediately
beneath the bases of the ectoderm cells. A large band of fibres is given
off to each podium, which is covered with minute elevations, each with
pointed sense-hairs in the centre. As the animal grows, another nervous
system makes its appearance, which is developed from the coelomic wall, the
cells of certain tracts of which multiply and bud off ganglion cells from
which the fibres grow out.
[Illustration: FIG. 266.—Diagrammatic longitudinal section through one arm
and the opposite interradius of _Antedon_. _ax_, Central canal of
centro-dorsal, with prolongation of genital stolon; _B_, rosette,
consisting of coalesced basals; _Br^1_, _Br^2_, _Br^3_, _Br^4_, the first
four brachial ossicles; _chamb_, chambered organ; _coel.coe_, coelomic
canal of arm; _gen.coe_, genital canal of arm; _gen.r_, genital rachis;
_gen.st_, genital stolon; _m.p_, madreporic pores; _musc.long_,
longitudinal muscle; _nerv.rad.d_, dorsal radial nerve; _nerv.rad.v_,
ventral radial nerve; _p.c_, pore-canal; _pod_, podium; _R^1-R^3_, 1st to
3rd radials; _st.c_, stone-canal; _sub.coe_, sub-tentacular canal of arm;
_w.v.r_, radial water-vessel.]
This "aboral nervous system," as it is called, has its centre in the
"chambered organ" (Fig. 266, _chamb_), which is embedded in the
centro-dorsal ossicle, and is roofed over by a plate called the "rosette."
This represents the five coalesced "basals," a ring of plates which in
other forms alternate with the lowest radials, and it intervenes between
these and the centro-dorsal. The chambered organ consists of a ring of five
vesicles, which have originated as pouches of the aboral coelom (Fig. 266,
_chamb_). The walls of these vesicles develop nervous matter; from them
radiate out five great cords, deeply embedded in the {585}plates of the
patina. These cords rapidly fork, and one division of each of two adjacent
cords enters the lowest radial. In the third radial all the cords are
connected by a commissure which runs completely round the calyx. Each of
the cords in the third radial forks again, and one branch of each cord
enters each of the two arms connected with it, and the two branches
entering an arm coalesce to form a single cord. In the arms, as in the
calyx, the cords are deeply embedded in the ossicles, but branches extend
to the ventral surface of the arms and here unite to form two longitudinal
cords, one on each side of the groove. In the tegmen these cords are
connected by an outer nerve-ring, branches from which join the ectodermal
nerve-ring already described.
The researches first of W. B. Carpenter[512] and then of Marshall[513] have
proved that it is the aboral nervous system which really controls the
movements of the animal. If the chambered organ is destroyed by cautery,
the whole movements of the animal are paralysed; but it will carry out its
characteristic swimming movements just as well if the whole tegmen with the
ambulacral nerve-ring and the whole of the alimentary canal are torn away.
The commissure in the third radials co-ordinates the movements of the arms.
If it is cut they move independently of one another. The position of the
radial cords inside the ossicles is gradually acquired. At first they are
gutter-like evaginations of the coelom; by upgrowth of their sides the
gutters become canals, and are then surrounded by calcified tissue. The
cirri have each a cord traversing them which originates from the chambered
organ.
COELOM.—In the young stalked form the coelom consists of the water-vascular
system ("hydrocoel"), and underlying it an oral coelom, separated from an
aboral coelom by a horizontal mesentery. As the animal grows, this
horizontal mesentery becomes largely absorbed, and the coelom becomes
everywhere traversed by cellular cords (trabeculae), which are later
calcified.
Both oral and aboral coelom become, like the hydrocoel, bent into hoops,
and along the axis of the aboral coelom a cord of germ-cells is developed,
which constitutes the "genital stolon." The chambered organ is developed
from the aboral coelom, and {586}in the centre of its five chambers a
median pocket grows down into the centro-dorsal, along the side of which is
an extension of the genital stolon. In the arms the mesentery separating
the extensions of the oral and aboral coelom persists; the oral extension
consists of two parallel canals called "subtentacular" (Fig. 267, _s.c_),
whilst the aboral space is termed the "coeliac" canal (Fig. 267, _c.c_). In
the tip of the pinnule, that is to say at the extremity of a ramification
of the arm, the coeliac and subtentacular canals communicate. As portions
of the lining of both canals are ciliated, a circulation of the coelomic
fluid is thus kept up. The GENITAL STOLON gives rise at the level of the
remnant of the horizontal mesentery in the disc to a circular genital
rachis, whence cords pass down the arms in the tissue separating
subtentacular and coeliac canals (Fig. 267, _g.r_). Each cord is contained
in a special tube, the "genital canal," which is probably developed in the
same way as the aboral sinus of the Eleutherozoa, _i.e._ as a special
sheltering outgrowth of the coelom (Fig. 267, _g.c_). In the pinnule the
rachis swells out into a genital organ, from which a short duct is
developed when the organ is mature. The eggs are large (3 mm. in diameter),
and adhere for a considerable period of their development to the pinnules.
[Illustration: FIG. 267.—Diagrammatic transverse section of arm of
_Antedon_. To compare it with the section of an Ophiuroid arm it is
inverted from its natural position. _br_, Brachial ossicle; _c.c_, coeliac
canal; _c.p_, covering plate; _g.c_, genital canal; _g.r_, genital
rachis; _n.r.d_, dorsal nerve-cord; _n.r.v_, ventral nerve-cord; _pod_,
podium; _s.c_, subtentacular canal; _w.v.r_, radial water-vessel.]
{587}The MUSCLES of _Antedon_ are of two kinds. Those of the water-vascular
system are, as in Eleutherozoa, basal outgrowths of the cells forming the
walls of the system. The muscles moving the joints of the arms appear to be
modifications of connective-tissue cells. When the brachials are isolated
their terminal faces, strikingly long, recall those of Ophiuroid vertebrae.
There is a ventral groove for the coelomic canal. Above this groove the
face is divided by ridges into four areas for attachment of the muscles.
Dorsal to this is the pit for the strong ligament which binds the ossicles
together; then comes the canal for the aboral nerve-cord, whilst dorsal to
this is the pit for what is called the "dorsal elastic ligament." The
theory underlying this name is that the muscles bend the arms ventrally,
and the ligament by its elasticity restores them to their places; but there
seems reason to believe that the "ligament" is really a dorsal muscle. It
is particularly to be noted that similar muscles occur between the first
and second radials, proving that the primary arm really begins with the
first radial. The second and third radials, as also the first two ossicles
and certain others of each arm, are closely united by calcified fibres, and
this kind of union is called a "syzygy" (Fig. 265, _syz_). The cirri have
all their ossicles united by muscular attachment, and can move rapidly.
The BLOOD SYSTEM (see pp. 449-451) forms a ring consisting of a network of
strings round the oesophagus. This is termed the "labial plexus." From this
cords can be traced to the wall of the stomach and to the surface of the
genital stolon. The assertion that radial strands intervene between the
ectodermic nerve-cord and the radial water-vascular canal, though usually
made, does not appear to be justified, since what is termed the vessel
appears to be a crevice formed by shrinkage in preservation.
The process of respiration is doubtless largely carried out by the podia,
but it must be assisted by the constant instreaming of fresh sea-water
through the pore-canals. The process of EXCRETION has not been directly
observed in _Antedon_, but structures called "sacculi" may be connected
with this function. These are spherical masses of amoebocytes embedded in
the tegmen. During life they are colourless, but after death they become
coloured, showing that they secrete a peculiar compound. These sacculi
abound in the disc, and a row of them is to be found at each side of the
ambulacral groove in the arms. When, as in {588}tropical species, the
groove is supported by side-plates, these are notched for the reception of
the sacculi.
Turning now to survey the group Crinoidea as a whole, lack of space forces
us to confine our attention mainly to the living forms. These differ
amongst themselves chiefly in the following points: (1) the condition of
the stem; (2) the structure of the calyx; (3) what is intimately connected
with this, the method of branching of the arms; and (4) the length of the
alimentary canal.
CONDITION OF THE STEM.—This is represented by a centro-dorsal stump in
_Antedon_ and most of its allies, but in _Actinometra_ it becomes a flat
plate, and in some species in old age all the cirri drop off. In
_Uintacrinus_ and _Marsupites_ (fossil genera) there is no trace of cirri.
In Pentacrinidae there is a long stem, pentagonal in cross-section, in
which alternate ossicles carry whorls of cirri; in Rhizocrinidae the stem
consists of compressed ossicles, elliptical in section, bearing cirri only
at the rooting tip, whilst in _Hyocrinus_ the stem is made up of
cylindrical ossicles, cirri being apparently absent. Finally, in _Holopus_
the stem is represented by an uncalcified leathery outgrowth from the
calyx.
SKELETON OF CALYX AND ARM.—In living Crinoidea, with the doubtful
exceptions of _Holopus_ and _Hyocrinus_, the calyx is supposed to be built
up originally of four whorls of plates, viz. "infra-basals," "basals,"
"radials," and "orals," the last named forming the skeleton of the oral
valves round the mouth. In the two exceptions named there is no certain
evidence of the existence of infra-basals. In living forms the infra-basals
coalesce with the uppermost joint of the stem; the basals remain large and
conspicuous, though they are fused into a ring in Rhizocrinidae,
_Atelecrinus_, and _Thaumatocrinus_, whilst in Pentacrinidae this ring is
nearly, and in _Antedon_ and its allies completely, hidden when the calyx
is viewed from the outside. In _Hyocrinus_ the basals are represented by
three ossicles. The lowest radials are an important element in the patina
in every case, but the upper radials, the incipient portions of the arms,
may be incorporated in the calyx (_Pentacrinus_, _Antedon_) or may be free
(Rhizocrinidae and _Hyocrinus_); in _Metacrinus_ there are five to eight
radials in each column, all incorporated.
The oral plates are very large in _Hyocrinus_, _Holopus_, and
{589}_Thaumatocrinus_, small in _Rhizocrinus_, vestigial or absent in
_Bathycrinus_, and completely absorbed in _Antedon_. In addition to these
main elements, in many species small accessory plates are developed (_a_)
at the sides of the ambulacral groove, over which they can close down (many
species of _Antedon_, _Hyocrinus_, _Holopus_, Rhizocrinidae, some species
of Pentacrinidae); these are "covering plates," and correspond in function
to the adambulacrals of Asteroidea; (_b_) supporting the sides of the
groove and corresponding to the ambulacrals of Asteroidea;[514] these are
"side-plates," and the covering plates articulate with them; (_c_) on the
surface of the tegmen; these are the interradial plates, which in
_Thaumatocrinus_ alone among recent forms, but in many fossil forms, are
continued into the patina, where they separate the radial plates.
MODE OF BRANCHING OF THE ARMS.—All modern Crinoids have pinnules, and this,
as has already been explained, is due to a suppressed dichotomy. The extent
of the suppression determines the number of arms, which varies within the
same genus.
ALIMENTARY CANAL.—In _Hyocrinus_ there is no dilatation which could be
called a stomach; in _Actinometra_ the mouth is excentric, and the anal
papilla occupies the centre of the tegmen. The intestine is elongated, and
describes several turns round the papilla before ending in the anus.
The CLASSIFICATION of Crinoidea cannot properly be considered without
taking account of fossil forms, but to do so at all adequately is
impossible on account of limitations of space. Less regret may be felt
because the three specialists in this branch, viz. Bather in England,
Springer in America, and Jaekel in Germany, come to fundamentally different
conclusions on the subject. If we confine our attention to living forms we
may, with P. H. Carpenter,[515] select the stem as the basis of
classification. As the method of gaining food is the same in all cases, the
Crinoidea have probably split on the method of attachment to the
substratum. These families—it is impossible, in view of the greater range
of variety in fossils, to dignify them with the name of orders—are as
follow:—
{590}FAM. 1. HYOCRINIDAE.—Stem long and persistent; cirri absent; stem
ossicles cylindrical—ligaments uniting them not specialised. Arms (five)
short, but with extremely long pinnules. Patina composed of long exposed
basals and a ring of five spade-shaped radials. Five large persistent
orals. Interradials and covering plates present. One species (_Hyocrinus
bethellianus_, Figs. 268, 269) dredged up in the Southern Pacific Ocean.
[Illustration: FIG. 268.—_Hyocrinus bethellianus._ × 2. (From Wyville
Thomson.)]
[Illustration: FIG. 269.—Tegmen of _Hyocrinus_, viewed from above after
removal of the arms, × 8. (From Wyville Thomson.)]
FAM. 2. RHIZOCRINIDAE.—Stem long and persistent; cirri confined to a few
near the root or replaced by rooting branches of the stem. Stem-ossicles
thin and pentagonal at the summit, but lower down compressed, elliptical in
section, and united by two ligaments separated by a transverse ridge for
articulation. Patina composed of exposed basals and a ring of five short
radials. Orals large {591}or vestigial. Covering plates and interradials
present. Two genera, both from great depths in the Atlantic—_Rhizocrinus_
(Fig. 270), with five arms and well developed orals (attachment by
branching root-cirri); and _Bathycrinus_, with ten arms and vestigial
orals; attachment by root-like branches of stem (this is essentially the
same as root-cirri).
[Illustration: FIG. 270.—_Rhizocrinus lofotensis._ × 1½. (From Wyville
Thomson.)]
FAM. 3. PENTACRINIDAE.—Stem consisting of ossicles which are pentagonal in
section, united in pairs by syzygy, the upper one of each pair bearing a
whorl of cirri and united by five bundles of fibres of petal-like section
with the lower one of the pair above it. No rooting processes. Patina
consists almost entirely of columns of radials, the basals being almost or
completely hidden. Orals absent, but side-plates in the ambulacral grooves.
Two recent genera, _Pentacrinus_ (_Isocrinus_), with three radials in each
column; _Metacrinus_, with five to eight radials in a column, but the third
{592}radial bears a pinnule. _Pentacrinus_ is found in both the Caribbean
Sea and the Pacific Ocean; _Metacrinus_ in the Pacific. It appears that the
Pentacrinidae when young are attached by a foot-plate at the apex of the
stem; but when adult, the stem is broken in two and the animals, like
_Antedon_, swim by movements of the arms, dragging a large part of the stem
after them, by which they effect temporary attachment. As in other stalked
forms, the cavities of the chambered organ are prolonged into canals which
traverse the stalk; but in this family there is the peculiarity that a
repetition of the chambered organ is found opposite every whorl of cirri.
[Illustration: FIG. 271.—_Pentacrinus asteria._ × ¼. (From Wyville
Thomson.)]
FAM. 4. HOLOPODIDAE.—Stem represented by a leathery noncalcified outgrowth
from the base of the calyx; one circle of radials indistinguishably fused
with the basals and with each other to form the walls of the calyx. Large
oral plates, ten short arms. One genus, _Holopus_, in shallow water in the
Caribbean Sea.
{593}[Illustration: FIG. 272.—Arms and portion of stem of _Pentacrinus
maclearanus_, slightly enlarged. In this species the basals can be seen.
(From Wyville Thomson.)]
{594}[Illustration: FIG. 273.—Calyx of _Actinocrinus_, one of the Camerata,
broken open to show structure. _amb_, Ambulacral groove enclosed in
covering plates; _B_, basal; _R^1_, _R^2_, _R^3_, the three radials of a
column. (After Zittel.).]
FAM. 5. COMATULIDAE.—Stem in the adult broken off, leaving only a stump,
the centro-dorsal, covered with cirri. Six genera. _Antedon_ (= _Comatula_)
has already been described; many tropical species have numerous arms and
often side-plates and covering plates. _Actinometra_ is distinguished by
its excentric mouth, and by the fact that the centro-dorsal is flat and has
cirri only round its edges; _Atelecrinus_ has an acorn-shaped
centro-dorsal, and the basals are externally visible; _Eudiocrinus_ differs
from _Antedon_ only in having five arms; _Promachocrinus_ is a remarkable
form, having ten radii (this is a unique feature in Crinoidea); finally,
_Thaumatocrinus_ has basals externally visible, large persistent orals and
interradial plates, and in addition a short free appendage of several
plates on the anal interradius. _Antedon_ and _Actinometra_ are almost
world-wide. Six species of the first have been recorded from British
waters, of which the commonest is _Antedon rosacea_; four others are
distinguished by having longer cirri, and do not seem to be well defined;
but _A. eschrichtii_, a northern form, is larger, and is distinguished by
having long proximal pinnules. The other genera are rare, and occur in deep
water.
When we turn to survey FOSSIL CRINOIDEA, we are met with a bewildering
variety of forms ranging from the Lower Cambrian to the present day. As
already mentioned, there is no agreement amongst experts as to how they
should be classified. Bather makes the fundamental cleavage depend on the
possession of two whorls of plates in the base (Fig. 274), or of only one
whorl. These two divisions he calls DICYCLICA and MONOCYCLICA respectively.
He admits that in many forms allied to Dicyclica the infra-basals have
disappeared; these he terms "pseudomonocyclic" forms, and believes that he
is able to discriminate them from true Monocyclica.
{595}[Illustration: FIG. 274.—_Crotalocrinus pulcher._ × 1. _B_, basal;
_Br_, arm-fan of adhering branches; _col_, ossicle of stem; _IB_,
infra-basal; _R_, radial. (After Zittel.)]
The present author is utterly unable to believe that the Crinoidea diverged
into two groups on what is a trifling point of meristic variation
comparable to the varying number of rows of plates in the interradial areas
of the older Echinoidea; and he is equally sceptical as to the validity of
Jaekel's division of the group into CLADOCRINOIDEA and PENTACRINOIDEA,
leading to the view that organs like pinnules represent totally different
structures in different groups. Wachsmuth and Springer adopt as bases of
classification the extent to which the arms and their branches are
incorporated in the disc, and they recognise three main divisions:
INADUNATA, in which the arms are completely free from the calyx;
ARTICULATA, in which the arms are partly incorporated but the tegmen
remains flexible; and finally CAMERATA, in which the arms and their first
branches are largely incorporated in the cup; the tegmen is converted into
a rigid dome and the ambulacral grooves on it become closed, as does the
mouth, by the meeting of overarching folds; the grooves remaining, of
course, open in the distal portions of the arms (Fig. 273). This
classification, founded as it is on physiological factors, seems to the
present author more satisfactory. Speaking generally, the points in which
fossil Crinoids may differ from living genera are: (1) the total absence or
irregular nature of the branching in the arms, so that pinnules may be said
to be absent; (2) the closure of the ambulacral grooves and mouth already
alluded to, and (3) the adhesion of the arms in the same ray to produce
net-like structures (_Crotalocrinus_, Fig. 274), or a fan-shaped structure
(_Petalocrinus_); (4) the frequent presence of two rows of brachials in one
arm (biserial structure); (5) the {596}development of an enormous anal
tube, so large that in extreme cases (_Eucalyptocrinus_) the arms may be
lodged in grooves of it.
CLASS II. THECOIDEA (EDRIOASTEROIDEA, BATHER)
These remarkable Pelmatozoa are the most primitive known. They have
sac-like or sometimes cushion-shaped or even disc-shaped bodies, covered
with numerous irregular plates without any symmetry in their arrangement.
There is no stem, but when they are fixed this is effected by an adhesion
of the aboral pole. There are no arms, but on the upper surface is to be
seen the impression of five ambulacral grooves radiating from a central
mouth. These grooves are bordered by covering plates, which in the earliest
form (_Stromacystis_) are seen to be slight modifications of the plates
covering the upper surface of the body, but in the later genera (Fig. 275,
_Thecocystis_) become specialised. The anus is situated on the side, as is
also the madreporite. It has been suggested that Eleutherozoa were derived
from this group; that individuals were occasionally overturned by the waves
or currents, and in this way compelled to use their podia for locomotion.
When Eleutherozoa, however, have a fixed stage in their development, they
are fixed by the oral, not the aboral, surface, and hence can have no close
affinity to Thecoidea. Thecoidea begin in the Middle Cambrian, but
according to Jaekel impressions in the Lower Cambrian, referred to Medusae,
may be casts of this group.
[Illustration: FIG. 275.—_Thecocystis saeculus._ × 6. _a._ Anus; _m_,
mouth; _p_, madreporite(?) (After Jaekel.)]
CLASS III. CARPOIDEA
Pelmatozoa with a well-developed stem; body bilaterally compressed; only
two rays apparently developed. These are indicated only by grooves
radiating from the mouth; but in {597}some cases slight horn-like
outgrowths of some of the plates of the calyx may support prolongations of
the grooves.
This group, which, like the foregoing, commences in the Cambrian, is
perhaps more primitive than the Thecoidea in showing less influence of the
water-vascular system on the skeleton; but in the presence of a
differentiated stem and the development of only two rays, it is more
differentiated. The anus is on one of the flat sides, covered with a flat
plate acting as a valve. The members of this group were formerly confounded
with Cystoidea, from which they differ in the absence of the characteristic
pores. _Trochocystis_, the genus figured, is devoid of any horn-like
outgrowths of the calyx.
[Illustration: FIG. 276.—_Trochocystis bohemicus_, viewed from two sides.
_o_, Mouth. (After Jaekel.)]
CLASS IV. CYSTOIDEA
Pelmatozoa with respiratory organs in the form of "diplopores" or
"pore-rhombs." In a great many cases there is a stalk, but in other cases
this is atrophied, and the animal is attached by the base of the calyx. The
radial canals run for a shorter or longer distance over the calyx, but the
plates of the calyx themselves are not modified for them. Either they run
in simple grooves, or they are protected by a special series of plates
lying above the plates of the calyx. The terminal portions of the radial
canals are in all cases free, supported by unbranched arms consisting
usually of a double row of ossicles. These arms are termed "fingers."
It will be gathered from the description just given that the fingers and
the respiratory organs distinguish Cystoidea from {598}the two foregoing
classes. Formerly this class was a lumber-room in which were placed all
primitive irregular Pelmatozoa. The labours of Jaekel[516] have, however,
dispelled the mist which enveloped this group, and in his monograph all
that can be extracted both from superficial examination and dissection of
these fossils is contained. It seems possible to the present author that
the class may eventually require to be divided into two, corresponding to
the two main divisions which Jaekel recognises, viz. DICHOPORITA, with
pectinated rhombs, and DIPLOPORITA, with diplopores.
[Illustration: FIG. 277.—_Echinosphaerites aurantium._ A, from above; B,
from the side; C, neighbourhood of mouth, enlarged. _amb_, Ambulacral
groove with side-plates and covering plate; _mad_, madreporite. The short
parallel lines across the sutures are the "pore-rhombs." (After Zittel.)]
The pore-rhombs of the Dichoporita (indicated in Fig. 277 by the small
parallel lines crossing the boundaries of the plates) were, according to
Jaekel, nothing but a series of folds of thin integument projecting into
the interior, the outer opening of which in most cases adhered in the
middle, leaving two pores connected by a groove. The inner boundaries of
the folds are sometimes preserved, but in many cases they were entirely
devoid of calcification, and so were lost. The radial vessels either
branched a great deal, giving rise to a multitude of fingers, or, as in
_Echinosphaerites_ (Fig. 277), there were a few long fingers supporting a
reduced number of radial canals. In some cases the calyx can be analysed
into a regular series of cycles of plates, consisting of basals, orals, and
three intervening whorls, thus including one more ring than the calyx of
Crinoidea. Jaekel regards this as a primitive arrangement, believing that
the irregularity seen in _Echinosphaerites_ secondary. This is a doubtful
hypothesis.
{599}The diplopores of the Diploporita appear to consist of two canals
traversing the body-wall, opening close together into a common pit
externally, but diverging internally. Since in some cases, as in
_Aristocystis_ (Fig. 278), this common pit is proved to have been closed
externally by a very delicate layer of calcification, it is probable that
the pores represent in other cases the points of origin of finger-like
gills similar to those of Asteroidea. Where they were closed by
calcification this was so thin and porous that the diffusion through it
sufficed for respiration. Jaekel regards the Diploporita as a group derived
from Dichoporita, but this seems to be extremely doubtful.
[Illustration: FIG. 278.—_Aristocystis._ In the upper part of the calyx the
heavy dots are "diplopores," seen owing to removal of the superficial
layer. (After Zittel.)]
CLASS V. BLASTOIDEA
Pelmatozoa with respiratory organs in the form of longitudinal calcified
folds, termed "hydrospires," radiating from the mouth. Stem well developed;
calyx regular, consisting of a whorl of basals surmounted by a whorl of
forked radiais, in the clefts of which lay the recumbent radial
water-vascular vessels, supported each on a special plate ("lancet plate"),
and giving off two rows of branches supported by short fingers (Fig. 279).
Side-plates and covering plates were also developed; five orals
("deltoids") completed the calyx. The anus was at the side, just beneath
one of the orals.
The hydrospires, which are the great characteristic of the class, are seen
in section in Fig. 279, B (_hyd_). They consist of a varying number of
parallel folds on each side of each "pseudambulacrum," as the lancet plate
with its adhering side-plates and covering plates has been termed. In the
most primitive genus, _Codaster_, they appear to have opened directly to
the exterior, and to have been placed at right angles to the lines of union
of the radial and oral plates, just like the grooves of a pectinated rhomb.
In more modified forms, such as _Pentremites_ and _Granatocrinus_ (Fig.
279), the outer openings were overarched {600}by the extension of the
side-plates of the radial vessel, and the whole group of folds has a common
opening near the mouth; indeed, in the highest form there is one common
"spiracle" for the two groups of folds in an interradius, which in one
interradius is confluent with the anus. The hydrospires, when they reach
this form, irresistibly recall the genital bursae of Ophiuroidea (Fig. 214,
p. 490), and very possibly served the same purpose.
[Illustration: FIG. 279.—_Granatocrinus norwoodi._ A, view of whole animal;
B, section of radius; C, an isolated finger. _hyd_, Hydrospire; _l_, lancet
plate; _pinn_, finger; _p.p_, covering plate; _R_ and _D_ both signify
radial plate. (After Zittel.)]
Reviewing the whole group of the Pelmatozoa, we see that in the Cambrian
they begin with the extremely primitive Thecoidea and Carpoidea, together
with some obscure forms which, combining a stem with pentamerous symmetry
in the calyx, are supposed to be the forerunners of the Crinoidea. In the
Lower Silurian or Ordovician the two groups of the Cystoidea make their
appearance, possibly independently developed from either Carpoidea or
primitive Crinoidea, which in this period are present in unmistakable form.
In the Upper Silurian the Blastoidea appear, distinguishable from the most
regular Cystoidea only by their hydrospires. It seems practically certain
that they were developed from Cystoidea, and we follow Jaekel in believing
that they arose from Dichoporita. The Carpoidea do not extend beyond the
Ordovician, and by the end of the Carboniferous period Cystoidea and
Blastoidea die out, leaving only the Crinoidea, which at that period were
at their maximum development. From the Carboniferous to the present day the
Crinoidea have continually decreased, leaving in recent seas, as sole
representatives of the Pelmatozoa, only the few forms described at the
beginning of this chapter.
{601}CHAPTER XXI
ECHINODERMATA (_CONTINUED_): DEVELOPMENT AND PHYLOGENY
In Chapter XVI. it was stated that whilst a more or less perfectly
developed radial symmetry was one of the characteristic features of the
phylum Echinodermata when in the adult condition, yet in the immature or
larval condition the members of the group have a strongly marked bilateral
symmetry. In this feature larval Echinodermata resemble the other Phyla of
the animal kingdom which have a well-developed coelom, such as Annelida,
Mollusca, Vertebrata, etc. Since, then, the peculiar radial symmetry is
gradually acquired during the growth of the Echinoderm, we may possibly
discover by a close scrutiny of the life-history what is the nature and
meaning of this departure from the ordinary type of structure among
coelomate animals.