ECHINODERMATA.
THIS class consists of five orders, all of which are marine. They are,
with one exception, creeping animals, and the whole class is remarkable
for having most of their members and general structure either in fives
or multiples of five. Their skin is hardened by calcareous deposits,
sometimes of beautiful microscopic structure: they have a digestive
cavity, a vascular fluid system, and some distinct respiratory organs,
so that they are comparatively of a high grade.
_Echinodermata Asteroïdea._
The Asteroïdea, or Star-Fishes, which are the highest order, form two
natural families, the Stelleridæ and Ophiuridæ, which comprise
twenty-two genera.
The simplest form of the Stelleridæ is the common star-fish, with its
flat regularly five-sided disk. A tough membrane, strengthened by
reticulated calcareous matter, covers the back, and bends down along the
sides, while the under-side of the body or disk, on which the animal
creeps, is soft and leathery, with the mouth in its centre. In the other
genera, although the body is still a flat, five, equal-sided disk, the
angles are extended into long arms, broad whence they diverge from the
disk, but decreasing rapidly in width to their extremities, so that the
animal is exactly like a star with five long, equal, and flexible rays.
The backs of all the star-fishes are covered with most minute movable
spines, and with microscopic organs like minute pincers, called
pedicellariæ, which are diffused generally over the surface, and form
dense groups round the spines. They have a slender, contractile,
calcareous stem, and a head formed of two blades, which they continually
open and shut, the whole being coated with a soft external tissue. They
grasp anything very firmly, and are supposed to be used to free the
star-fish from parasites. In some species of Goniaster the pedicellariæ
resemble the vane of an arrow, and are so numerous as to give a villous
appearance to the skin of the back.
On the under-side of each ray of a star-fish, a central groove or furrow
extends throughout its whole length, and the semi-calcareous flexible
membrane which covers the back and rays not only bends down round the
sides of the rays, but borders both edges of the grooves. Upon these
edges ridges of small calcareous plates beset with spines are placed
transversely: they are larger near the mouth, and gradually decrease in
size as they approach the point of the ray.
Interior to the spines, these ridges are pierced by alternate rows of
minute holes for the long rows of feet, which diminish in size to the
end of the ray. The feet are contractile muscular tubes communicating
through the holes with internal muscular sacs, which are regarded as
their bases. The sacs are full of a liquid, and when the animal
compresses them the liquid is forced through the holes into the tubular
feet, and stretches them out; and when the muscular walls of the hollow
feet are contracted, the liquid is forced back again into the sacs, and
the feet are drawn in. The liquid is furnished by a circle of small
vascular tentacles, or sacs, surrounding the mouth, which are both
locomotive and prehensile. From these a canal extends through the centre
of each ray, which in its course sends off lateral branches to the bases
of the feet to supply them with liquid. The whole of this system of
vessels and feet are lined with vibratile cilia, which maintain a
perpetual circulation in the liquid.
The toothless mouth on the under-side of the disk dilates so as to admit
large mollusca with their shells. The short gullet and stomach are
everted, protruded through the mouth, and applied round the object to be
swallowed, which is then drawn in, digested, and the shell is discharged
by the mouth. However, in three orders of this family there is a short
intestine and vent. From the large stomach, which occupies the central
part of the disk of the star-fish, a couple of tubes extend to the
extremity of each ray, where they secrete a substance essential for
digestion: the stomach is in fact a radiating organ, partaking the form
of the animal it sustains.
A pulsatory vessel near the gullet propels the yellow blood into a
system of fine tubes, that are spread over the walls of the stomach and
its rays. Through these walls the blood receives a nutritious liquid,
which it carries with it into a network of capillary vessels, widely
extended throughout the body, being propelled by the contractile powers
of the vessels themselves, and after having supplied the tissues with
nourishment, it is carried by tubes to the point from whence it started,
to begin a new course. The capillary network passes immediately under a
portion of the skin of the star-fish, through which an exchange of the
respiratory gases takes place. Besides, the star-fishes breathe the
sea-water through numerous conical tubes, that project in patches from
the back. Through these tubes, which can be opened and shut, the water
is readily admitted into the cavity containing the digestive organs,
with which they are in communication. The star-fish slowly distends
itself with water, and then gives out a portion of it, but at no regular
time. The cavity is never empty of water, and as its lining is densely
bristled with cilia, their vibrations keep the vascular surface of the
digestive organs perpetually bathed with the respiratory medium.
The star-fishes have a radiating system of nerves suited to their form.
A ring of slender nerve-cords surrounds the mouth, from whence three
nerves are sent off at the commencement of each ray: two of these, which
are filaments, go to the organs in the central disk, while the middle
one, which is a great trunk, passes through the centre of the rays, and
terminates in a nerve-centre, or ganglion, placed under a coloured
eye-speck at their extremity. The structure of the rays, the eye-specks,
and the nerve-centres below them, are so similar, that they are merely
repetitions of one another; hence no nerve-centre can control the
others, but they are all connected by the ring encircling the mouth,
which is a common bond of communication. How far the movements of these
animals indicate sensation we have not the power to determine, but they
feel acutely, for the mouth, the feet, and especially the pedicellariæ,
are highly sensitive, and shrink on the least touch. The eye-specks are
probably sensitive to light, and as the star-fishes often feed on putrid
matter, they are supposed to be endowed with the sense of smell.
The family of the Ophiuridæ, or Snake Stars, are widely distributed in
the ocean. The genus Euryales with branching rays, and that of Ophiura
with simple rays, comprising the Brittle and Sand Stars, are abundant in
the British seas. In the sand stars there are cavities full of sand at
the points from whence the rays diverge, which appear like warts on the
surface of the disk. Their rays are exceedingly long, thin, and
flexible; they have no central groove nor feet, but they are employed as
organs of locomotion and prehension, for by their alternate strokes the
sand stars can elevate or depress themselves in the water, creep on the
bottom, and by twisting them round objects they can fix themselves,
firmly aided by spines or bristles on their edges. The Ophionyx has the
addition of movable hooks beneath bristled spines. The rays are bent by
the contraction of internal muscles, and extended again by the
elasticity of the external leathery coat. The Ophiuridæ, like the Luidia
fragilissima belonging to the preceding order, cast off a ray if
touched, and even all the five if rudely handled; but they can replace
them with as much ease. If only a fragment of a disk remains attached to
a ray the whole animal may be reproduced.
The Ophiuridæ have an internal calcareous skeleton or framework, in the
form of spicules, scattered in their tissues. They have a capacious
mouth with tentacles and ten small chisel-shaped teeth, five on each
side, which meet and close the mouth. The mouth is separated from the
stomach by a circular muscle that opens and shuts the passage, but no
canal diverges from the stomach through the rays. The nervous system and
the circulation of the blood are similar to those in the Stelleridæ; and
respiratory organs, in the form of from two to four plates, or lamellæ,
project from each of the spaces between the bases of the rays into the
central cavity, by which sea-water has free access to bathe the
digestive organs and aërate the blood.
The colour of the star-fishes, as well as of other marine invertebrate
animals, seems to be independent of light. The Ophiuridæ that had been
living at a depth of 1,260 fathoms in the North Atlantic were coloured,
though not a ray of light could reach their dark home, and those dredged
up from 100 to 300 fathoms on the coast of Norway were of brilliant
hues—red, vermilion, white, and yellow. In general, both plants and
animals of the lower kinds become of a sickly white when kept in
darkness.
The Stelleridæ are male and female, and form fertilized eggs of an
orange or red colour. These eggs are first converted into a mass of
cells and then into larvæ, not radiating symmetrically like their
parents, but of a bilateral form, the two sides being perfectly alike
and bordered by a ciliated fringe nearly throughout their whole length.
These two fringes are united by a superior and inferior transverse
ciliated band, and between the two the mouth is placed. A stomach,
intestine, and vent are formed; the creatures can provide for
themselves, and swim about as independent zooids. A young star-fish is
gradually developed by a succession of internal growths, part of the
original zooid is retained, and the rest is either thrown off or
absorbed; then the star-fishes having lost the power of swimming, crawl
slowly away and acquire their full size. There is great diversity in the
external form of the zooids of the different genera, as well as in the
portion of them retained in the adult star-fish.
Fossil star-fishes have a very wide range. They are found among the
earliest Silurian organic forms, but they scarcely bear any resemblance
to existing genera. The Ophiuridæ, fished up from the bottom of the
North Atlantic, come nearest to them. Five genera are found in the
Oolitic formation, all extinct; three genera range from the Lias to the
present seas; and five genera belonging to the Cretaceous period are
represented by living species.
_Echinodermata Crinoïdea._
The Crinoid Echinoderms, or Stone-Lilies, are like a tulip or lily on an
upright stem, which is firmly fixed to a substance at the bottom of the
sea. During the Jurassic period, miniature forests of these beautiful
animals flourished on the surface of the Oolite strata, then under the
ocean. Myriads of their fossil remains are entombed in the seas, and
extensive strata of marble are chiefly composed of them. Their hollow
joints are known in several parts of England as wheel stones, and as St.
Cuthbert’s beads on the Northumbrian coast, in honour of the patron
saint of Holy Island, where they abound. The Crinoïdea are of two kinds:
the Encrinites, which chiefly flourished in the Palæozoic period and are
now represented by a minute species (Rhizocrinus Lofotensis) lately
discovered on the coast of Norway by Professor Sars, have a smooth,
cylindrical, jointed stem; and the Pentacrinites, which began at the
Lias, and have a five-sided jointed stem, the present representative of
which is the Pentacrinus caput-Medusæ, found in the West Indian seas.
The hollow, five-sided, calcareous, jointed stem of the living
Pentacrinite is filled with a spongy substance, and supports a cup on
its summit, containing the digestive organs, mouth, and tentacles of the
animal. The cup is formed of a series of calcareous plates, and from its
margin five long many-jointed rays diverge, each of which is divided
into two-jointed branches. Groups of curled filaments, called cirri, are
placed at regular distances from the bottom of the stem to the extremity
of the rays, while, on the opposite side of the rays, there are groups
of feathery objects called pinnæ at each joint. Food is caught by the
tentacles and digested by the stomach and viscera at the bottom of the
cup, from whence vessels diverge through a system of canals in the axes
of the rays, pinnæ, and down the stem, all of which convey sea-water
mixed with nutritious liquid, for the nourishment of the animal.
The genus Comatula are star-fishes, believed to have alternately a fixed
and a free state. Mr. J. V. Thomson discovered that the Pentacrinus
Europæus is merely the fixed state of a Comatula. These star-fishes have
pairs of pinnæ placed at regular distances along their long-jointed
rays, and in the pinnæ sacs containing eggs are placed as far as the
fifteenth or twentieth pair. The eggs yield active ciliated larvæ, which
attach themselves in the form of flat oval disks to corallines and
sea-weeds. By degrees they develop a stem, about three-fourths of an
inch high, with twenty-four distinct joints. Its expanded top bears five
sulphur-coloured bifurcating rays with their pinnæ and dorsal cirri. A
mouth is formed in the centre with its tentacles, and a lateral
prominent vent. The actual change of a Pentacrine into a Comatula has
not been seen, but as the small Pentacrinites disappear in September, at
which season the Comatulæ appear, it is believed that when full grown
the top of the fixed Pentacrinite falls off and becomes a Comatula,
which swims backwards with great activity by striking the water
alternately with its long rays. The Pentacrinus caput-Medusæ, which is
fixed by its stem to sea-weeds and zoophytes, forms a most beautiful
object for the lower magnifying powers when viewed in a fluid by a
strong refracting light.
_Echinodermata Echinoïdea._
The family of Echinidæ, commonly known as Sea-Eggs or Sea-Urchins, have
a beautiful but complicated structure. The calcareous shell of an
Echinus is a hollow spheroid with large circular openings at each pole.
In the larger of the two, called the corona, the mouth of the animal is
situated; in the lesser circle the vent is placed. The spheroid itself
is formed of ten bands extending in a meridional direction from the
corona to the lower ring; that is, from one polar circle to the other.
Each band consists of a double row of pentagonal plates increasing in
size from the poles to the equator, nicely dovetailed into one another,
and the bands are neatly joined by a zigzag seam. Every alternate band
is perforated by a double series of minute double holes for the passage
of the tubular feet of the animal. The five perforated or ambulacral
bands have rows of tubercules parallel to the series of feet holes,
supporting spines movable in every direction. The five imperforated
bands are characterized by a greater number of spines, but there are
none within the polar circles. The spines may be long rods, or merely
prickles, or stout, club-shaped bodies, according to the genera.
[Illustration: Fig. 138. Section of Shell of Echinus. _a_, portions of a
deeper layer.]
The microscopic structure of the shell of the Echinus is everywhere the
same; it is composed of a network of carbonate of lime, with a very
small quantity of animal matter as a basis. In general, the network
extends in layers united by perpendicular pillars, but so arranged that
the open spaces, or meshes, in one layer correspond to the solid
structure in the next.
The spheroid of the Echinus is covered with spines, and both outside and
inside by a contractile and extensile transparent membrane, which
supports the shelly plates at the poles, and dips between the bands but
does not penetrate them. Its extensile nature admits of the addition of
calcareous matter to the edges of the plates when the animal is
increasing in size. The membrane lining the interior of the shelly globe
is tough; it encloses the digestive organs, and forms a muscular lip to
the mouth, which is armed with five triangular, sharp-pointed, white
teeth, and surrounded by five pairs of pinnate tubular tentacles. The
outer margin of the lip is fringed with a circle of snake-headed
pedicellariæ visible to the naked eye.
The five teeth, whose sharp tips meet in a point when closed, are
triangular prisms, the inner edge is sharp and fit for cutting. Each
tooth is planted upon a larger triangular socket, two sides of which are
transversely grooved like a file, and as these two sides are in close
contact with the sides of the opposite socket, the food when cut by the
small teeth is ground down by the sockets, and a salivary secretion
finishes the mastication. The sockets of the teeth are connected by ten
additional solid pieces, placed two and two between them, which
completes the pyramidal apparatus called Aristotle’s lantern; it
consists of forty solid calcareous pieces arranged in fives, and moved
by forty muscles attached to five calcareous ridges, and five arches
near the internal edge of the corona.
Five pairs of these muscles when acting together protrude and retract
the teeth; when acting separately they draw them to one side or to the
other; five pairs separate the five teeth, five pairs shut them, and the
remaining five pairs work the bruising machine. The masticated food
passes through a short gullet into the stomach, where it is digested,
and the indigestible part is carried by an intestine to the vent in the
smaller polar circle.
The smaller polar circle is formed of ten triangular plates, five are
attached to the bands containing the feet holes, and five to the
intermediate bands. The last five are perforated, and are the
reproductive plates: the other five are also perforated for the
discharge of the liquid that moves the tubular feet, and which, after
having circulated in the body, is no longer of use. In five of these
polar plates there are red specks, the rudiments of eyes, the only
organs of sense these creatures seem to possess except that of touch and
probably smell. The nervous system is a slender, equal-sided pentagon
round the gullet, from the sides of which five nerves are sent to the
muscles of the mouth, and others, extending along the ambulacral or feet
bands, end in nerve-centres under the eye-specks.
[Illustration: Fig. 139. Sucker-plate of Sea-Egg.]
[Illustration: Fig. 140. Section of a sucker-plate.]
The mechanism for extending and retracting the feet by a liquid, is the
same with that in the star-fishes, but the pores which admit the liquid
into the feet are double. The tubular feet swell at their extremity into
a fleshy sucker, within which there is a thin glassy reticulated rosette
(fig. 139), of which fig. 140 is a highly magnified segment. It is
perforated in the centre by a large round opening. The sea-urchins can
stretch their feet beyond the spines, and by means of the suckers they
can attach themselves even to smooth objects, or aided and directed by
their spines they roll themselves along with a rotatory motion head
downwards.
The circulation of the bright yellow blood is like that of the
star-fishes. It is aërated both internally and externally. The external
respiratory organs are short, branched, and highly vibratile bodies
attached in pairs to the oval extremities of the fine imperforated
bands.
There are pedicellariæ scattered among the spines of the sea-urchins
which are in constant motion, protruding themselves beyond the spines
and withdrawing again, snapping their pincers, and grasping firmly
anything that comes within their reach, or that is presented to them.
The pedicellariæ vary much in form and position in the different genera
of the Echinidæ; but they invariably consist of a long, slender,
calcareous stem, and generally tripartite head, the whole coated with a
gelatinous fibrous transparent substance. The head of the Pedicellaria
globosa is a formidable weapon; at the apex of each of its three
serrated and toothed blades there is a strong sharp spine directed
horizontally inwards, so that the three spines cross each other when the
blades close, which they do so energetically that nothing could escape
from such a grasp. The pedicellariæ are curious microscopic objects;
they are extremely irritable, and although their use is unknown, they
must be essential to the well-being of the animals, since hundreds are
scattered over their shells.
The spines of the Echinidæ vary in shape and structure in the different
genera and species. Those of the Scutella form merely a velvety pile. On
the common sea urchin the spines are simple, and shed twice in the year;
those on the Amphidetus are both club and spoon-shaped; and, on the
Cidaris, they are large formidable clubs moved by a ball and socket. All
the spines, whatever their form may be, are moved in that manner; for
there are little tubercules on the surface of the shell on which a cup
at the bottom of the spines is pressed down by the muscular skin which
covers the shell and spines, and by its contractile power it enables the
animal to move the spines in any direction.
The microscopic structure of the calcareous spines is often beautifully
symmetrical. Those of the Acrocladia mamillata consist of concentric
alternate layers of network and sheaths of pillars; so that a section of
the spine perpendicular to its axis exhibits a succession of concentric
rings like those of an exogenous tree. The cup at the bottom of the
spine is very dense network, and the last of a sheath of encircling
pillars form the ribs, sometimes seen on the exterior of the spines.
[Illustration: Fig. 141. Spine of Echinus miliaris.]
The spines of the Echinus miliaris, of which fig. 141 represents the
segment of a section highly magnified, are fluted columns of calcareous
glass, the grooves of which are filled with solid glassy matter curved
on the exterior. The innumerable hair-like objects attached to the
shells of some of the Echinidæ, the almost filamental spines of others,
and the pedicellariæ themselves, are formed of a regularly reticulated
substance. When the Echinidæ are stripped of their spines and all their
appendages, their shells show 2,400 plates united with the symmetry of a
tesselated pavement.
[Illustration: Fig. 142. Pluteus of the Echinus:—_a_, mouth; _b_,
stomach; _c_, echinoid disk; _d d d d_, four arms of the pluteus-body;
_e_, calcareous framework; _f_, ciliated lobes; _g g g g_, ciliated
processes of the proboscis.]
The Echinidæ are male and female, and the eggs are excluded through the
five perforated productive plates at the posterior end of the shell.
According to the observations of Prof. Fritz Müller the embryo, soon
after issuing from the egg, takes a form represented (magnified) in fig.
142.
All parts of this creature, which is called a Pluteus, are strengthened
by a framework of calcareous rods tipped with orange colour, all the
rest being transparent and colourless. It swims freely, back foremost,
by means of its cilia.
[Illustration: Fig. 143. Larvæ of Echinus in various stages of
development within the Pluteus, which is not represented:—B, disk with
the first indication of the cirrhi; C, disk with the origin of the
spines between the cirrhi; D, more advanced disk with the cirrhi, _g_,
and spines, _x_, projecting from the surface.]
While in this active state a circular disk (_c_, fig. 142), covering the
stomach (_b_, fig. 142), appears within it, which gradually expands, and
sends through the skin of the Pluteus spines, pedicellariæ, and
tubercules, ultimately developed into hollow feet. Then the feet are
pushed out and drawn in, the pedicellariæ (D, fig. 143) snap their
pincers; and while the half-formed Echinus is making these motions
within the Pluteus, the mouth and gullet of the Pluteus itself are in
constant activity; and, while it swims about, the unformed Echinus
within it gets a globular shape, the shell is formed, and when the
Echinus is complete, the rest of the Pluteus is thrown off, and the
young animal rolls away.
The free swimming larval zooids of the Echinodermata are generally
hyaline, and some are phosphorescent. The Pluteus is also the larval
zooid of the ophiurid star-fishes; they may be seen in great numbers on
the surface of the sea in August and September. The young star-fish is
formed in them by a process analogous to that described. The motions of
the Echinidæ are reflex; nothing indicates volition.
The fossil Echinidæ first appeared in the lower Ludlow limestone, and
attained their maximum in the Cretaceous strata. A species of Diadema,
with annulated hollow spines, common in the Chalk, still exists.
Numerous species of the genus Clypeaster, remarkable for their flattened
form, and known as lake urchins, are peculiar to the Tertiary strata and
existing seas; and, lastly, five species of Spatangidæ, heart-shaped
urchins, which lived in the Tertiary periods, still exist. In
consequence of the porous texture of the solid calcareous parts of the
Echinidæ, their fossil remains are commonly impregnated with pyrites or
silex, without altering their organic structure, so that they exhibit a
fracture like that of calcareous spar.
_Echinodermata Holothuroïdea._
The Holothuridæ, or Sea-Cucumbers, are of a higher organization than the
preceding Echinoderms. They are soft, worm-shaped, five-sided animals,
covered by a flexible, leathery integument or skin, in which are
imbedded a vast multitude of microscopic calcareous plates of
reticulated structure. The mouth, which is placed at one end of the
animal, is surrounded by ten bony plates forming a lantern, analogous to
that of the Echinus; they support branching, tubular, and retractile
tentacles, which encompass the mouth like a star. The tentacles are
connected with sacs at their bases, and are extended and retracted by
the injection of a watery liquid contained in them. Innumerable tubular,
suctorial feet, precisely similar to those of the Echinus, are protruded
and retracted through corresponding pores in the skin of the animal by a
watery liquid, in sacs, at their bases. The water is supplied by a
system of canals connected with an annular reservoir round the top of
the gullet, which is supplied with water by a bottle-shaped bag at the
mouth.
Besides transverse muscles, five pairs of muscles attached to the
lantern at the mouth, extend throughout the whole length of the animal.
Nerve-chords from the ring at the gullet accompany these, and such is
the irritability of this muscular system, that the Holothuriæ eject
their viscera when alarmed or caught; but they have the power of
reproducing them: sometimes they divide their whole body into parts.
The respiratory organs are two very long and beautifully arborescent
tubes veined with capillary bloodvessels. The circulation of the blood
is similar to that of the star-fishes, but more complicated.
The minute calcareous particles scattered independently in the tough
leathery skin of the Holothuridæ remain as fine dust when the flesh is
dissolved and washed away; but, upon microscopic observation, Mr. Gosse
found that the forms of these particles are remarkable for elegance,
regularity, and variety of structure, but that the normal form is an
ellipse of open work built up of five pieces of a highly refractive,
transparent, glassy material, having the shape of dumb-bells.
The Holothuriæ found under stones at low spring tides, on the British
coasts, are small; those dredged up from deep water are five or six
inches long, and not unlike a well-grown warty cucumber; they do not
form an article of food in Europe, but they are highly esteemed by the
inhabitants of the Indian Archipelago and in China, where many shiploads
of the trepang are imported annually. It is a species that swarms in the
lagoons of the coral islands, the reefs of the coral seas, and at
Madagascar. Some species are two feet long, and six or eight inches in
circumference.
The order of the Holothuridæ form eggs like all the other Echinoderms;
the larval zooid has the same form as that of the star-fishes, and
changes its form twice, while the members of the Holothuria are forming
within it; at last they combine with those of the zooid, and no part is
cast off.
_Echinodermata Synaptidæ._
The Synaptidæ are five-sided creatures, similar in structure to the
Holothuriæ, though more worm-like. The whole order, which consists of
the two genera of Synapta and Chirodota, have twelve calcareous plates
round the mouth, five of which are perforated for the passage of the
vascular water canals, which convey the liquid for the protrusion of the
feet.
[Illustration: Fig. 144. Skeleton of Synapta.]
The calcareous particles imbedded in the skin of the genus Synapta are
anchor-shaped spicules fixed to elliptical or oval plates, (fig. 144).
The plates are reticulated and sometimes leaf-shaped, and the flukes of
the anchors are either plain or barbed. All the anchors are fixed
transversely to the length of the animal, lying with great regularity in
the interspaces of the longitudinal muscular bands. Sometimes a thousand
anchors are crowded into a square inch, each elegant in form, perfectly
finished, and articulated to an anchor-plate, whose pattern as well as
that of the anchor itself is characteristic of the species to which it
belongs. In the Synapta digitata, which has four fingers and a small
thumb on each of its twelve oval tentacles, the anchors are but just
visible to the naked eye;[36] in all the other species they are
microscopic. Besides the anchors, the skin of the genus Synapta contains
innumerable smaller particles, ‘miliary plates,’ which are crowded over
the muscular bands. The muscular system of the Synapta digitata is so
irritable that, on being touched, it divides itself into a number of
independent fragments, each of which keeps moving for a time, and
ultimately becomes a perfect animal like its parent. Specimens of this
Synapta have been found on the southern coasts of England and in the
West of Scotland, but the genus is rare, although containing several
species in the British seas; it is more common in the Adriatic; but they
cannot be compared, as to size, with the great Synapta of Celebes, which
is sometimes a yard in length, and is known among the natives as the Sea
Serpent.
[Illustration: Fig. 145. Wheel-like Plates of Chirodota violacea.]
The calcareous particles imbedded in the skin of the allied genus
Chirodota are wheel-shaped when viewed with a microscope (fig. 145). One
species is British, but they are mostly inhabitants of warm seas. In
Chirodota violacea, a Mediterranean species, the skin is full of groups
of broad thin hyaline wheels lying upon one another and connected by a
fine thread. The wheels have five or six flat radiating spokes.[37] The
wheels are exceedingly small in the Chirodota lævis, and are arranged in
groups; in the C. myriotrochus they are imbedded in myriads, as the name
implies.
_Echinodermata Sipunculidæ._
The Sipunculidæ, which form the last order of the Echinoderms, consist
of several genera of marine worm-shaped animals which burrow in the
sand, and form a link between the Holothuridæ and the true sea-worms.
They have no calcareous particles in their flexible skins, nor have they
any tubular feet, or special respiratory organs, but a vascular liquid
is kept in motion in the internal cavity by the cilia with which it is
lined. The mouth of the Sipunculus is a kind of proboscis with a
circular fringed lip and two contractile vessels, supposed to serve for
raising the fringes. An alimentary canal extends to the end of the
animal, turns back again, and the intestine ends in a vent near the
mouth, so that the creature need not leave its burrow and expose itself
to enemies in order to eject the refuse of its food. The locomotive
larval zooids from the rose-coloured eggs undergo two metamorphoses; at
last the young Sipunculus unites with the zooid, and no part is thrown
off.