They may be close set and small, or few and large, and often bear spines of
the second order, or spinelets, attached to them. In _Asterias_ and its
allies they are {455}comparatively short, blunt tubercles, covered with
thick skin. In the Echinasteridae and Asterinidae they are short and blunt,
but they are very numerous and thick set. In the Solasteridae they are
long, and arranged in bundles diverging from a common base. Such bundles
may be termed sheaves, and starting from an arrangement like this, two
distinct lines of modification may be traced. Thus (1) the members of a
sheaf become connected by a web of skin, so that the sheaf becomes an
umbrella, and successive umbrellas may adhere, so that a supra-dorsal tent
is formed (a structure characteristic of the Pterasteridae), or (2) the
members of a sheaf may become arranged in a circle round a central vertical
axis so that a structure like a capstan is produced, which is called a
"paxilla" (characteristic of Astropectinidae, Porcellanasteridae, and
Archasteridae). The axis,[453] as shown by its development, represents the
plate which bore the bundle of spines. Again, the skeleton may consist of
plates with a close covering of granules (Pentagonasteridae, etc.). Lastly,
in _Porania_ spines are absent, the plates being deeply embedded in a thick
leathery skin.
[Illustration: FIG. 194.—Views of portions of the aboral surface of
different genera of Asteroidea in order to show the main varieties of
skeleton. A, _Solaster_, showing spines arranged in sheaves; B,
_Pteraster_, showing webs forming supra-dorsal membrane supported by
diverging spines; C, _Astropecten_, showing paxillae; D, _Nardoa_, showing
uniform plating of granules. × 8. (After Sladen.)]
{456}(_c_) PEDICELLARIAE.—These are to be looked on as spines of the second
order. In _Asterina_ and its allies they are not present, but groups of
little spines arranged in twos and threes, each group being attached to a
special small plate, are scattered over the aboral surface; and these on
irritation approach one another, and represent the rudiment out of which
pedicellariae have been developed. The most perfect form, termed
"forcipulate," in which there is a basal ossicle, is found in Asteriidae,
Brisingidae, Heliasteridae, Pedicellasteridae, Zoroasteridae,
Stichasteridae. There are two varieties of forcipulate pedicellariae, the
"crossed" and the "straight," which have been described on p. 432. In all
other cases the pedicellariae are devoid of the basal ossicle, and the two
or more spinelets forming the jaws are directly attached to one of the main
plates of the skeleton.
[Illustration: FIG. 195.—Different forms of pedicellariae (excluding the
forcipulate form, for which see Fig. 186). A, pectinate; B, pectinate; C,
valvate; D, pincer-shaped; E, alveolate, from the side; F, alveolate, from
above. × 10. (After Sladen.)]
The simplest variety is termed "pectinate"; these pedicellariae are
composed of two parallel rows of small spines opposed to each other. They
are found in the Archasteridae, and are hardly more advanced in structure
than the groups of spines found in _Asterina_. In _Leptogonaster_ and its
allies there are pincer-shaped pedicellariae composed of two curved rods
articulating with one of the plates of the skeleton, and also "alveolate"
pedicellariae, composed of two short prongs which are implanted on a
concave tubercle borne on one of the plates of the skeleton. In the
Antheneidae every plate of the ventral surface bears a large "valvate"
pedicellaria consisting of two horizontally elongated ridges, which can
meet one another. It is possible that valvate pedicellariae have been
derived from a pectinate form in which successive spinules of one row have
become adherent.
(_d_) AMBULACRAL SKELETON.—In every case, whether spines are developed
elsewhere or not, the adambulacral plates bear spines. Where the spines are
elsewhere represented by granules (_Nardoa_ and its allies) (Fig. 194, D)
the adambulacral spines are {457}short and blunt. The terms "monacanthid"
and "diplacanthid" are used to express the occurrence of one or two rows of
spines respectively on each adambulacral plate.
In the Zoroasteridae the adambulacral plates are curved, and are
alternately convex and concave towards the ambulacral groove, so that this
groove presents a wavy outline.
In the description of _Asterias_ it was pointed out that the first
adambulacral plates in adjacent radii are closely approximated to one
another, and bear spines which can to some extent form a trellis-work over
the mouth. In very many species not only is this the case, but the plates
themselves project inwards over the mouth so as to form prominent
"mouth-angles." This is not the case in the Asteriidae or the allied
families.
PAPULAE.—In Asteriidae and many allied families these organs are found both
on the upper and under surface of the disc, but in another large group
consisting of Astropectinidae, Pentacerotidae, and allied families, papulae
are only borne on the dorsal surface, and, in some cases, are restricted to
a few groups at the base of the arms. In most Asteroidea the papulae are
arranged singly, that is to say, each occupies one of the interspaces
between the plates of the skeleton, but in _Asterias_ and some other genera
they are arranged in tufts of two or three.
WATER-VASCULAR SYSTEM.—In its general structure this system of organs is
very constant, the two most important variations being found, one, in
Asteriidae and a few allied families, and the other, in the Astropectinidae
and the families allied to them.
The first of the variations alluded to concerns the number of the tube-feet
in a radius. In _Asterias_ and its allies these are so numerous that there
is not room for them one behind the other, but they follow one another in a
zigzag line, the transverse canals connecting them with the radial canals
being alternately longer and shorter. In this way the appearance of four
rows of tube-feet is produced, and the advantage of this increase in number
can be recognised by any one who has compared the quick movements of
_Asterias_ and the slow ones of a _Cribrella_, for instance.
The second important variation referred to is the complete loss of the
sucker of the tube-foot, and, concomitantly, the loss {458}of the power of
climbing. Starfish which have undergone this change live on sandy bottoms
and run over the surface of the sand. They are also incapable of forcing
asunder the valves of Molluscs, and hence are compelled to swallow their
prey whole.
"Polian vesicles," or stalked sac-like outgrowths of the water-vascular
ring, are absent from the Asteriidae, but are found in many families—the
Asterinidae, Solasteridae, Astropectinidae, for example. They project
outwards from the water-vascular ring in the interradii; when there are
several present in one interradius they often arise from a common stalk.
Cuénot believes that their sole function, like that of Tiedemann's bodies,
is to produce amoebocytes, but this appears unlikely. It is more probable
that they act as store-houses of fluid for the water-vascular ring.
[Illustration: FIG. 196.—Dissection of _Ctenodiscus_ to show the Polian
vesicles. _amp_, Ampullae of the tube-feet; _nerv.circ_, nerve-ring; _Pol_,
Polian vesicle; _sept_, interradial septum; _stone c_, stone-canal; _T_,
Tiedemann's body; _w.v.r_, water-vascular ring. × 1.]
The stone-canal is rarely repeated, but this occurs in the aberrant genus
_Acanthaster_, where there may even be several in one interradius, and each
stone-canal has an axial sinus, genital stolon, and madreporite annexed to
it. According to Cuénot, in _Asterias_, when 6-rayed specimens occur in a
species normally 5-rayed, there are two stone-canals, suggesting that the
repetition of stone-canals is a suppressed effort at multiplication by
division. This is also true of _Echinaster_, but in _Ophidiaster_ two
madreporites may occur in an individual with five arms. In the Asterinidae
the Y-shaped fold which projects into the cavity of the stone-canal is
feebly developed, whereas in the Pentacerotidae it meets the opposite side
of the stone-canal, and in _Culcita_ gives out branches which reduce the
cavity of the canal to a series of channels. In Echinasteridae and some
Asterinidae, and in Astropectinidae and Pentacerotidae the ampullae become
so deeply indented as to be almost divided into two, so that each tube-foot
has virtually two ampullae.
The ALIMENTARY CANAL has a remarkably constant structure. {459}The only
important variation from the type, as described in _Asterias_, is found
amongst the Astropectinidae and Porcellanasteridae, where the anus is
wanting. In _Astropecten_ the rectum and the rectal caeca still persist,
but in _Luidia_ even these have disappeared. The rectal caeca are
remarkably variable structures. In _Asterias_ there are two, but in
Pentacerotidae there are five forked caeca, in _Asterina_ five simple
caeca, and in the Echinasteridae and Astropectinidae one large flat
slightly 5-lobed caecum. In the Asterinidae the pyloric caeca are
remarkable for the size of the enlarged basal portion in each radius, which
serves as a reservoir for the juices secreted by the branched forks of the
caecum. In _Porcellanaster pacificus_ the pyloric caeca are vestigial, and
in _Hyphalaster moseri_ they are absent.[454]
The GENITAL ORGANS are, as we have seen, outgrowths from radial branches of
the genital rachis. In most species, as in _Asterias_, they are limited to
a single cluster of tubes on each branch of the rachis, but in the
Astropectinidae and Pentacerotidae each branch gives rise to a large number
of clusters, arranged in longitudinal series, each cluster having its
independent opening to the exterior.
ASEXUAL REPRODUCTION, as a regular occurrence, is not common amongst
Asteroidea. If, however, a Starfish loses some of its arms, it has the
power of regenerating the missing members. Even a single arm will
regenerate the whole Starfish. Now in some cases (Astropectinidae,
Linckiidae) Starfish will readily snap off their arms on irritation. In
_Linckia_ this occurs at regular intervals and the separated arm forms a
new individual. In one of the Asterinidae, _Asterina wega_, a small
Starfish with seven arms, transverse fission regularly occurs, a portion
with three arms separating from one with four. The same is believed to
occur in two species of _Asterias_, and as has already been pointed out,
the repetition of the madreporite and stone-canal is, in many cases,
possibly connected with this tendency to transverse fission.
CLASSIFICATION OF ASTEROIDEA.
Whilst there is considerable agreement amongst the authorities as to the
number of families, or minor divisions of unequivocal {460}relationship, to
be found in the class Asteroidea, there has been great uncertainty both as
to the number and limits of the orders into which the class should be
divided, and also as to the limits of the various species. The difficulty
about the species is by no means confined to the group Echinodermata; in
all cases where the attempt is made to determine species by an examination
of a few specimens of unknown age there is bound to be uncertainty; the
more so, as it becomes increasingly evident that there is no sharp line to
be drawn between local varieties and species. In Echinodermata, however,
there is the additional difficulty that the acquisition of ripe genital
cells does not necessarily mark the termination of growth; the animals can
continue to grow and at the same time slightly alter their characters. For
this reason many of the species described may be merely immature forms. In
proportion, however, as the collections from various localities increase in
number and size, difficulties connected with species will tend to
disappear.
The disputes, however, as to the number of orders included in the
Asteroidea proceed from a different cause. The attempt to construct
detailed phylogenies involves the assumption that one set of structures,
which we take as the mark of the class, has remained constant, whilst
others which are regarded as adaptive, may have been developed twice or
thrice. As the two sets of structures are often of about equal importance
it will be seen to what an enormous extent the personal equation enters in
the determination of these questions.
Where, as in Asteroidea, the internal organisation is very uniform, the
best method of classification is to take as our basis the different methods
in which the demands of the environment have been met. It is in this way,
we hold, that divergence of character has been produced, for whilst species
may differ in trifling details, families and orders differ in points of
functional importance. The fact that one of the orders may have sprung from
several allied species instead of one may be admitted, and at the same time
the hopelessness of trying to push phylogenetic inference into details
asserted.
Sladen, in his Monograph of the Asteroidea collected by the "Challenger"
expedition, took for the basis of his system the presence or absence of
distinct pavement-like marginal plates along the edges of the arms and the
restriction of the papulae to {461}the aboral surface, or their
distribution over the whole surface of the body. What connexion, if any,
the presence of these pavement-like plates has with the habits it is
impossible to say, but it is unlikely to be of the high importance with
which it was regarded by Sladen, for in the same family we have genera with
inconspicuous marginals (_Asterina_) and others with conspicuous marginals
(_Palmipes_). The restriction of the papulae to the back also varies within
the same family (Linckiidae), and whilst, on the whole, it is perhaps a
primitive arrangement, it is in many cases connected with burrowing habits,
which can scarcely be deemed to have been the original mode of life of the
class.
A far better basis is supplied by the system of Perrier,[455] who divides
the Asteroidea into five orders according to the character of the dorsal
skeleton; and this classification really corresponds with the different
habits assumed by groups of Asteroidea in order to meet what must be
regarded as one of their chief dangers, viz. assaults by other animals,
especially parasites, on their soft and delicate skins. Since the food (so
far as is known) of all Asteroidea is more or less similar, the great
differentiating factor in their development must have been the means they
adopt to shelter themselves from their enemies. Perrier's classification,
which we shall adopt, is as follows:—
ORDER 1. SPINULOSA.—Asteroidea in which the plates of the dorsal skeleton
bear spines arranged singly or in groups. The tube-feet have suckers and
there are no pedicellariae. Marginals sometimes conspicuous, sometimes
rod-like.
ORDER 2. VELATA.—Asteroidea in which the dorsal surface of the animal is
concealed from view by a false membrane composed of the webs of skin
stretched between diverging groups of spines united at the base with one
another. No pedicellariae. Tube-feet with suckers.
ORDER 3. PAXILLOSA.—Asteroidea in which the dorsal surface is beset with
paxillae (upright spines bearing two or three circles of horizontal
spinelets). Pedicellariae, when present, few, and never of the forcipulate
variety; often absent. Marginals large. Papulae only on dorsal surface.
Tube-feet mostly devoid of suckers.
ORDER 4. VALVATA.—Asteroidea in which the dorsal surface {462}is protected
by plates covered with a mail of minute granules. Pedicellariae of the
valvate or alveolate type. Marginals large.
ORDER 5. FORCIPULATA.—Asteroidea in which the dorsal surface is beset with
small spines surrounded by numerous forcipulate pedicellariae. Tube-feet
with suckers and arranged in four rows. Marginals rod-like and
inconspicuous.
ORDER I. SPINULOSA.
This is by far the most primitive order of Asteroidea. The tube-feet are
arranged in two rows only, and there is no special means of protecting the
back, other than the small close-set plates bearing spines, with which it
is covered. In some cases, as _Asterina_, these spines have a tendency to
converge when irritated, and thus act somewhat like pedicellariae. This
circumstance suggests strongly the manner in which pedicellariae have been
developed from small groups of spines. The order is divided into six
families, of which four have common representatives on the British coast.
FAM. 1. ECHINASTERIDAE.—Spinulosa in which the aboral skeleton is composed
of close set plates bearing comparatively small spines. This family is
represented on the British coasts by the beautiful scarlet Starfish
_Cribrella_ (_Henricia_) _sanguinolenta_. It is also found on the Norwegian
coast and on the east coast of North America. On the Pacific coast it is
replaced by a larger species, _C. laeviuscula_. The narrow ambulacral
grooves and sluggish movements at once distinguish it from the Starfish
described as the type. Indeed, all the Spinulosa seem to be slow in their
movements in contrast to the comparatively active _Asterias_ and its
allies. _Cribrella_ is remarkable for its large eggs, which have a rapid
development. The larva never swims at the surface but glides only for a
short time over the bottom. _Echinaster_ is an allied genus in which each
plate bears a single somewhat enlarged spine. It possesses on the skin of
the aboral surface numerous pits lined by glandular walls, which probably
secrete a poisonous fluid which defends it. _Acanthaster_ has thorny
spines, more than ten arms, and several stone-canals and madreporites.
FAM. 2. SOLASTERIDAE.—Spinulosa in which the aboral skeleton is a network
of rods. Spines arranged in diverging bundles {463}(sheaves) attached to a
basal button. This family includes the well-known "Sun-stars," with
numerous arms and a wide peristome. There are two species found on both
sides of the Atlantic. _Solaster papposus_, with thirteen or fourteen arms
and long bundles of spines on the dorsal surface, which is of an orange
colour variegated with yellow, and _S. endeca_ with eleven rays and shorter
spines and of a reddish violet colour. _Rhipidaster_ has eight arms. Some
genera have, however, only five arms, as, for instance, _Peribolaster_ and
_Korethraster_ (Fig. 197). In this family there are conspicuous "Polian
vesicles" attached to the water-vascular ring.
[Illustration: FIG. 197.—_Korethraster hispidus._ × 2. (From Wyville
Thomson.)]
FAM. 3. ASTERINIDAE.—Spinulosa in which the aboral skeleton consists of
overlapping plates, each bearing a few small spines. The common British
representative of this family is the small _Asterina gibbosa_, in which the
arms are short and stout and of somewhat unequal length. This Starfish
differs from most of its allies in being littoral in its habit. At low tide
on the south and west coasts of England it can be found on the underside of
stones feeding on the Sponges and Ascidians with which they are covered.
Like _Cribrella sanguinolenta_ this species has a modified development. The
larva resembles that of _Cribrella_, and the larval stage only lasts about
a week. Owing to the fact that {464}_Asterina_ lays its eggs in accessible
localities, its development has been more thoroughly worked out than that
of any other species. _Palmipes membranaceus_, an animal of extraordinary
thinness and flatness, is sometimes dredged up off the coast of Britain in
deeper water. Its arms are so short that the general form is pentagonal.
The infero-marginal plates are long and rod-like, and form a conspicuous
border to the body when viewed from below.
FAM. 4. PORANIIDAE.—Spinulosa allied to the Asterinidae but possessing a
thick gelatinous body-wall in which the plates and spines are buried, the
marginals forming a conspicuous border to the body. This family is
represented in British waters only by _Porania pulvillus_, a cushion-shaped
Starfish with very short arms and of a magnificent reddish-purple colour.
It is occasionally, but rarely, exposed at low tide.
FAM. 5. GANERIIDAE.—Spinulosa allied to the Asterinidae but distinguished
by the large marginals and by the fact that the skeleton of the oral
surface consists of plates each bearing a few large spines. _Ganeria_,
_Marginaster_.
FAM. 6. MITHRODIIDAE.—Spinulosa with a reticulate aboral skeleton. The
spines are large and blunt, covered with minute spinules. _Mithrodia_, sole
genus.
These last two families are not represented in British waters.
ORDER II. VELATA.
This is a very extraordinary group of Starfish, about the habits of which
nothing is known, since they all live at very considerable depths. Their
nearest allies amongst the Spinulosa must be looked for amongst the
Solasteridae. If the sheaves of spines with which the latter family are
provided were to become adherent at their bases, and connected with webs of
skin so as to form umbrella-like structures, and if then these umbrellas
were to become united at their edges, we should have a supra-dorsal
membrane formed such as is characteristic of the order.
FAM. 1. PYTHONASTERIDAE.—Velata in which each sheaf of spines is enveloped
in a globular expansion of the skin and is not united with the neighbouring
sheaves. _Pythonaster_, sole genus.
FAM. 2. MYXASTERIDAE.—Velata with numerous arms in which the sheaves of
spines are long and form with their connecting "umbrellas" web-like
expansions which do not fuse with one another. _Myxaster_, sole genus.
{465}[Illustration: FIG. 198.—Aboral view of _Pteraster stellifer_. _mars_,
Dorsal brood-pouch, × 1½. (From Sladen.)]
[Illustration: FIG. 199.—Oral view of _Hymenaster pellucidus_. × 1. (From
Wyville Thomson.)]
{466}FAM. 3. PTERASTERIDAE.—Velata in which the membranes supported by the
sheaves of spines are united so as to form a continuous supra-dorsal tent.
The Pterasteridae are represented in British waters by a single species,
_Pteraster militaris_, which is occasionally dredged in deep water off the
British coast, and is found also in the Norwegian fjords and off the east
coast of Canada. This interesting Starfish has five short, blunt arms, and
its general appearance at first sight recalls that of _Asterina_. Closer
inspection reveals the "false back." The anus is surrounded by five
fan-like valves, supported by spines (Fig. 198), underneath which is a
space in which the young complete their development, _Pteraster_ being one
of the genera in which the normal larval form is not developed. The
tendency towards the union of adjacent spines by webs is deeply rooted in
the organisation of the animal. It is seen on the under side where the
spines borne by the ventral plates are united so as to form transverse
combs. In _Hymenaster_ (Fig. 199) the spines borne by the ventral plates
are long and free.
ORDER III. PAXILLOSA.
This is an exceedingly well-marked order. The armature of the upper surface
consists of paxillae. These organs as already mentioned are probably to be
traced back to sheaves of spines like those of the Solasteridae. The same
end as that striven after in the case of the Velata has been attained, but
in a different way. The horizontal spinelets of the paxillae meet one
another and form a close-fitting mail which is almost as efficient a
protection as the webs and umbrellas of the Velata. Pedicellariae are
occasionally present, but they are always of the pectinate or pincer
variety, never forcipulate.
FAM. 1. ARCHASTERIDAE.—Paxillosa in which the anus is still retained and in
which the tube-feet have suckers.
The Archasteridae are a most interesting family. Thus _Pararchaster_ has no
true paxillae, but only small isolated groups of spines. The pectinate
pedicellariae are composed each of two parallel rows of somewhat smaller
spines. The members of this family are to some extent intermediate in
structure between the {467}Spinulosa, such as Echinasteridae, and the other
families of the Paxillosa—some genera, indeed, might almost be classed as
Spinulosa. At the same time they are apparently closely allied with the
more primitive Valvata such as _Astrogonium_ and its allies, some of which
have paxillae on the upper surface; although the retention of the anus and
of the suckers on the tube-feet (in which characters they agree with the
Archasteridae) distinguishes them from the more typical Paxillosa, in which
both anus and suckers are lost. _Archaster_ (Figs. 200, 201).
_Leptogonaster._
[Illustration: FIG. 200.—Aboral view of _Archaster bifrons_. × ¾. (From
Wyville Thomson.)]
FAM. 2. ASTROPECTINIDAE.—Paxillosa which have lost the anus, but which
possess neither aboral protuberance nor interradial grooves. The marginal
plates are thick, covered with spinules and placed horizontally. The
tube-feet have no suckers.
This family is the only one of the order which occurs in British waters,
where it is represented by two genera, _Astropecten_ and _Luidia_. In
_Astropecten_ the inferior marginal plate is in {468}immediate contact with
the adambulacral, whilst in _Luidia_ it is separated from it by a small
intermediate plate.
[Illustration: FIG. 201.—Oral view of _Archaster bifrons_. × ¾. (From
Wyville Thomson.)]
_Astropecten irregularis_ is a very common species on the coast of Britain,
and a study of its habits when in captivity has thrown a great deal of
light on many obscure points in the anatomy of the Paxillosa. Owing to the
loss of suckers it is unable to climb over rocks and stones like the
ordinary species, but it runs over the surface of the hard sand in which it
lives by means of its pointed tube-feet. The arms are highly muscular, and
the animal when laid on its back rights itself by throwing the arms upwards
and gradually overbalancing itself. The loss of suckers has also rendered
_Astropecten_ and its allies incapable of feeding in the manner described
in the case of _Asterias rubens_. They are unable forcibly to open the
valves of shell-fish, and the only resource left to them is to swallow
their prey whole. The mouth is consequently wide, and the {469}unfortunate
victims, once inside the stomach, are compelled by suffocation to open
sooner or later, when they are digested.[456]
[Illustration: FIG. 202.—Oral view of _Psilaster acuminatus_. × 4/3.
_adamb_, Adambulacral spines; _pax_, paxillae; _pod_, pointed tube-feet
devoid of sucker. (After Sladen.)]
Many interesting experiments have been made on _Astropecten_ by Preyer and
other investigators, but one important fact[457] has escaped their notice,
that _Astropecten_, when at rest, lies buried in the sand, whilst the
centre of the aboral surface is raised into a cone which projects above the
surface. On the sides of this cone the few papulae which this species
possesses are distributed. This raising of the aboral surface is obviously
an expedient to facilitate respiration. It loosens the sand over the region
of the papulae, and thus allows the water to have access to them. We can
thus understand how the restriction of the papulae to the dorsal surface,
so characteristic of the Paxillosa, is not always as Sladen imagined, a
primitive characteristic, but often an adaptation to the burrowing habits
which in all probability are characteristic of the whole order. In both
_Luidia_ and _Astropecten_ Cuénot has described short spines covered with
cilia in {470}the interspaces between the marginal plates, these also
subserve respiration by drawing a current of water over the gills.
_Psilaster_ (Fig. 202).
FAM. 3. PORCELLANASTERIDAE.—Paxillosa which have lost the anus. There is a
conical prominence in the centre of the dorsal surface termed the
epiproctal cone, and in the interradial angles there are vertical grooves
bordered by folds of membrane produced into papillae, the so-called
"cribriform organs." The marginal plates are thin and form the vertical
border of the thick disc. The tube-feet have no suckers.
[Illustration: FIG. 203.—_Porcellanaster caeruleus._ A, aboral view; B,
oral view, × 1. (From Wyville Thomson.)]
Comparing the Porcellanasteridae with the Astropectinidae we see at once
that the "epiproctal cone" is a permanent representative of the temporary
aboral elevation in _Astropecten_, and we are inclined to suspect that the
cribriform organs are grooves lined with cilia which keep up a respiratory
current like the ciliated {471}spines of _Luidia_. In all probability the
Porcellanasteridae are more habitual burrowers than even the
Astropectinidae.
_Ctenodiscus_ (Fig. 196), a genus in which there is a short epiproctal cone
and numerous feeble cribriform organs in each interradius, is found in deep
water north of the Shetland Islands. _Porcellanaster_ (Fig. 203) is a more
typical genus, with one large cribriform organ in each interradius.
_Hyphalaster_ has long arms, on which the supero-marginal plates meet
above.
ORDER IV. VALVATA.
The Starfish included in this order are characterised by the absence of
prominent spines and by the superficial covering of minute granules. The
skeleton consists, in most cases, of plates, and these plates with their
covering of granules probably represent the first stage in the evolution of
paxillae.
The tube-feet possess well-developed suckers. No members of this order can
properly be said to be British.
FAM. 1. LINCKIIDAE.—Valvata with long arms, the marginals being developed
equally throughout the whole length. These Starfish are distinguished by
their long narrow arms and small disc. It is possible that these forms, so
different in many respects from the other families of the order, have been
directly derived from the long-armed Echinasteridae. _Ophidiaster_,
_Nardoa_, _Linckia_.
FAM. 2. PENTAGONASTERIDAE.—Valvata with short arms, the marginals being
especially developed at the base and in the interradial angles. The aboral
skeleton consists of close-fitting plates. _Pentagonaster_ (Fig. 204),
_Astrogonium_.
FAM. 3. GYMNASTERIDAE.—Valvata allied to the foregoing but distinguished by
possessing a very thick skin in which the plates are completely buried.
_Dermasterias_, _Asteropsis_.
FAM. 4. ANTHENEIDAE.—Valvata with short arms. The dorsal skeleton is
reticulate and each ventral plate bears one or several large valvular
pedicellariae (Fig. 195, C). _Hippasterias_, _Goniaster_.
FAM. 5. PENTACEROTIDAE.—Valvata with arms of moderate length. The dorsal
skeleton is reticulate but the ventral plates bear only small pedicellariae
or none. The upper marginals are smaller than the ventral ones.
The Pentacerotidae include both short-armed and long-armed {472}forms.
Amongst the former is _Culcita_, in which the body is a pentagonal disc,
all outer trace of the arms being lost; _Pentaceros_ is a long-armed form.
[Illustration: FIG. 204.—_Pentagonaster japonicus._ × ⅔. (After Sladen.)]
The family Pentagonasteridae furnishes the key to the understanding of most
of the forms contained in this order. It contains genera such as
_Astrogonium_ which possess on the back unmistakable paxillae, whilst on
the under surface they have the characteristic covering of granules; these
genera seem to be closely allied to the short-armed species of the
Archasteridae, from which they are distinguished chiefly by the granular
covering of the marginals. From a study of these cases it seems clear that
the plates of the dorsal skeleton of the Valvata correspond to the
supporting knobs of the paxillae much broadened out, and the granules
correspond to the spinelets of the paxillae increased in number and
diminished in size.
{473}As mentioned above, Ludwig has proved that the paxillae develop in the
life-history of the individual out of ordinary plates, the axis of the
paxilla representing the plate.
ORDER V. FORCIPULATA.
This order, which includes the most highly developed members of the class
Asteroidea, is at once distinguished by the possession of forcipulate
pedicellariae which, as we have seen, possess a well-marked basal piece
with which the two plates articulate. The pedicellariae are consequently
sharply marked off from the spinelets, and no intermediate forms occur. The
first conjoined adambulacrals, which in other orders form the "teeth" or
mouth-angles, do not here project beyond the first pairs of ambulacral
plates.
FAM. 1. ASTERIIDAE.—Forcipulata in which the tube-feet are apparently
arranged in four rows. Aboral skeleton a loose reticulum.
The general features of the family Asteriidae have been explained in the
description of _Asterias rubens_ (p. 432). There are five well-marked
species of the genus found on the British coasts. Of these _A. glacialis_
is found chiefly in the south-western parts of the English Channel. It is a
large Starfish of a purplish-grey colour, with large spines surrounded by
cushions of pedicellariae arranged in one or two rows down each arm. _A.
muelleri_ resembles the foregoing species, but is of much smaller size, and
is further distinguished by having straight pedicellariae in the
neighbourhood of the ambulacral groove only. It is found on the east coast
of Scotland, and carries its comparatively large eggs about with it until
development is completed. _A. rubens_ is the commonest species, and is
found on both east and west coasts. Its colour is a bright orange, but
varies to almost a straw colour. It is at once distinguished from the
foregoing species by the spines of the dorsal surface, which are small and
numerous, an irregular line of somewhat larger ones being sometimes seen
down the centre of each arm. _A. murrayi_ is a peculiar species restricted
to the west coast of Scotland and Ireland. It has flattened arms, with
vertical sides, and only three rows of small spines on the dorsal surface.
It is of a violet colour. _A. hispida_ is also a western species. It is a
{474}small Starfish with short stout arms; there are no straight
pedicellariae, and only a few sharp spines on the dorsal surface.
On the eastern coast of North America there are several species of
_Asterias_, of which the most noteworthy is the 6-rayed _A. polaris_ of the
Gulf of St. Lawrence. This species exhibits a marvellous range of
colour-variation, ranging from bluish-violet through purple to red and
straw-coloured. This variation seems to show that colour, as such, is of no
importance to the animal, but probably depends on some compound of slightly
varying composition which is being carried by the amoebocytes towards the
exterior. On the Pacific coast there is a rich fauna of Starfish, among
which we may mention as members of this family _Asterias ochracea_, a large
violet species, so strong that it requires a severe wrench to detach it
from the rock, and _Pycnopodia_ with twenty-two arms.
FAM. 2. HELIASTERIDAE.—Forcipulata allied to the Asteriidae, but with very
numerous arms and double interradial septa. _Heliaster._
FAM. 3. ZOROASTERIDAE.—Forcipulata with the tube-feet in four rows at the
base of the arm, in two rows at the tip. Aboral skeleton of almost
contiguous plates bearing small spines or flattened scales. _Zoroaster_,
_Pholidaster_.
FAM. 4. STICHASTERIDAE.—Forcipulata with the tube-feet in four rows. Aboral
skeleton of almost contiguous plates covered with granules. _Stichaster_,
_Tarsaster_.
The Stichasteridae and Zoroasteridae have acquired a superficial
resemblance to some of the long-armed Valvata, from which they are at once
distinguished by their pedicellariae. It would be exceedingly interesting
if more could be found out concerning the normal environment of these
animals; it might then be possible to discover what is the cause of the
assumption of this uniform mail of plates.
FAM. 5. PEDICELLASTERIDAE.—Forcipulata with two rows of tube-feet. The
aboral skeleton bears projecting spines surrounded by cushions of straight
pedicellariae. _Pedicellaster_, _Coronaster_.
FAM. 6. BRISINGIDAE.—Forcipulata with numerous arms and only two rows of
tube-feet. Aboral skeleton largely rudimentary and confined to the base of
the arms. The small blunt spines are contained in sacs of skin covered with
pedicellariae.
The Brisingidae, including _Brisinga_ and _Odinia_, are a very
{475}remarkable family, chiefly on account of the smallness of the disc and
of the extraordinary length of the arms. The arms have what we must
consider to have been the primitive arrangement, since there is no lateral
adhesion between them, and interbrachial septa are consequently entirely
absent. The reduction of the skeleton is a very marked peculiarity and,
like the tendency to the reduction of the skeleton of deep-sea fish, may
stand in some relation to the great pressure under which the animals live.
[Illustration: FIG. 205.—Aboral view of _Odinia_. × ⅔. (After Perrier.)]
FOSSIL ASTEROIDEA.
The Asteroidea occur somewhat plentifully as fossils. In the Lower Jurassic
_Asterias_, _Astropecten_, _Luidia_, _Solaster_, and _Goniaster_ have
already made their appearance. In the Cretaceous {476}_Pentaceros_ appears.
In the older rocks occur a number of forms of different character from any
now existing. Of these _Aspidosoma_ (Fig. 206), with short lancet-shaped
arms sharply distinguished from the disc and continued along its under
surface, seems to be intermediate between Asteroidea and Ophiuroidea. The
skeleton of the arm is composed of alternating ambulacral ossicles bordered
by adambulacral ossicles, which are at the same time marginals and sharply
distinguished from the marginals forming the edge of the disc.
_Palaeaster_, on the other hand, is a true Asteroid; there are marginals
distinct from the adambulacrals, but the disc is reduced to its smallest
dimensions, there being only one plate on the ventral side of each
interradius. There are a number of genera (_Palaeocoma_, for instance) with
a large disc and very short arms and very shallow ambulacral grooves; all
have alternating ambulacral plates. Some genera appear to have had the
madreporite on the ventral surface of an interradius. On the other hand, in
the Devonian occurs _Xenaster_, which was a fairly normal Asteroid, with
pavement-like marginals, deep ambulacral grooves, and broad arms.
[Illustration: FIG. 206.—Three views of _Aspidosoma_, a fossil Asteroid. A,
oral view; B, aboral view of one arm; C, enlarged view of a portion of the
ambulacral groove. _adamb_, Adambulacral plate; _amb_, ambulacral plate;
_marg_, marginal plate; _pod_, aperture for extension of tube-foot.]
Thus it will be seen that already in Jurassic times the three orders,
Forcipulata, Paxillosa, and Spinulosa were differentiated from each other,
but how these are related to the older Palaeozoic forms it is at present
impossible to say.
{477}CHAPTER XVII
ECHINODERMATA (_CONTINUED_): OPHIUROIDEA = BRITTLE STARS
CLASS II. OPHIUROIDEA
The second class of Eleutherozoa are familiarly known as "Brittle Stars,"
on account of their tendency, when seized, to escape by snapping off an
arm, although this habit is by no means confined to them, but is shared in
a marked degree by many Asteroidea, such as _Luidia_, for instance. Like
the Asteroidea, they are "starfish," that is to say, they consist of a disc
and of arms radiating from it; but the scientific name Ophiuroidea really
expresses the great dominating feature of their organisation. Literally it
signifies "Snake-tail" (ὄφις, snake; οὐρά, tail), and thus vividly
describes the wriggling, writhing movements of the long thin arms, by means
of which the Ophiuroid climbs in and out of the crevices between the stones
and gravel in which it lives. This feature, viz. the effecting of movement
by means of muscular jerks of the arms, instead of by the slow protrusion
and retraction of the tube-feet, is the key to the understanding of most of
the points wherein the Brittle Stars differ from the true Starfish.
Asteroidea and Ophiuroidea agree in the common ground-plan of their
structure, that is, they both possess arms; but the most obvious difference
in their outer appearance is that whereas in Asteroidea the arms merge
insensibly into the disc, in Ophiuroidea the disc is circular in outline
and is sharply marked off from the arms. Closer inspection shows that in
the Ophiuroid the arms are continued inwards along grooves, which run on
the under surface of the disc, and that they finally coalesce to form a
buccal framework surrounding the mouth. In {478}the very young Ophiuroid
the arms melt into a small central disc, as in the Starfish, but the disc
of the adult is made up of a series of interradial dorsal outgrowths which
meet one another above the arms.
[Illustration: FIG. 207.—Aboral view of _Ophiothrix fragilis_. × 1. _r_,
Radial plate.]
{479}[Illustration: FIG. 208.—Oral view of the disc of _Ophiothrix
fragilis_. _g.b_, Opening of the genital bursa; _m.p_, madreporite; _pod_,
podia; _t.p_, tooth-papillae; _v.p_, ventral plates of the arms. × 1.]
One of the commonest British Ophiuroids is _Ophiothrix fragilis_ (Figs.
207, 208), which is found in swarms in shallow water off the west coast of
England and Scotland. We may therefore select it as the type, and, since
the ARM is the most characteristic organ of an Ophiuroid, we may commence
by studying it. Speaking generally, an Ophiuroid either drags itself
forward by two arms and pushes itself by the other three (Fig. 207),[458]
or else it drags itself by one and pushes with the other four (Fig. 217).
The arms during this process are bent into characteristic curves, by the
straightening of which in the posterior arms the animal is pushed onwards,
whilst the intensification of these curves in the anterior arms causes the
animal to be dragged forwards. The grip of the arm on the substratum is
chiefly in the distal portion of the curve. The alteration of the curvature
is due to the contraction of the muscles on one side of the arms. There is
no ambulacral groove such as is found on the under side of the arms of all
Asteroidea, for the arm is completely ensheathed by four series of plates,
an upper row of dorsal plates, an under row of ventral plates, and two
lateral rows of lateral plates. The last named, which in all probability
correspond to the adambulacral plates of Starfish, bear each a transverse
row of seven spines with roughened surfaces; these enable the animal to get
a grip on the substratum over which it moves. The podia in Ophiuroidea are
termed "tentacles"; they are totally devoid of suckers, being simple
conical papillae used as sense-organs, and are of little, if any, service
in locomotion. They issue from openings called "tentacle-pores" situated
between the edges of the {480}ventral and lateral plates, guarded each by a
valve-like plate called the "tentacle-scale." In _Ophiothrix_ they are
covered with sense-organs, each consisting of a hillock-like elevation of
the ectoderm, in which are cells carrying long stiff sense-hairs. In most
Ophiuroids such organs are not present, though abundant scattered
sense-cells occur, and the outer surface of the tube-feet and the lining of
certain pockets called "genital bursae" (Fig. 208, _g.b_) are the only
portions of the surface where the ectoderm persists. Everywhere else,
although present in the young, it disappears, leaving as remnants a few
nuclei here and there attached to the under side of the cuticle.[459]
[Illustration: FIG. 209.—Diagrammatic transverse section of the arm of an
Ophiuroid. _coe_, Dorsal coelomic canal; _ect_, ectoderm covering the
tube-foot; _ep_, epineural canal; _gang.p_, pedal ganglion; _L_,
nerve-cord; _musc_, longitudinal muscles attaching one vertebra to the
next; _nerv.rad_, radial nerve-cord; _perih_, radial perihaemal canal;
_pod_, podium (tube-foot); _sp_, lateral spines; _w.v.r_, radial
water-vascular canal.]
The greater part of the section of the arm is occupied by a disc-like
ossicle called the "vertebra." Each vertebra articulates with its
predecessor and successor by cup-and-ball joints, and it is connected to
each of them by four powerful longitudinal muscles. Above, its outline is
notched by a groove, in which lies an extension of the coelom of the disc
(Fig. 209, _coe_), but contains no outgrowth of the alimentary canal, as is
the case in Asteroidea. The vertebra is also grooved below, and in this
lower groove are contained the radial water-vascular canal {481}(Fig. 209,
_w.v.r_), and below it perihaemal canals as in Asteroidea; below this again
the radial nerve-cord (_L_), and beneath this again a canal called the
"epineural canal" (_ep_), which represents the missing ambulacral groove.
This canal in the very young Brittle Star is an open groove, but becomes
closed by the approximation of its edges. The vertebra, which has a double
origin, represents a pair of fused ambulacral ossicles. In _Ophiohelus_
these are only slightly adherent to one another (Fig. 216).
[Illustration: FIG. 210.—Proximal and distal views of the three types of
vertebra found amongst Ophiuroidea. A, _Ophioteresis_, a type of the
Streptophiurae (after Bell), × 24; B, _Astroschema_, a type of the
Cladophiurae (after Lyman), × 10; C, _Ophiarachna_, a type of the
Zygophiurae (after Ludwig), × 3. The upper figure in all cases represents
the distal aspect, the lower the proximal aspect of the vertebra. _v.g_,
Ventral groove.]
When the surface of a vertebra is examined it is found that it can be
divided into a thin border, to which are attached the four muscles by which
it is connected to its successor and predecessor, and a central portion, on
which are situated the knobs and pits, by means of which it articulates
with the next vertebra.
The simultaneous contraction of the two upper muscles causes the arm to
bend upwards. The contraction of the two lower bend it downwards, whilst a
sideward movement is effected by the contraction of the upper and lower
muscle of the same side. On the proximal surface of the central portion of
the vertebra there is a central knob and two ventro-lateral knobs, {482}a
median ventral pit and two dorso-lateral pits, and on the distal surface
there are pits corresponding to the knobs on the proximal side and _vice
versa_ (Fig. 210, C). These knobs and pits restrict the movement of one
vertebra on the next, so that although the arms can undergo an unlimited
amount of flexion from side to side, they cannot be rolled up in the
vertical plane. When the under surface of the vertebra is examined there is
seen on each side of the central groove two round holes, a distal and a
proximal. The distal pair are for the passage of the canals connecting the
radial water-vessel with the tentacles, these canals traversing the
substance of the vertebra for a part of their course; the proximal pair are
for nerves going to the longitudinal muscles, which likewise perforate part
of the ventral border of the vertebra.
In order to understand the anomalous circumstance that the canals going to
the tentacles actually perforate the vertebrae, it must be clearly borne in
mind that the basis of the body-wall in all Echinoderms is a mass of jelly
with amoebocytes in it, to which we must assign the power of secreting
carbonate of lime, and all we have to assume in the case of Ophiuroids is
that calcification spread outwards from the original ambulacral ossicles
into the surrounding jelly, enclosing any organs that happened to traverse
it.
When the ossicles of the arm are followed inwards towards the MOUTH, they
are seen to undergo a profound modification, so as to form, by union with
the corresponding ossicles of adjacent arms, a structure called the
mouth-frame. The general character of this modification is similar to that
affecting the first ambulacral and adambulacral ossicles in the arms of an
Asteroid, but in the Ophiuroid the change is much more profound. The first
apparent vertebra consists of two separated halves, and each is fused with
the first adambulacral (lateral) plate, which in turn is firmly united with
the corresponding plate in the adjoining arm. Thus is formed the "jaw," as
the projection is called. The extensions of the mouth-cavity between
adjacent jaws are termed "mouth-angles." To the apex of each jaw is
attached a plate bearing a vertical row of seven short blunt spines called
"teeth" (Fig. 212, _p_). The plate is called the "torus angularis" (Fig.
211, _T_), and on its ventral edge there is a tuft of spines which are
termed "tooth-papillae" (Fig. 208, _t.p_). On the upper aspect of the jaw
{483}are a pair of plates termed "peristomial plates." These discs—of which
there are two in each radius, one on each jaw which flanks the
radius—possibly represent the separated halves of the first vertebra, the
apparent first vertebra being really the second. On the flank of the jaw
there is dorsally a groove for the water-vascular ring and nerve-ring (Fig.
212, _n.r_), and beneath this a groove for the first tentacle and a pore
for the second, both of which spring directly from the ring-canal; below
these, in most Ophiuroidea, but not in _Ophiothrix_, there is a row of
blunt triangular spines called "mouth-papillae" (Fig. 212, _p^1_).
[Illustration: FIG. 211.—Diagrams to show the modification of the
ambulacral and adambulacral ossicles to form the armature of the mouth. A,
Asteroid; B, Ophiuroid. _A_{1}-A_{4}_, the first four ambulacra ossicles;
_Ad_{1}-Ad_{4}_, the first four adambulacral ossicles; _J_{1}_, the first
plate of the interradius (in the Ophiuroid the _scutum buccale_); _P_, the
spines borne by the jaw (in the Ophiuroid the teeth); _T_, the torus
angularis; _W_, the water-vascular ring; _Wr_, the radial water-vessel;
_I_, _II_, the first two pairs of tube-feet. (After Ludwig.)]
The words "jaw" and "tooth" are misleading. There is no evidence that the
jaws of a Brittle Star are ever used for crushing food, but by means of the
muscles attaching them to the first {484}complete vertebra in the arm they
can be rotated downwards so as greatly to enlarge the mouth, and again
rotated upwards and inwards, when they form an excellent strainer to
prevent the entrance of coarse particles. To permit this extensive movement
the articulatory facets on the proximal surface of the first vertebra have
been much modified; the median knob and pit have disappeared, and the
dorso-lateral pits are raised on to the surface of processes, so that there
are in all four processes, two of which articulate with one half of a jaw.
[Illustration: FIG. 212.—Lateral view of mouth-frame of _Ophiarachna
incrassata_. × 4. _A^1_?, peristomial plate, possibly the half of the first
vertebra; _A^2_, the half of the second vertebra; _A^3_, the third
vertebra; _F^1_, pores for pair of tentacles; _gen_, genital scale lying
beside opening of genital bursa; _musc_, longitudinal muscles connecting
vertebrae; _n.r_, groove for nerve-ring; _p_, tooth; _p^1_, mouth-papilla;
_t_, torus angularis. (After Ludwig.)]
The mouth can be narrowed and the jaws forced inwards towards the centre by
the simultaneous contraction of five muscles (_musc. tr_, Fig. 213) each,
which unite the two halves of a jaw.
Turning now to the skeleton of the DISC, we notice that dorsally it
consists of a closely-fitting mosaic of small plates, which are usually
concealed from view by a covering of minute spines. Opposite the insertion
of each arm there are, however, a pair of large triangular plates
("radials"), which extend outwards to the periphery and strengthen it, much
as the ribs do in an umbrella. These radial plates are always exposed, in
_Ophiothrix_, even when the rest of the dorsal plates are concealed by
spines. On the under surface there is a similar plating; but adjoining the
jaws are five large, more or less rhomboidal, plates {485}termed "scuta
buccalia" (Fig. 211, _J_{1}_), on one of which open the few madreporic
pores which the animal possesses. Attached to the sides of the scuta
buccalia are the "lateral mouth shields," which are in fact the
adambulacral plates belonging to the second pair of ambulacral plates which
form the main mass of the jaws. Further out, on the under side of the disc,
there is, on each side of each arm, a long narrow slit—the opening of the
genital bursa (Fig. 208, _g.b_), so that there are ten genital bursae. The
"genital bursa" (Fig. 214) is a sac lined by ciliated ectoderm projecting
into the interior of the disc. It is called genital because the openings of
the genital organs are situated on its surface; its main function, however,
is respiratory, the cilia bringing about a constant inward current of fresh
sea-water, the oxygen contained in which diffuses through the thin wall of
the sac into the coelomic fluid. The opening of the bursa is strengthened
on its radial side by a rod-like ossicle, the "genital plate," and on its
interradial side by an ossicle called the "genital scale" (Fig. 212,
_gen_), and in _Ophiothrix_ the outer end of the radial plate articulates
with the outer end of the genital plate. Muscles connect the two plates
running on either side of the articulation.
Observations on _Ophiothrix_[460] show that in this species at any rate the
radial plates can be raised or lowered. When they are raised the centre of
the disc is lifted into a cone and water is sucked into the genital bursae,
whereas when they are lowered the bursae are compressed and water is
expelled. This forced respiration appears to come into play when the supply
of oxygen is getting scanty.
The ALIMENTARY CANAL of _Ophiothrix_ is a simple flattened sac (Fig. 213).
It is devoid of an anus and cannot be everted through the mouth. There is a
horizontal pouch given off into each interradial lobe of the disc. The sac
is attached to the dorsal wall of the coelom by numerous mesenteries,
fibrous cords traversing the coelomic cavity and clothed on the outer side
by coelomic epithelium. To the mouth-frame it is attached by a circular
membrane, which we have reason for believing is a {486}functionless remnant
of the retractor muscles of the stomach of Asteroidea. In the young
Asteroid there is a similar sheet of membrane, which later becomes resolved
into the ten retractor bands.
The simple structure of the alimentary canal appears to be correlated with
the exceedingly simple character of the food. _Ophiothrix_ feeds on the
most superficial layer of mud at the bottom of the sea. This deposit
consists partly of microscopic Algae and partly of decaying organic matter,
and is much more easily disposed of than the living animals on which the
Starfish preys. The food is shovelled into the mouth by the first two or
"buccal" pairs of tube-feet in each ray.
[Illustration: FIG. 213.—Longitudinal section through the disc of a young
Ophiuroid passing along one arm and the middle of the opposite interradius.
(Diagrammatised from an actual section of _Amphiura squamata_.) _ab_,
Aboral sinus (dorsal in the radius, ventral in the interradius); _ax_,
axial sinus; _coe_, dorsal coelomic canal of the arm; _ep_, epineural
canal; _gang.rad_, ganglion of the radial nerve; _gen.r_, genital rachis
contained in the aboral sinus; _gen.st_, genital stolon; _mp_, madreporic
pore; _musc.long_, longitudinal muscle of the arm; _musc.tr_, transverse
muscle uniting the two halves of each jaw; _mv_, madreporic vesicle;
_nerv.r_, nerve-ring; _p.c_, pore-canal; _perih_, perihaemal canal; _vert_,
vertebra; _w.vr_, radial water-vessel.]
The WATER-VASCULAR SYSTEM has undergone a most interesting set of
modifications, which can be explained by noticing the fact that the
tube-feet have almost, if not quite, lost their locomotor function and are
now used as tactile organs. The ampulla, or swollen inner end of the
tube-foot, has disappeared, and the upper end of the organ is directly
connected with the radial canal by means of a curved canal, which traverses
the outermost flange of the vertebra, appearing on its {487}surface in a
groove on the outer side of the dorsal lateral knob on the distal side of
the ossicle. As in Asteroidea there are valves, which regulate the entrance
of fluid into the tube-foot. The stone-canal is a curved tube of simple
circular section and excessively narrow bore which extends from the
water-vascular ring _downwards_ to the madreporite (Fig. 213, _mp_)
situated on one of the scuta buccalia. The madreporite, in _Ophiothrix_ as
in most Brittle Stars, is an exceedingly rudimentary structure, consisting
of one or two pores leading into as many pore-canals. From each
interradius, except that in which the stone-canal lies, a large Polian
vesicle hangs down from the water-vascular ring into the coelom.
We saw that in the Asteroid the ampulla was used like the bulb of a pipette
to force the fluid in the tube-foot down into the tip, so as to press the
sucker against the substratum. But when the tube-foot is used as a
sense-organ, a few circular fibres around its upper end suffice to bring
about all the extension that is needed. Since the extension is no longer a
very vigorous act, the loss of fluid by transudation has probably been
rendered insignificant, and hence the stone-canal and madreporite, whose
function it is to repair the loss, have been reduced in size. The curious
ventral curvature of the stone-canal is, however, due to another cause. In
the very young Ophiuroid the madreporite is on the edge of the disc, and
the stone-canal extends horizontally outwards; and in some Asteroidea there
is a similar outward direction in its course. As development proceeds the
dorsal interradial areas of the disc of the young Ophiuroid grow out into
lobes, building up the conspicuous adult disc and forcing the madreporite,
and with it the stone-canal, downwards towards the ventral surface.
The pores of the madreporite in _Ophiothrix_, like some of those in the
Asteroid, open not directly into the stone-canal but into the AXIAL SINUS
(Fig. 213, _ax_). This is a large ovoid sac, lined with thin epithelium,
lying between the stone-canal and the mouth-frame, since of course it has
shared in the ventral rotation of the stone-canal. Its open connexion with
the stone-canal was easily recognised by Ludwig, who termed it, on this
account, the "ampulla."[461] The name "axial sinus" was bestowed
{488}mistakenly on another cavity, which will be mentioned in connexion
with the genital organs.
The radial PERIHAEMAL SPACES of the arms open into a "perihaemal ring"
representing the outer perihaemal ring of Asteroids; but the axial sinus
does not have any such extension as constitutes the inner perihaemal ring
in Starfish. So-called oral circular and radial BLOOD STRANDS are to be
found in similar positions to the corresponding structures in _Asteroidea_.
The NERVOUS SYSTEM might have been expected to have become very much
modified, since the activities of the Brittle Stars are so different from
those of the Starfish. It is indeed a universal rule in the Animal Kingdom
that, concomitantly with the increase in size and activity of a muscle,
there is a corresponding increase in the number of ganglion-cells which
control it. An accurate radial section of an arm shows that there is,
corresponding to the interspaces between the two vertebrae, a ganglionic
swelling of the nerve-cord. As in Asteroids, there are not only ectodermic
ganglion-cells on the under surface of the cord abutting on the epineural
canal, but also coelomic ganglion-cells derived from the floor of the
radial perihaemal canal. Both these categories of cells are largely
increased in number in the ganglion. From the dorsal-cells arise a pair of
large nerves which pass directly up and supply the great intervertebral
muscles. From the interspace between the ganglia a direct prolongation of
the ventral part of the nerve-cord, the so-called pedal nerve, extends out
along the side of the tentacle, as in Asteroids. In Ophiuroids it swells
out into a ganglion, completely surrounding the tentacle and giving off
nerves to the surfaces of the arm which terminate in the cuticle.
There is a large ganglion where the radial cord joins the nerve-ring, and,
owing to the more specialised condition of the nervous system, a severed
arm in an Ophiuroid is much more helpless than an arm of an Asteroid. It
will not carry out "escape movements," and is for a long time rigid under
the shock of section; at last it simply gives reflex movements on
stimulation.
Preyer[462] endeavoured to test the "intelligence" of Ophiuroids by
observing how they would adapt themselves to circumstances which it might
be fairly assumed they had never encountered {489}in their ordinary
experience. To this end he passed over the arm of a specimen a piece of
indiarubber tubing, which clung to it tightly. He found that the animal
first tried walking off, pressing the encumbered arm against the ground, so
that the piece of tubing was rubbed off. It was then replaced more tightly
than before; the animal, having tried the first method without result,
waved the arm to and fro in the water till the rubber floated off. In a
third experiment the animal held the rubber against the ground by a
neighbouring arm, and drew the encumbered arm out. When the rubber was
replaced a fourth time, the animal kicked it off by alternately pressing
neighbouring arms against it. Finally, when the rubber was put on so firmly
that all the above-mentioned methods failed, the arm was broken off. Preyer
concludes from this that Ophiuroids have a high degree of intelligence; but
this may be doubted, and the reader is referred to the account of Uexküll's
experiments given in the next chapter. There is, however, no doubt at all
that Ophiuroidea are by far the most active of all Echinoderms, and one
would naturally correlate this with higher psychic development.
The radial nerve ends in a terminal tentacle sheltered by a median plate at
the end of the arm; but eyes, such as are found in Asteroids, are wanting,
and the animal does not appear to be sensitive to light.
The REPRODUCTIVE SYSTEM in Ophiuroids consists of a genital stolon giving
rise at its distal end to a genital rachis, which extends in a circular
course round the disc, ensheathed in an "aboral sinus" (Fig. 213, _ab_) and
swelling out so as to form the gonads (testes or ovaries), where it passes
over the inner side of the genital bursae. The genital stolon (Fig. 213,
_gen.st_) is a compact ovoid organ, often termed on account of its shape
the "ovoid gland." It is situated close to the stone-canal, and, as in
Starfish, it indents the outer wall of the axial sinus; but, unlike the
stolon of the Asteroid, it is separated from the general coelom by a space,
of which it forms the inner wall, but whose outer wall is formed by a sheet
of membrane. This cavity must be carefully distinguished from the axial
sinus of Asteroidea, to which it was supposed at one time to correspond; it
is really formed by a pocket-like ingrowth of the general coelom into the
septum dividing it from the axial sinus. The cells forming the inner side
of this pocket form the primitive germ-cells, which {490}constitute the
main mass of the ovoid gland; those of the outer side remain thin. The
cavity of the ingrowth is shut off from the general coelom, but persists
throughout life. In Asteroids a similar ingrowth takes place, but both
walls thicken and become converted into germ cells, and the cavity
disappears, and, as in Asteroidea, a considerable number of the germ-cells
in the stolon degenerate.
[Illustration: FIG. 214.—Diagram of a tangential section through the edge
of the disc of an Ophiuroid to show the relations of the disc, _arm_, and
genital bursae. _ep_, Epineural canal; _musc_, longitudinal muscle of the
arm; _nerv.rad_, radial nerve cord; _ov_, ovary; _perih_, radial perihaemal
canal; _w.v.r_, radial water-vessel.]
The genital rachis (Fig. 213, _gen.r_) is an outgrowth of the distal end of
the genital stolon, which extends in a complete circle round the disc. The
rachis does not, however, lie everywhere in the same plane, but by its
undulating course bears witness to the distortion which the disc has
undergone. In the radii it is, as in the Asteroid, dorsal; but in the
interradii it is ventral, this ventral portion having, like stone-canal and
axial sinus, been carried down by the preponderant growth of the dorsal
parts of the disc. It is everywhere ensheathed by the aboral sinus, which,
as in Asteroids, is an outgrowth of the coelom. The rachis is embedded in a
strand of modified connective tissue, to which we may (as in the case of
_Asterias_) apply the name "aboral blood-ring." Both on the central and
peripheral sides of this sinus are vertical muscles connecting the genital
and the radial plates, which bring about the respiratory movements already
referred to. Just above the madreporite, at the end of the genital stolon,
is a small, completely closed space, which by its position corresponds with
the madreporic vesicle of Asteroids and represents the right hydrocoel
(Fig. 213, _mv_). As the rachis passes over the genital bursa it gives off
branches, which swell up to form the genital organs. In _Ophiothrix_ there
is {491}one such organ on each side of each bursa, but in other genera (cf.
_Ophiarachna_) a large number of small ones. The genital products are shed
into the water through the bursae.
CLASSIFICATION OF OPHIUROIDEA.
Before proceeding to study the classification of Brittle Stars, it is
necessary to give some account of the range of structure met with in the
group.
NUMBER OF RADII.—The number of arms is rarely increased, and hardly ever
exceeds six; a few species (each an isolated one in its genus) have six
arms, and in one case (_Ophiactis virens_), at any rate, this is associated
with the power of transverse fission. In many Cladophiurae the arms fork
repeatedly, so that although there are only five radii, there is quite a
crowd of terminal branches.
VERTEBRAE.—The vertebrae differ in the manner in which they articulate with
one another. In _Ophiothrix fragilis_ taken as the type, which in this
respect resembles the vast majority of species (Zygophiurae), the knobs and
pits on the faces of the vertebrae prevent the arms from being coiled in
the vertical plane. In _Ophioteresis_ (Fig. 210, A) and some allied genera
(Streptophiurae) the knobs are almost obsolete, and the arms are free to
coil in the vertical plane; whilst in _Gorgonocephalus_ and _Astrophyton_
(Cladophiurae) the arms are repeatedly branched and the vertebrae have
saddle-shaped articulating surfaces, so that they have quite a snake-like
capacity for coiling themselves round external objects. In _Ophiohelus_
(Fig. 216) each vertebra consists of two rod-like plates placed parallel
with the long axis of the arm and fused at both ends, but divergent in the
middle, leaving a hole between them.
COVERING PLATES OF THE ARMS.—The upper arm-plates are the most variable.
They may be surrounded by small supplementary plates (_Ophiopholis_) or
double (_Ophioteresis_). In all (?) Cladophiurae and most Streptophiurae
they are absent, being replaced by minute calcareous granules. Under
arm-plates are absent in _Ophioteresis_ and in the distal portion of the
arms in many Cladophiurae. Side arm-plates are constantly present, and in
most Cladophiurae meet in the middle line below.
ARM-SPINES.—The spines borne by the lateral covering plates of the arms
vary greatly in character. In _Ophiura_ and its {492}allies they are short
and smooth, and are borne by the hinder edge of the arm and directed
backwards; but in the larger number of genera they are borne nearer the
centre of the plate, and are directed outwards at right angles to the arm.
They may be covered by small asperities, as in _Ophiothrix_ (Fig. 215, C),
when they are said to be rough; or these asperities may become secondary
spines, as in _Ophiacantha_ (Fig. 215, B), when they are said to be thorny.
In _Ophiopteron_ all the spines borne by a single plate are united by a web
of skin so as to constitute a swimming organ. The small plates guarding the
ends of the tentacles (tentacle-scales) may be absent, or more rarely
double. In Cladophiurae there is a regular transition from tentacle-scale
to arm-spine; the tentacle-scale being merely the smallest of the series of
lateral spines.
True PEDICELLARIAE are unknown amongst Ophiuroidea, since there is no
longer a soft ectoderm to protect, but in some cases, as for instance in
_Ophiohelus_, small hooks movable on a basal piece attached to the arms are
found which may represent the vestiges of such organs (Fig. 216). Similar
hooks are found in the young _Ophiothrix fragilis_ just after metamorphosis
and in all Cladophiurae, replacing in the latter case the arm-spines in the
distal portion of the arm.
[Illustration: FIG. 215.—Three types of mouth-frame found in Zygophiurae.
A, _Ophioscolex_, × 10; B, _Ophiacantha_, × 6; C, _Ophiothrix_, × 6. (After
Lyman.)]
MOUTH-FRAME.—In its broad outlines there is practically no variation in
this organ throughout the group, but in respect of {493}the spines, which
are borne on the flanks of the jaws (mouth-papillae) and on their apices
(teeth and tooth-papillae) there is very great variation. Teeth are always
present. Mouth-papillae are very frequently present, tooth-papillae are
rarer, and it is only in a restricted number of genera (_Ophiocoma_ and its
allies) that both mouth-papillae and tooth-papillae are present at the same
time.
[Illustration: FIG. 216.—A portion of an arm of _Ophiohelus umbella_, near
the distal extremity, treated with potash to show the skeleton, × 55. The
vertebrae are seen to consist of two curved rods united at their ends. The
triangular side-plates bear a row of movable hooks which articulate with
basal outgrowths of the plate. (After Lyman.)]
SKELETON OF THE DISC.—This is typically composed of a mosaic of plates of
different sizes, but in some cases (_Ophiomyxa_, most Streptophiurae, and
Cladophiurae) these, with the exception of the radials and genitals, are
entirely absent, and the disc is then quite soft and covered with a
columnar epithelium, the persistent ectoderm. Even the scuta buccalia may
disappear. Radial shields are absent in _Ophiohelus_. In many cases
(_Ophiothrix_ and _Ophiocoma_) all the dorsal plates except the radials are
concealed from view by a covering of small spines. In some genera
(_Ophiopyrgus_) there are five large plates in the centre of the upper part
of the disc, which have been termed "calycinals" from a mistaken comparison
with the plates forming the cup or calyx of the Pelmatozoa, but there is no
connexion between the two sets of structures.
The MADREPORITE is usually quite rudimentary, but in Cladophiurae there may
be five madreporites, each with about 200 pores, and, of course, five
stone-canals.
{494}The number of GENITAL ORGANS varies very much. In the small _Amphiura
squamata_ there are two gonads, an ovary and a testis, attached to each
bursa, but in the larger species there may be very many more.
We follow Bell's classification,[463] according to which the Ophiuroidea
are divided, according to the manner in which the vertebrae move on one
another (cf. Fig. 210), into three main orders, since these movements are
of prime importance in their lives.
(1) STREPTOPHIURAE, in which the faces of the vertebrae have rudimentary
knobs and corresponding depressions, so that the arms can be coiled in the
vertical plane. These are regarded as the most primitive of Ophiuroidea.
(2) ZYGOPHIURAE, in which the vertebral faces have knobs and pits which
prevent their coiling in a vertical plane.
(3) CLADOPHIURAE, in which the arms can be coiled as in (1) and are in most
cases forked. No teeth; the arm-spines are papillae, the covering plates of
the arms are reduced to granules.
ORDER I. STREPTOPHIURAE.
This is not a very well defined order; it includes a few genera
intermediate in character between the Cladophiurae and the Zygophiurae, and
believed to be the most primitive Ophiuroids living. It is not divided into
families. The vertebrae have rudimentary articulating surfaces, there being
two low bosses and corresponding hollows on each side, and so they are
capable of being moved in a vertical plane, as in the Cladophiurae; the
arms never branch, and further, they always bear arm-spines and lateral
arm-plates at least. No species of this order are found on the British
coast, but _Ophiomyxa pentagona_, in which the dorsal part of the disc is
represented only by soft skin, is common in the Mediterranean.
_Ophioteresis_ is devoid of ventral plates on the arms, and appears to
possess an open ambulacral groove, though this point has not been tested in
sections. _Ophiohelus_ and _Ophiogeron_ have vertebrae in which traces of
the double origin persist (see p. 491).
{495}ORDER II. ZYGOPHIURAE.
This group includes all the common and better-known British forms. They are
divided into five families, all of which are represented in British waters.
[Illustration: FIG. 217.—Aboral view of _Ophioglypha_ (_Ophiura_)
_bullata_. × 3. (From Wyville Thomson.)]
FAM. 1. OPHIOLEPIDIDAE.[464]—Arm inserted in a definite cleft {496}in the
disc, or (expressing the same fact in another way) the interradial lobes
out of which the disc is composed are not completely united. Radial shields
and dorsal plates naked. Arm-spines smooth and inserted on the posterior
border of the lateral arm-plates.
[Illustration: FIG. 218.—Oral view of _Ophioglypha_ (_Ophiura_) _bullata_.
× 5. (From Wyville Thomson.)]
This family includes all the Brittle Stars of smooth porcelanous aspect and
provided with only short spines. Forbes[465] called them Sand-stars, since
their short spines render these animals incapable of burrowing or of
climbing well, and hence they appear to move comparatively rapidly over
firm ground, sand, gravel, or muddy sand, and they are active enough to be
able to capture small worms and Crustacea. The prey is seized by coiling
one of the arms around it.
One genus, _Ophiura_, is fairly common round the British coast, {497}being
represented by _O. ciliaris_ and _O. albida_; the former is the commoner.
An allied species dredged by H.M.S. "Challenger" is represented in Figs.
217 and 218.
_Ophiomusium_ (Fig. 219) is a very peculiar genus. The mouth-papillae on
each side of each mouth-angle are confluent, forming a razor-like
projection on each side of each mouth-angle (Fig. 220). The arms are short,
and the podia are only developed at the bases of the arms. _Ophiopyrgus_
has the dorsal surface raised into a conical elevation protected by a
central plate surrounded by five large plates.
[Illustration: FIG. 219.—Aboral view of _Ophiomusium pulchellum_. × 7.
(From Wyville Thomson.)]
In the remaining four families the arms are inserted on the under surface
of the disc; in other words, the interradial lobes which make up the disc
have completely coalesced dorsally; and the spines stand out at right
angles to the arm.
FAM. 2. AMPHIURIDAE.—Mouth-papillae present, but no tooth-papillae; radial
shields naked; small scuta buccalia.
The most interesting Brittle Star belonging to this family is _Amphiura
squamata_ (_elegans_), a small form, with a disc about ¼ inch in diameter
covered with naked plates. It is hermaphrodite and viviparous, the young
completing their development inside the bursae of the mother. Occasionally
the whole disc, with the exception of the mouth-frame, is thrown off and
regenerated. This appears to be a device to enable the young to escape.
Three other species of _Amphiura_ are found in British waters.
{498}[Illustration: FIG. 220.—Oral view of _Ophiomusium pulchellum_. × 7.
(From Wyville Thomson.)]
_Ophiactis_ is another genus belonging to this family, distinguished from
_Amphiura_ by its shorter arms and smoother arm-spines. It lives in the
interstices of hard gravel. The British species, _O. balli_, presents no
special features of interest, but the Neapolitan _O. virens_ is an
extraordinary form. It has six arms, three of which are usually larger than
the other three, for it is always undergoing a process of transverse
division, each half regenerating the missing part. It has from 1 to 5
stone-canals, the number increasing with age; numerous long-stalked
Polian-vesicles in each interradius, and in addition a number of long
tubular canals which spring from the ring-canal, and entwine themselves
amongst the viscera.[466] All the canals of the water-vascular system,
except the stone-canals, contain non-nucleated {499}corpuscles, carrying
haemoglobin,[467] the respiratory value of which compensates for the loss
of the genital bursae, which have entirely disappeared.
_Ophiopholis_ is distinguished from the foregoing genera by the granular
covering of its dorsal plates; whilst in _Ophiacantha_ these granules
develop into prominent spinelets, and the arm-spines are also thorny.
_Ophiopholis aculeata_ occurs in swarms in the branches of the Firth of
Clyde, and presents a most remarkable series of variations in colour.
_Ophiopsila_ is a closely allied form, distinguished by its large
peristomial plates.
[Illustration: FIG. 221.—Oral view of _Ophiacantha chelys_. × 4. (From
Wyville Thomson.)]
FAM. 3. OPHIOCOMIDAE.—Both mouth-papillae and tooth-papillae are
present;[468] the arm-spines are smooth, and the disc is covered with
granules.
_Ophiocoma nigra_ is the only common British representative of this family.
In this species the plates of the dorsal surface are completely hidden from
view by a covering of granules. _Ophiarachna_.
FAM. 4. OPHIOTHRICIDAE.—Tooth-papillae alone present, mouth-papillae
absent; arm-spines roughened or thorny.
This family is represented only by _Ophiothrix fragilis_, which is perhaps
the most abundant of all British Ophiuroids, and has been selected as the
type for special description.
The back is covered with spinules, having, however, the triangular radial
plates bare. This produces a contrast-effect, which suggested the name
_pentaphyllum_, formerly used by some naturalists for the species. It
occurs in swarms, and presents variations in colour nearly as marked as
those of _Ophiopholis_. {500}_Ophiopteron_ is probably a swimming
Ophiuroid, as the lateral spines of each segment of the arm are connected
by a web of skin.
ORDER III. CLADOPHIURAE.
[Illustration: FIG. 222.—Aboral view of young _Astrophyton linckii_,
slightly enlarged. (From Wyville Thomson.)]
These, like the Streptophiurae, have the power of rolling the arms in a
vertical plane, but the articulating surfaces of the vertebrae are
well-developed and saddle-shaped. The dorsal surface of the disc and arms
is covered with a thick skin with minute calcifications. Upper-arm plates
wanting. Radial plates always present, though occasionally represented by
lines of scales. {501}The order is divided into three families, two of
which are represented in British waters.
FAM. 1. ASTROSCHEMIDAE.—Arms unbranched. _Astronyx_ is comparatively common
in the sea-lochs of Scotland. There are a series of pad-like ridges on the
arms, representing the side-plates and bearing the spines. _Astroschema_.
FAM. 2. TRICHASTERIDAE.—Arms forked only at the distal ends. _Trichaster_,
_Astrocnida_.
FAM. 3. EURYALIDAE.—Arms forked to their bases. _Gorgonocephalus_ is
occasionally taken in deep water off the north coast of Scotland. In it the
arms repeatedly fork, so that a regular crown of interlacing arms is
formed. The animal obviously clings to external objects with these, for it
is often taken in fishermen's nets with its arms coiled around the meshes.
The genital bursae are said to be represented by slits which open directly
into the coelom. (Lyman describes the coelom as divided into ten
compartments by radiating septa; it is possible—even probable—that these
are really the bursae.) An allied species is common in the Bay of Fundy,
being found in comparatively shallow water. _Astrophyton_ (Fig. 222) is
closely allied to _Gorgonocephalus_, differing only in trifling points. It
is doubtful whether the separation of these two genera is justified.
FOSSIL OPHIUROIDEA.—The Ophiuroidea are rather sparsely represented among
fossils, but in the Silurian and Devonian a series of very interesting
forms occur which are intermediate in character between Starfish and
Brittle Stars, and which were therefore in all probability closely allied
to the common ancestors of modern Ophiuroids and Asteroids. Jaekel[469] has
recently added largely to our knowledge of these primitive forms, and has
described a number of new genera. Thus _Eophiura_ from the Lower Silurian
has an open ambulacral groove, and the vertebrae are represented by an
alternating series of quadrate ossicles, each deeply grooved on its under
surface for the reception of the tentacle, which was not yet (as in modern
forms) enclosed in the vertebra. The lateral or adambulacral plates
extended horizontally outwards, and each bore a series of spines at its
outer edge.
A remarkable fact is that where the halves of the vertebrae (_i.e._ the
ambulacral ossicles) diverge in order to form the {502}mouth-angles, no
less than five or six vertebrae are thus affected, instead of only two as
in modern forms. The actual "jaw," however, seems, as in modern forms, to
consist only of the first adambulacral fused to the second ambulacral, so
that instead of concluding with Jaekel that the "jaws" of modern forms
result from the fusion of five or six vertebrae, a conclusion which would
require that a number of tentacles had disappeared, we may suppose that the
gaping "angles" of these old forms have, so to speak, healed up, except at
their innermost portions.
In _Bohemura_, which belongs to a somewhat younger stratum, the structure
is much the same, but the groove in the ambulacral ossicle for the tentacle
has become converted into a canal, and the ambulacral groove itself has
begun to be closed at the tip of the arm by the meeting of the
adambulacrals.
In _Sympterura_, a Devonian form described by Bather,[470] the two
ambulacral plates of each pair have thoroughly coalesced to form a
vertebra, but there is still an open ventral groove, and no ventral plates.
In the Trias occurs the remarkable form _Aspidura_, which had short
triangular arms, in which the tentacle pores were enormous and the ventral
plates very small. The radial plates formed a continuous ring round the
edge of the disc. _Geocoma_ from the Jurassic is a still more typical
Ophiuroid; it has long whip-like arms, and the dorsal skeleton of the disc
is made of fifteen plates, ten radials, and five interradials. In the
Jurassic the living genus _Ophioglypha_, appears.
The Cladophiurae are represented already in the Upper Silurian by
_Eucladia_, in which, however, the arms branch not dichotomously, as they
do in modern forms, but monopodially. There is a large single madreporite.
_Onychaster_, with unbranched arms, which occurs in the Carboniferous, is a
representative of the Streptophiurae.
It will therefore be seen that the evolution of Ophiuroidea must have begun
in the Lower Silurian epoch. The Streptophiurae are a few slightly modified
survivors of the first Ophiuroids. By the time the Devonian period had
commenced, the division of the group into Zygophiurae and Cladophiurae had
been accomplished.
{503}CHAPTER XVIII
ECHINODERMATA (_CONTINUED_): ECHINOIDEA = SEA-URCHINS
CLASS III. ECHINOIDEA
The Sea-urchins or Echinoidea (Gr. ἐχῖνος, Hedgehog or Sea-urchin), which
constitute the third class of the Eleutherozoa, have derived both their
popular and scientific names from the covering of long spines with which
they are provided. At first sight but little resemblance is to be discerned
between them and the Starfish and Brittle Stars. They are devoid of any
outgrowths that could be called arms; their outline is generally either
circular or that of an equilateral pentagon, but as their height is almost
always smaller than their diameter, they are never quite spherical;
sometimes it is so small that the animals have the form of flattened discs.
All doubt as to the relationship of the Echinoidea to the Starfish is at
once dispelled in the mind of any one who sees one of the common species
alive. The surface is beset with delicate translucent tube-feet, terminated
by suckers resembling those of Starfish, although capable of much more
extension. The animal throws out these organs, which attach themselves by
their suckers to the substratum and so pull the body along, whilst the
spines are used to steady it and prevent it from overturning under the
unbalanced pull of the tube-feet. When moving quickly the animal walks on
its spines, the tube-feet being little used. The tube-feet are distributed
over five bands, which run like meridians from one pole of the animal to
the other. These bands are termed "radii," and they extend from the mouth,
which is situated in the centre of the lower surface, up to the
neighbourhood of the aboral pole. The radii must be compared to the
ambulacral grooves on the {504}oral surface of the arms of Starfish, and
hence in Urchins the aboral surfaces of the arms have, so to speak, been
absorbed into the disc, so that the oral surfaces have become bent in the
form of a semicircle. The radii are separated from one another by
meridional bands called "interradii," which correspond to the interradial
angles of the disc of a Starfish and to the sides of its arms. The small
area enclosed between the upper terminations of the radii is called the
"periproct," and this corresponds to the entire dorsal surface of the
Starfish, including that of the arms.
One of the commonest species of British Sea-urchin is _Echinus esculentus_.
In sheltered inlets, such as the Clyde, it is often left exposed by the
receding tide, whilst everywhere on the coast in suitable localities it may
be obtained by dredging at moderate depths on suitable ground. In the Clyde
it is easy to observe the habits of the animal through the clear still
water. It is then seen to frequent chiefly rocky ground, and to exhibit a
liking for hiding itself in crevices. Often specimens will be seen clinging
to the rock by some of their tube-feet, and, as it were, pawing the under
surface of the water with the others. In the Clyde it feeds chiefly on the
brown fronds of _Laminaria_, with which the rocks are covered. In more
exposed situations, such as Plymouth Sound, it does not occur in shallower
water than 18 to 20 fathoms. At this depth it occurs on a rocky ridge; but
in 1899, after a south-west gale, all the specimens had disappeared from
this ridge, showing at what a depth wave disturbance is felt.
A full-grown specimen is as large as a very large orange; its under surface
is flattened, and it tapers somewhat towards the aboral pole. The outline
is that of a pentagon with rounded angles. The spines in _Echinus
esculentus_ are short in comparison to the diameter of the body, and this
is one of the characteristics of the species.
The animal is provided with a well-developed skeleton, consisting of a mail
of plates fitting closely edge to edge, and carrying the spines. This
cuirass bears the name "corona" (Fig. 227). It has two openings, an upper
and a lower, which are both covered with flexible skin. The upper area is
known as the "periproct" (Fig. 227, 2); it has in it small isolated plates,
and the anus, situated at the end of a {505}small papilla, projects from it
on one side of the centre. The lower area of flexible skin surrounds the
mouth, and is called the "peristome" (Fig. 229), though it corresponds to
considerably more than the peristome of Asteroidea. In the mouth the tips
of the five white chisel-like teeth can be seen.
The plates forming the corona are, like all the elements of the skeleton of
Echinodermata, products of the connective tissue which underlies the
ectoderm, which in Echinoidea remains in a fully developed condition
covering the plates, and does not, as in Ophiuroidea, dry up so as to form
a mere cuticle. The ectoderm consists of the same elements as that of
Asteroidea, viz. delicate tapering sense-cells with short sense-hairs,
somewhat stouter supporting cells and glandular cells. It is everywhere
underlaid by a plexus of nerve fibrils, which, in part, are to be regarded
as the basal outgrowths of the sense-cells and partly as the outgrowths of
a number of small bipolar ganglion-cells, found intermixed with the fibres.
[Illustration: FIG. 223.—Aboral view of _Echinus esculentus_. × ½. (After
Mortensen.)]
Just as the MUSCULAR ARM has been the determining factor in the structure
of the Ophiuroidea, so the MOVABLE SPINE has been {506}the leading factor
in the evolution of Echinoidea. The SPINES have cup-shaped basal ends,
which are inserted on special projections of the plates of the skeleton
called tubercles. The tubercle is much larger than the cup, and hence the
spine has a great range of possible motion. The spines differ from those of
Starfish and Brittle Stars in being connected with their tubercles by means
of cylindrical sheaths of muscle fibres, by the contraction of which they
can be moved in any direction. The muscles composing the sheath consist of
an outer translucent and an inner white layer. The former are easily
stimulated and soon relax; they cause the movements of the spines. The
latter require stronger stimulation, but when aroused respond with a
prolonged tetanus-like contraction, which causes the spines to stand up
stiffly in one position; these muscles can be torn across sooner than
forced to relax. Uexküll[471] has appropriately named them "block
musculature." These sheaths, like everything else, are covered with
ectoderm, which is, however, specially nervous, so that we may say that the
muscular ring is covered by a nerve-ring from which stimuli are given off
to the muscles.
The spines are, speaking generally, of two sizes, the larger being known as
"primary spines" and the smaller as "secondary." In many Echinoidea these
two varieties are very sharply contrasted, but in _Echinus esculentus_
there is not such a great difference in length, and intermediate kinds
occur. The forest of spines has an undergrowth of PEDICELLARIAE. All
Echinoidea possess pedicellariae, which are much more highly developed than
those of any Asteroid. With few exceptions all the pedicellariae of
Echinoidea possess three jaws and a basal piece. This latter is, however,
drawn out so as to form a slender rod, which articulates with a minute boss
on a plate of the skeleton.
Of these pedicellariae there are in _E. esculentus_ four varieties, viz.
(1) "tridactyle" (Fig. 225, C; Fig. 226, B): large conspicuous
pedicellariae with three pointed jaws, each armed with two rows of teeth on
the edges. There is a flexible stalk, the basal rod reaching only half way
up. These are scattered over the whole surface of the animal.
(2) "Gemmiform" (Fig. 225, A, B; Fig. 226, A), so called from the
translucent, almost globular head. The appearance of {507}the head is due
to the fact that there is on the outer surface of each jaw a sac-like gland
developed as a pouch of the ectoderm. From it are given off two ducts which
cross to the inner side of the blades and, uniting into one, run in a
groove to near the tip. The gland secretes a poisonous fluid. The basal rod
reaches up to the jaws, so that this form of pedicellaria has a stiff
stalk. On the inner side of each blade, near the base, there is a slight
elevation (Fig. 225, B, _s_), consisting of cells bearing long cilia; this
is a sense-organ for perceiving mechanical stimuli. The gemmiform
pedicellariae are particularly abundant on the upper surface of the animal.
[Illustration: FIG. 224.—View of the apical region of _Echinus esculentus_,
showing spines and pedicellariae; drawn from the living specimen, × 3. _a_,
Anus; _g.p_, genital pore; _i_, interradius; _mp_, madreporite; _per_,
periproct; _p.gemm_, gemmiform pedicellaria; _pod_, podia; _p.trid_,
tridactyle pedicellaria; _p.trif_, trifoliate pedicellaria; _r_, radius;
_t.t_, pore for terminal tentacle of the radial water-vascular canal.]
{508}(3) "Trifoliate" (Fig. 225, E; Fig. 226, D): these are very small
pedicellariae, in which the jaws are shaped like leaves with the broad end
projecting outwards. They are scattered over the whole surface of the body.
(4) "Ophicephalous" (Fig. 225, D; Fig. 226, C): pedicellariae in which the
jaws have broad rounded distal ends fringed with teeth; these ends bear a
resemblance to a snake's head, whence the name. The bases are also broad
and thin, with a strong median rib and a peculiar semicircular hoop beneath
the spot where they articulate with one another. The three hoops of the
three jaws work inside each other in such a way as to cause the jaws to
have a strong grip and to be very difficult to dislocate from their mutual
articulation.
The ophicephalous pedicellariae are in _Echinus_ the most abundant of all;
and they alone extend on to the peristome, where a special small variety of
them is found.
A thorough investigation of the functions and reactions of the
pedicellariae has quite recently been made by von Uexküll.[472] He showed,
first of all, that there is a nervous centre in the stalk of each
pedicellaria (see below), which causes the organ to incline towards a weak
stimulus, but to bend away from a stronger stimulus. In the head there is
an independent nervous centre, which regulates the opening and closing of
the valves, and causes these to open on slight stimulus and close when a
stronger one is applied. The amount of stimulus necessary to cause the
pedicellariae to retreat varies with the kind of pedicellariae, being least
with the tridactyle and most with the gemmiform, so that when a chemical
stimulus, such as a drop of dilute ammonia, is applied to the skin, the
tridactyle pedicellariae may be seen to flee from and the gemmiform to
approach the point of stimulation. In a living Sea-urchin, if the attempt
is made to seize the tridactyle pedicellariae they will evade the forceps,
but the ophiocephalous are easy to catch.
The tridactyle pedicellariae open with the very slightest mechanical
stimulus and close with rather greater mechanical stimuli or with
exceedingly slight chemical ones. Uexküll calls them "Snap-pedicellariae,"
and their function is to seize and destroy the minute swimming larvae of
various sessile parasitic {509}animals, which would otherwise settle on the
delicate exposed ectoderm of the Sea-urchin.
The gemmiform pedicellariae are brought into action when a more serious
danger threatens the Sea-urchin, such as an attack of a Starfish. The
corrosive chemical influence, which it can be proved exudes not only from
the stomach but even from the tube-feet of the Starfish, causes the
gemmiform pedicellariae to approach and open widely. When the foe
approaches so closely as to touch the sense-organs (Fig. 225, B, _s_)
situated on the inner side of the valves of these pedicellariae, the blades
close violently, wounding the aggressor and causing its juice to exude,
thus producing a renewed and severe chemical stimulation which irritates
the poison glands and causes the poison to exude. The virulence of the
poison may be gauged from the fact that the bite of a single gemmiform
pedicellaria caused a frog's heart to stop beating.
[Illustration: FIG. 225.—The pedicellariae of _Echinus acutus_, drawn from
a living specimen. A, gemmiform pedicellaria, closed. B, gemmiform
pedicellaria, open; _g_, poison gland; _s_, sense-organ, × 3. C, tridactyle
pedicellaria, × 6. D, ophicephalous pedicellaria, × 9. E, trifoliate
pedicellaria, × 12; _a_ (in all figures), axial rod of the stalk. (After
Uexküll.)]
Prouho[473] has described a combat between a Sea-urchin and a Starfish.
When the latter approached, the spines of the {510}Sea-urchin diverged
widely (strong form of reaction to chemical stimulus), exposing the
gemmiform pedicellariae. These at once seized the tube-feet of the enemy
and the Starfish retreated, wrenching off the heads of these pedicellariae;
then the Starfish returned to the attack and the same result followed, and
this was repeated till all the pedicellariae were wrenched off, when the
Starfish enwrapped its helpless victim with its stomach.
The minute trifoliate pedicellariae are brought into play by any prolonged
general irritation of the skin, such as bright light or a rain of particles
of grit or mud. They have the peculiarity that not all the blades close at
once, so that an object may be held by two blades and smashed by the third.
They may be seen in action if a shower of powdered chalk is poured on the
animal, when they seize the particles and by breaking up any incipient
lumps reduce the whole to an impalpable powder, which the cilia covering
the skin speedily remove. In thus assisting in the removal of mechanical
"dirt" they earn the name which Uexküll has bestowed on them, of "cleaning
pedicellariae."
[Illustration: FIG. 226.—Views of a single blade of each kind of
pedicellaria. A, blade of gemmiform pedicellaria of _Echinus elegans_; _g_,
groove for duct of poison gland; B, blade of tridactyle pedicellaria of the
same species; C, blade of ophicephalous pedicellaria of the same species;
_r_, ring for clamping this blade to the other blades; D, blade of
trifoliate pedicellaria of _E. alexandri_. (After Mortensen.)]
The ophicephalous pedicellariae, with their powerful bull-dog grip, assist
in holding small animals, such as Crustacea, till the tube-feet can reach
them and convey them to the mouth.
{511}The number and variety of the pedicellariae, then, is an eloquent
testimony to the dangers to which the soft sensitive skins of the
Sea-urchin and other Echinodermata are exposed, and afford confirmatory
evidence in support of the view expressed above, that the method adopted to
defend the skin was one of the great determining features which led to the
division of the Asteroidea into different races.
[Illustration: FIG. 227.—Dried shell of _Echinus esculentus_, showing the
arrangement of the plates of the corona. × 1. 1, The anus; 2, periproct,
with irregular plates; 3, the madreporite; 4, one of the other genital
plates; 5, an ocular plate; 6, an interambulacral plate; 7, an ambulacral
plate; 8, pores for protrusion of the tube-feet; 9, tubercles of the
primary spines, _i.e._ primary tubercles.]
The CORONA consists of five radial or "ambulacral" bands of plates and five
interradial, or as they are usually termed, "interambulacral" bands of
plates—ten in all. Each of the ten consists of two vertical rows of plates
throughout most of its extent, and each plate is studded with large bosses,
or "primary tubercles" for the primary spines, smaller bosses called
"secondary tubercles" for the secondary spines, and finally, minute
elevations called "miliary tubercles" for the pedicellariae.
{512}[Illustration: FIG. 228.—The so-called calyx and the periproct of
_Echinus esculentus_. × 4. 1, Genital plates with genital pores; 2, ocular
plates with pores for terminal tentacles of the radial water-vascular
canals; 3, madreporite; 4, periproct with irregular plates; 5, anus. (After
Chadwick.)]
Even in the dried skeleton, however, the ambulacral plates can be
discriminated from the interambulacral by the presence of pores to permit
the passage of the tube-feet. These pores are arranged in pairs, and each
pair corresponds to a single tube-foot, since the canal connecting the
ampulla with the external portion of the tube-foot is double in the
Echinoidea. In _Echinus esculentus_ there are three pairs of such pores in
each plate, in _Strongylocentrotus droëbachiensis_ four pairs. The
ambulacral plate is really made up of a series of "pore-plates," each
carrying a single pair of pores, and these become united in threes in
_Echinus_ and fours in _Strongylocentrotus_, while in primitive forms like
the Cidaridae they remain separate. Each ambulacral and interambulacral
area ends at the edge of the periproct with a single plate. The plate
terminating the ambulacral band is pierced by a single pore for the exit of
the median tentacle, which, as in Asteroids, terminates the radial
water-vascular canal. Thus the aboral end of the radius in an Echinoid
corresponds to the tip of the arm in an Asteroid. The plate is termed
"ocular," because the terminal tentacle has a mass of pigmented cells at
its base; but no eye-cups can be seen, and there is no evidence that this
spot is specially sensitive to light. Species which show special
sensitiveness to light have often a large number of what we may perhaps
term secondary eyes. The plate terminating the interambulacral {513}series
is termed the "genital plate," because it is pierced by the duct of one of
the five genital organs. One of the genital plates is also pierced by the
madreporic pores. Some zoologists have separated the ocular and the genital
plates under the name of "calyx" from the rest of the corona, under a
mistaken idea that they are homologous with the plates of the body or calyx
of a Crinoid.
[Illustration: FIG. 229.—The peristome of _Echinus esculentus_. × 2. 1,
Tube-feet of the lower ends of the radii; 2, gill; 3, teeth; 4, buccal
tube-foot; 5, smooth peristomial membrane. (After Kükenthal.)]
The PERIPROCT (Fig. 228, 4) is covered with small plates and bears a few
pedicellariae. The PERISTOME (Fig. 229) is covered by flexible skin with
abundant pedicellariae; it terminates in a thick lip surrounding the mouth,
from which the tips of five white teeth are just seen projecting. There are
ten short tube-feet projecting from the peristome—one pair in each
radius—and each tube-foot terminates in an oval disc and is capable of
little extension, and each has around its base a little plate. The presence
of these tube-feet shows that in _Echinus_ the {514}peristome extends
outwards beyond the water-vascular ring, whereas in Asteroidea it is
contained entirely within the ring. In the primitive Cidaridae (Fig. 235)
the whole peristome down to the lip surrounding the mouth is covered with a
series of ambulacral and interambulacral plates similar to those forming
the corona, though smaller and not immovably united, and the series of
tube-feet is continued on to it. It is thus evident that the peristome is
merely part of the corona, which has become movable so as to permit of the
extension of the teeth. In _Echinus_ the peristome is continued in each
interradius into two branched outgrowths called gills, the relation of
which to the respiratory function will be described later. These gills
(Fig. 229, 2) are situated in indentations of the edge of the corona called
"gill-clefts" (Fig. 230, _g_).
[Illustration: FIG. 230.—The dried peristome of _Echinus esculentus_ and
the surrounding portions of the corona. × 1. _amb_, Ambulacral plate;
_b.t_, buccal tube-foot; _g_, gill-cleft; _inter_, interambulacrum; _per_,
peristome.]
The most conspicuous plates in the peristome are those {515}surrounding the
buccal tube-feet; besides these, however, there are in _Echinus
esculentus_, and probably in most species, a large number of thinner
irregularly-scattered plates (Fig. 230).
The term ambulacral plate, applied to the plate pierced by the pores for
the tube-feet, conveys a misleading comparison with the ambulacral plate of
an Asteroid. In Echinoids the ambulacral groove has become converted into a
canal called the "epineural canal," and the ambulacral plates form the
floor, not the roof, of this canal; they may perhaps correspond with the
adambulacral plates of the Starfish, which one may imagine to have become
continually approximated as the groove became narrower until they met.
[Illustration: FIG. 231.—Dissection of _Echinus esculentus_. × 1. The
animal has been opened by a circumferential cut separating a small piece of
the skeleton at the aboral end, which is turned outwards exposing the
viscera on its inner surface. The other viscera are seen through the hole
thus made. _amp_, Ampullae of the tube-feet; _aur_, auricle; _b.v_,
so-called "dorsal blood-vessel"; _comp_, "compasses" of Aristotle's
lantern, often termed "radii" by English authors; _comp.elv_, elevator
muscles of the compasses; _comp.ret_, retractor muscles of the compasses;
_eph_, epiphyses of the jaws in Aristotle's lantern; _gon_, gonad;
_g.rach_, genital rachis; _int_, intestine; _oe_, oesophagus; _prot_,
protractor of Aristotle's lantern; _rect_, rectum; _ret_, retractor of
Aristotle's lantern; _siph_, siphon; _st_, stomach; _stone.c_,
stone-canal.]
{516}The internal organs of the Urchin can best be examined by making a
horizontal incision about one-third the distance from the mouth and pulling
the two parts gently asunder. A large amount of fluid escapes from the
exceedingly spacious coelomic cavity, the alimentary canal being
comparatively narrow.
The ALIMENTARY CANAL commences with a short vertical tube which has been
shown to be a stomodaeum; this is surrounded by the upper ends of the teeth
and their supporting ossicles, which are collectively termed "Aristotle's
lantern." The oesophagus leads into a baggy, flattened tube, the stomach,
which runs horizontally round the animal, supported by strings of tissue
from the coelomic wall, so that it hangs down in a series of festoons.
Having encircled the animal, it bends directly back on itself and
immediately opens into the intestine, which is also a flattened tube, which
runs round the circumference of the animal, but in the opposite direction,
the festoons of the second circle alternating with those of the first. The
intestine opens into a short rectum which ascends vertically to open by the
anus. The stomach is accompanied by a small cylindrical tube called the
"siphon" (Fig. 231, _siph_), which opens into it at both ends; this
represents merely a gutter which has been completely grooved off from the
main intestine; it is lined by cilia, and its function is believed to be
that of keeping a stream of fresh water flowing through the gut, so as to
subserve respiration.
_Echinus esculentus_ seems to feed chiefly on the brown fronds of
_Laminaria_ and the small animals found thereon, which it chews up with its
teeth, but it may regale itself on the same diet as Brittle Stars, as
Allen[474] has shown to be the case in Plymouth Sound. Dohrn[475] has
described the Neapolitan _Sphaerechinus granularis_ attacking and capturing
Crustacea such as _Squilla_.
The WATER-VASCULAR SYSTEM presents several features of great interest. The
ring-canal is situated at a considerable distance above the nerve-ring, and
is separated from it by the whole of the jaws and teeth. It has five small
interradial pouches on it, which apparently correspond to Tiedemann's
bodies in an Asteroid. The stone-canal (Fig. 231) opens as {517}usual into
the ring-canal, and is accompanied by the axial sinus and genital stolon.
The name "stone-canal" is very unsuitable in this order, for there are no
calcifications in its walls; it is a simple membranous tube of circular
section. On reaching the upper wall of the test it expands into an ampulla,
into which the numerous ciliated pore-canals traversing the madreporite
open. The radial canals, starting from the ring-canal, pursue a downward
course till they come into contact with the radial nerve-cords, and they
then bend upwards and run along the centre of the ambulacral region,
finally terminating in the small terminal tentacles. In the just
metamorphosed Echinoid these are well-developed tube-feet, each with a
well-developed sucker, in the centre of which is a conical sensory
prominence, but as development proceeds they become enclosed in a circular
outgrowth of the test, so that only the tip projects in the adult.
The long extensible tube-feet are connected by transverse canals with the
radial canal. Instead of the pair of valves which in Asteroids prevent the
reflux of liquid into the canal, there is a perforated diaphragm[476] with
circular muscles, which by contraction close the opening in the diaphragm,
while when they are relaxed fluid can return from the tube-foot. The
ampulla is flattened, and is contracted by muscular fibres called
"trabeculae" stretching across its cavity. These muscular strands are
developed by the cells lining the ampulla. The external portion of the
tube-foot, as in Asteroids, is provided with powerful longitudinal muscles,
and there is the same alternate filling and emptying of the ampulla as the
tube-foot is contracted and expanded. The tube-foot is connected by a
double canal with the ampulla, the object of which is to assist in
respiration. The cells lining it are ciliated, and produce a current up one
side of the tube-foot and down the other, and the double canal leading to
the ampulla separates these two currents and prevents them interfering with
one another. Thus water is continually transported from the ampulla to the
tube-foot, through the thin walls of which it absorbs oxygen, and it is
then carried back to the ampulla, and transfers its oxygen to the fluid of
the general body-cavity through the walls of the ampulla. The disc of the
tube-foot is supported by a calcareous plate {518}(Fig. 232, _oss_), a
circumstance which enabled Johannes Müller to recognise the Echinoid larva
when the form of the adult was as yet unrecognisable. Below the edge of the
disc there is a well-marked nerve-ring, from which two bundles of
nerve-fibres go to the disc itself, in the edge of which there is an
abundance of sense-cells.
The buccal tube-feet (Fig. 229, 4) are much shorter than the rest, and are
provided with oval discs which are highly sensory. These feet are not used
for seizing, but for tasting food; when a piece of food is placed near them
they are thrown into the most violent agitation.
[Illustration: FIG. 232.—Diagrammatic transverse section of the radius of
an Echinoid. _amb.oss_, Ambulacral ossicle; _amp_, ampulla of the
tube-foot; _ep_, epineural canal; _musc_, muscles attaching spine to its
boss; _nerv_, nervous ring in base of spine; _n.r_, radial nerve-cord;
_oss_, ossicle in sucker of tube-foot; _ped_, tridactyle pedicellaria;
_perih_, radial perihaemal canal; _pod_, tube-foot; _wv.r_, radial
water-vascular canal.]
The NERVOUS SYSTEM has the same form as in an Asteroid, viz. that of a ring
surrounding the mouth and giving off radial nerve-cords (Fig. 232, _n.r_),
one of which accompanies each water-vascular canal to the terminal
tentacle, where it forms a nervous cushion in which pigmented cells are
embedded.
A large band-like nerve is given off from the radial nerve-cord to each
tube-foot. This pedal nerve, as it is called, contains bipolar neurons, and
is really an extension of the nerve-cord itself. Beneath the sucker it
branches out to form a sensory ring. From the base of the pedal nerve,
branches are given off {519}which run to the ectoderm and enter into
connexion with the plexus there. Romanes[477] scraped away the radial cords
and found that the spines still converged when a point on the ectoderm was
stimulated, but that, on the other hand, if definite locomotor movements
were to be carried out, the presence of these cords was a necessity; hence
he concluded that the superficial plexus sufficed for ordinary reflexes,
but that for purposeful movements the central nervous system was necessary.
Von Uexküll[478] has made an exhaustive study of the physiology of the
nervous system in the Echinoidea. He points out that all the organs
controlled by the nervous system, spines, pedicellariae, tube-feet, and
(see below) Aristotle's lantern, give two opposite reactions in response to
the same stimulus according as it is strong or weak, bending away from the
point of stimulation when it is strong and towards it when it is weak. This
reversal of reaction can only be due to the action of the neuron in
altering the effect of the stimulus on the muscles, and this Uexküll
regards as its fundamental property. Thus in Preyer's[479] experiments with
Starfish the strong form of stimulation is obtained by directly applying
the stimulus to the radial cord or to the tube-feet, the weak form by
stimulating the back, when of course the stimulus has to traverse a longer
path before affecting the tube-feet, and is consequently weakened. Von
Uexküll also introduces the conception of "tone" with regard to the nervous
system. This term has been used to denote the amount of chronic contraction
in a muscle, and it is to be distinguished from the fleeting contractions
which cause movement. The more tone there is in a muscle the less
responsive it is to stimuli tending to bring about movement. As applied to
the nervous system "tone" denotes a condition when it is not receptive to
small stimuli, but when it is maintaining a condition of tone in a muscle
by which of course its own tone is measured. Tone in a neuron can therefore
be measured by the produced tone in the muscle, and the one is to be
discriminated from the other only by using stimulants, such as caffeine,
which have no direct action on muscle. Tone can also be measured by the
amount of stimulus necessary to irritate the neuron. When {520}muscles are
stretched the tone is lowered, and this loss of tone extends to the neuron
controlling the muscle, and _vice versa_. When the spines on being gently
stimulated bend towards the point of stimulation, this is due to the
contraction of the muscles on the side towards the point of stimulus, for
if the superficial plexus of nerve-fibres be cut through so that the
stimulus has to pursue a round-about course the spine will bend towards the
direction from which the stimulus comes. The bending of the spines away
from the stronger stimulus is likewise due to the muscles on the side
towards the stimulus. It is caused by a sudden fall of tone in these
muscles, which causes them to yield to the tone of the muscles on the
opposite side, and this fall of tone is due to a fall of tone in the
neurons, for it can be produced by chemicals, and the direct action of all
chemicals applied to muscle is to raise tone.
In _Arbacia_ this form of reaction cannot be produced; the spines respond
to stimuli of all degrees of intensity by convergence towards the point of
stimulation.
When a general skin-irritant like dilute acetic acid, or even strong light,
is applied to the skin of a Sea-urchin the spines bend alternately to all
points of the compass, or, in a word, rotate. This is due to the fact that
the weight of the inclined spine stretches the muscles of one side and so
renders them more open to the general stimulus; these muscles in
consequence, contract, and so move the spine to a new position in which
other muscles are stretched, and a similar result follows. A continuation
of this process brings about rotation.
When a piece of glass rod or other light object is laid on the spines of a
Sea-urchin, it naturally, by its weight, presses asunder the spines and
stretches their muscles on one side, thus lowering the tone. If now the
skin be stimulated at any point the piece of rod will be rolled by the
spines towards the point of stimulation. This is caused by the fact that
the muscles of the spines holding the rod are made more receptive by being
stretched, and therefore they contract more than do the others in response
to the stimulation, and so the rod is rolled onwards on to the next spines,
which then act in the same manner. This passage of stimulus is entirely
independent of direct nervous connexion between the bases of the spines,
for it will traverse at right angles a crack going clean through the shell;
it is merely the {521}result of the mechanical weight of the object and of
the juxtaposition of the spines.
If the stimulation be too violent the first spines affected diverge wildly
and strike their neighbours with vehemence, so arousing into activity the
block musculature of these. This causes them to stand rigidly up, and so
the path of the stimulus is barred.
Now the escape movements of the animal under strong stimulation which
Romanes[480] alludes to are just an example of this handing on of
stimulation from spine to spine, not by nervous connexion but by mechanical
touch only; the object in this case is the substratum on which the animal
lies, which is, so to speak, rolled towards the point of stimulation, or
putting it otherwise, the animal is rolled away from it. Righting when
upset is another example of the same phenomenon; the aboral spines are
stretched by the weight of the animal, and the animal acts as if it were
stimulated in the region of the periproct. When a Sea-urchin is in its
normal position and is stimulated in the periproct (as for instance by a
strong light), it would, according to this rule, tend to move downwards,
which is of course impossible; but as the stimulus never affects all sides
quite alike the result is that the Urchin rotates, turning itself ever away
from the point of strongest stimulation. In the case of _Strongylocentrotus
lividus_ when living on limestone, as on the west coast of Ireland, this
results in the animal excavating for itself holes in the rock, where it is
safe from the action of the breakers.[481]
But it may be objected that no account is taken in the above description of
the action of the "central nervous system," _i.e._ of the ring and the
radial cords, and yet Romanes found that when they were removed the escape
movements could not be carried out. The answer is that the central nervous
system is a store-house of _tone_, not, as in higher animals, a controlling
centre for co-ordinating the movements of the spines. When it is removed at
first the escape movements can be carried out, but in a day or two all tone
in the spine-muscles is lost, and then, since the tone of all is equally
low, there is no tendency in those that are stretched to be more responsive
than others, and hence the escape movements cannot be carried out.
Sea-urchins kept in the tanks {522}of an aquarium are apt to lose the tone
of their spines owing to the poisoning of the nervous system.
The central nervous system is, however, the system which controls the
movements of the tube-feet. As we have seen, extensions of the radial
nerves run to the tip of each podium. Tube-feet are chiefly used in
ordinary progression; when this is quickened the spines come into play
exclusively. The extent to which these two organs of locomotion are used
varies from genus to genus. Thus _Centrostephanus_ uses its spines a good
deal, _Echinus_ and _Strongylocentrotus_ very little. The last-named genus
sometimes walks on its tube-feet entirely without touching the ground with
its spines.
The faculty of vision in its simplest form may be defined as sensitiveness
to light and shade. Now strong light acts on all Sea-urchins as a general
skin irritant. They fly from it towards the darkest corner, and then if it
continues the spines rotate. A number of little violet spines on the aboral
pole of _Centrostephanus longispinosus_ are especially sensitive to light,
and hence are almost constantly in rotation. This is due, according to
Uexküll,[482] to a pigment of a purple colour, which can be extracted by
means of alcohol and which is decomposed by light, the products of
decomposition being supposed to irritate the nerves. _Centrostephanus_ when
exposed to light becomes darker in colour. This is due to the migration
outwards of amoebocytes, which carry a pigment which acts as a screen in
order to prevent the valuable visual purple being too rapidly decomposed.
Not all Sea-urchins, in fact very few of those living in northern waters,
give a reaction to shadow. _C. longispinosus_ is one of the few; it reacts
to a shadow by converging its spines towards it. A much larger number of
species inhabiting tropical waters show this reaction. It is entirely
stopped if the radial nerve-cords be removed, whereas the reaction to
strong light continues. The reaction to shade is strongest after a long
previous exposure to light, hence Uexküll has given the following
explanation of it. The continued irritation due to light, having spread to
all the spines, eventually reaches the radial cords and is there stored in
the bipolar nerve-cells as _tone_. When the light-stimulus is interrupted
some of {523}the stored tone spreads upwards to the spines, causing the
weak form of spine reaction, and the spines converge.
[Illustration: FIG. 233.—To show character and distribution of the
sphaeridia in _Strongylocentrotus droëbachiensis_. A, a portion of a
radius, with sphaeridia, and the adjoining edge of the peristome. _p_, Pair
of pores for a tube-foot; per, peristome; _t_, primary tubercle. B, an
isolated sphaeridium. (After Lovén.)]
It will be seen therefore that the so-called central nervous system of
_Echinus_ does not act in any sense as a brain, as indeed might have been
guessed from the absence of any differentiation in it. As Uexküll points
out, when an animal is covered all over with similar organs, such as spines
and pedicellariae, capable of acting automatically, a brain is not needed.
The object of a brain is to direct organs which are in a certain place to a
danger which may come from any quarter, but in the Sea-urchin any spine is
as good as any other spine, and such orientation is not needed. "In a dog
the animal moves its legs, in a Sea-urchin the legs move the animal." What
the Sea-urchin does need is a means to prevent its pedicellariae attacking
its own organs with which they may come into contact. Thus it possesses an
"autodermin," a chemical contained in the ectoderm which paralyses the
muscles of the pedicellariae, as may be seen by offering to them a spine of
the same animal. If, however, the spine be treated with boiling water, and
then offered, it is viciously seized, showing that this substance can be
dissolved out.
Just as in the case of the Starfish, when the nerve-ring is {524}cut
through, the tube-feet in the various radii are no longer co-ordinated with
one another.
Besides the tips of the tube-feet the Urchin possesses another kind of
sense-organ, the SPHAERIDIA (Fig. 233). These are minute glassy spheres of
calcareous matter attached by connective tissue to equally minute bosses on
the plates of the ambulacra, generally near the middle line. They are in
fact diminutive spines, and like the latter are covered with a thick layer
of ectoderm, beneath which is a particularly well-developed cushion of
nerve-fibrils. Only the layer of muscles which connects a normal spine with
its boss is wanting. Although definite experimental proof is lacking, the
whole structure of the sphaeridia shows that they belong to the category of
"balancing organs." As the animal sways from side to side climbing over
uneven ground, the heavier head of the sphaeridia will incline more to one
side or to another, and thus exercise a strain on different parts of the
sheath, and in this way the animal learns its position with regard to the
vertical.
Intervening between the radial nerve-cord and the radial vessel is a single
RADIAL PERIHAEMAL CANAL (Fig. 232, _perih_), representing the two parallel
canals found in the same position in the Asteroid. The five perihaemal
canals lead downwards to a space called the LANTERN-COELOM, surrounding the
oesophagus.[483] Since the skeleton of the corona is composed of plates
immovably connected together, muscles corresponding to the ambulacral
muscles of the Asteroids would be useless, and so the wall of the
perihaemal canal remains thin and the side of it turned towards the general
coelom develops no muscles, and that turned towards the nerve-cord no
nerve-cells. Where, however, the radial nerve enters the nerve-ring, and on
the ring itself, an inner layer of nerve-cells is developed from the
lantern-coelom which represents the lower or oral portions of the radial
perihaemal canals. These cells control the muscles moving the teeth. These
canals are originally parts of the lantern-coelom, but in the adult they
become closed off from it.
{525}[Illustration: FIG. 234.—_Echinus esculentus_ dissected in order to
display Aristotle's lantern, × 2. The whole upper part of the shell has
been cut away. 1, Upper growing end of tooth; 2, outer forked end of one
"compass"; 3, muscle joining adjacent compasses and acting as elevator of
these ossicles; 4, depressor of the compasses; 5, lower end of jaw; 6,
retractor of the whole lantern; 7, protractor of the whole lantern; 8,
auricle; 9, ampullae of the tube-feet; 10, interambulacral plate; 11, lower
part of tooth; 12, water-vascular ring; 13, meeting-point of a pair of
epiphyses; 14, so-called Polian vesicle, really equivalent to Tiedemann's
body in an Asteroid; 15, oesophagus; 16, so-called ventral blood-vessel;
17, genital stolon; 18, stone-canal; 19, rectum; 20, aboral sinus. (Partly
after Chadwick.)]
In the outer wall of this space are developed the calcareous rods forming
ARISTOTLE'S LANTERN. These are first: five teeth (Fig. 234, 11),
chisel-shaped ossicles of peculiarly hard and close-set calcareous matter,
the upper ends (1) pushing out projections of the upper wall of the
lantern-coelom. These projections are the growing points of the TEETH,
whose lower ends pierce the {526}ectoderm and project into the lower end of
the oesophagus. Each tooth is firmly fixed by a pair of ossicles inclined
towards one another like the limbs of a V and meeting below. Each ossicle
is called an "alveolus," and taken together they form a "jaw." Their upper
ends are connected by a pair of ossicles called "epiphyses" (13). These two
epiphyses meet in an arch above. The jaws and their contained teeth are
situated interradially. Intervening between successive alveoli are radial
pieces called "rotulae," which extend directly inwards towards the
oesophagus. Above the rotulae are pieces termed "radii" or "compasses" (2),
which are not firmly attached to the other pieces but lie loosely in the
flexible roof of the lantern-coelom.
The uses of the various components of this structure can be made out from
an inspection of the muscles which connect them together.
Overarching each radial perihaemal canal where it leaves the lantern is a
bridge of calcareous matter called the "auricula" (Fig. 234, 8). This
arises as two rods which meet each other in a pent-house over the canal. It
is the only part of the skeleton which can be compared to the ambulacral
ossicles of the Asteroidea, and like them it serves as the point of
insertion for important muscles. Thus we find (1) protractor (Fig. 234, 7)
muscles which arise from the upper ends of the alveoli and are inserted in
the auricula; when these contract they tend to push the whole "lantern"
outwards so as to expose the tips of the teeth. (2) The retractor muscles
(Fig. 234, 6) extend from the auriculae to the lower ends of the jaws and
restore the lantern when it has been extruded to its original position. (3)
The comminator muscles connect adjacent jaws with one another: these on
contraction approximate the pair of jaws into which they are inserted, and
it will easily be seen that by the successive contraction of the five
comminator muscles a rotating movement of the teeth would be produced which
would cause them to exert an action something like that of an auger; by
their simultaneous contraction the teeth are brought to a point. (4) The
internal and external rotula muscles: these are small muscles which connect
the outer side of the epiphysis with the rotula. There are two facets on
the epiphysis, which permit it to rock to and fro on the rotula under the
action of these muscles. This rocking action must greatly increase the
cutting power of the {527}tooth. These muscles are controlled by the
nerve-ring and the incipient portions of the radial nerves, which, as we
have seen, have an inner layer of nerve-cells. If the nerve-ring be gently
stimulated on one side the upper end of the lantern bends away from the
spot, causing the lower end, _i.e._, the teeth, to move towards it; but a
stronger stimulation produces the opposite effect, just as is the case with
spines. But besides these masticatory muscles there are others which have
nothing to do with moving the teeth. These muscles are attached to the rods
called radii or compasses (Fig. 234, 2),[484] which lie in the upper wall
of the lantern-coelom, and may be termed the compass muscles. There are two
sets:—(1) The elevator muscles (Fig. 234, 3), which connect the inner ends
of the compasses with one another. When these contract, the radii tend to
bend upwards at the inner ends and thus raise the roof of the coelom. (2)
The depressor muscles (Fig. 234, 4), which run downwards from the forked
outer ends of the compasses to the auriculae. Uexküll[485] has shown that
the function of these muscles and of the rods to which they are attached is
respiratory. These muscles are also controlled by the nerve-ring. If this
be stimulated by passing a pin-head into the oesophagus, the roof of the
lantern cavity is raised by the contraction of the elevator muscles. This
is followed by contraction of the depressor muscles lowering it; the same
result may be brought about by placing the animal in water with excess of
carbonic acid. The ten branched gills described on p. 514 are outgrowths of
the lantern-coelom. When the roof of this cavity is depressed the fluid
contents are driven out into the gills, which are thus expanded and then
absorb oxygen from the surrounding sea water. When, on the other hand, the
roof is raised the aerated water is sucked back into the lantern cavity,
and the oxygen passes easily through the thin walls of the lantern into the
fluid filling the main coelomic cavity. There are thus two independent
respiratory mechanisms in the Sea-urchin, the one being the compass
muscles, the other the cilia lining the interior of the tube-feet.
The function of EXCRETION is performed, as in Asteroidea, by {528}the
amoebocytes floating in the general coelomic cavity. These in part escape
through the thin bases of the gills. In other parts of the body they seem
not to succeed in reaching the exterior at all, but to degenerate and to
form masses of pigment; the colour of the animal is largely due to these
excrementitious substances.
The REPRODUCTIVE SYSTEM, as in the two preceding orders, consists of a
vertical pillar, the "genital stolon," and a circular "genital rachis"
giving off interradial branches from which the genital organs bud. The
genital stolon is developed from the wall of the general coelom near the
upper end of the axial sinus; it attains a great development and ultimately
completely surrounds the axial sinus, which then appears like the cavity of
a glandular tube, the walls of which are constituted by the genital stolon.
The compound structure consisting of stolon and axial sinus was actually
described as a nephridium by the Sarasins[486] in the case of
_Asthenosoma_. Its true nature, however, is shown when the upper end is
examined; it is then seen to open into the stone-canal and to be in
communication with the ampulla, into which the pore-canals open. Lying
alongside the upper end of the axial sinus is the somewhat elongated
"madreporic vesicle," or right hydrocoele, which was described by Sarasin
as the accessory kidney (_Nebenniere_), since like the axial sinus it is
partly enveloped by the genital stolon. Leipoldt,[487] however, showed
clearly that it is a completely closed space.
The genital rachis springs from the upper end of the stolon, and as in
Asteroids, it lies in the outer wall of a space called the "aboral sinus"
(Fig. 234, 20) intervening between it and the test. In adult specimens it
seems to degenerate. The genital organs are situated at the ends of five
interradial branches of the rachis (Fig. 231, _gon_). Each is an immense
tree-like structure consisting of branching tubes, which are lined by the
sexual cells. So enormous do they become in the breeding season that they
form an article of food among fishermen. The term _esculentus_ is derived
from this circumstance. Other species are regularly sold for food as
_Frutta di Mare_ (Fruit of the Sea) at Naples, and {529}as "sea eggs" in
the West Indian Islands. One female _Echinus esculentus_ will produce
20,000,000 eggs in a season.