The Echinodermata, including the Sea-urchins, Star-fishes, Sea-cucumbers, etc, form a very distinctly circumscribed group of the animal kingdom, and were formerly included in the old sub-kingdom Radiata. To Professor Huxley is due the credit of having first pointed out that the Echinoderms possess certain remarkable affinities with the lower Worms, and especially with those "Scolecids" which constitute the order of the Turbel-laria. So well marked are these affinities that the above-mentioned eminent zoologist at one time proposed to unite the Echinodermata with the Scolecida, to form a common division, or sub-kingdom, under the name of Annuloida; and there are many aspects in which this arrangement presents itself as a highly convenient one. The progress of modern Zoology has, however, shown that it is not possible to establish rigidly defined primary divisions of the animal kingdom; but that any such divisions must inevitably be more or less artificial, as including certain inosculating forms which lead by a more or less insensible gradation into neighbouring groups. Thus, in the group now in question, while there can be no doubt as to the affinities which subsist between the Echinodermata and the Scolecida, the latter, in turn, exhibit strong points of relationship with the lower Annulosa. While, therefore, we must not fail to recognise the points of resemblance between the Echinoderms and the Scolecids, it seems, upon the whole, best to separate the Echinodermata as a distinct primary division or "sub-kingdom," and to regard the Scolecida as a special section of the sub-kingdom Annulosa.
The Echinodermata may be defined as follows: -Simple marine organisms, the body of the adult more or less conspicuously radiate, that of the young often distinctly bilateral.
An alimentary canal, with or without a distinct anus, but never communicating with the body-cavity. The water-vascular (ambu-lacral) system often subserving locomotion. Nervous system radiate, composed of an oesophageal ring and radiating branches. Sexes generally distinct, rarely united.
The members of this class are known commonly as Sea-urchins, Star-fishes, Brittle-stars, Feather-stars, Sea-lilies, Sea-cucumbers, etc.; and though the fully-grown animal often exhibits distinct traces of bilaterality, this is usually more or less completely masked by the general radiate arrangement of the parts of the body. On the other hand, the embryonic Echino derm usually shows distinct bilateral symmetry. The outer layer of the general integument ("perisome") is ciliated, and the inner layer is more or less hardened by the deposition of carbonate of lime in the form of plates, granules, or spicules. In all adult Echinoderms there is a system of tubes, termed the "am-bulacral system," which generally subserves locomotion, and usually communicates with the exterior. This water-vascular system surrounds the commencement of the alimentary canal, and in almost all cases gives off secondary vessels in a radiating manner. An alimentary canal is always present, and is completely shut off from the body-cavity. A vascular (pseudo-haemal?) system is generally developed in addition to the true water-vessels. The nervous system in all the adult Echinoderms is a ring-like, usually gangliated cord, which surrounds the oesophagus and sends branches parallel to the radiating ambulacral canals.
The process of development is sometimes direct; but in the typical members of the class a characteristic form of metamorphosis occurs. The impregnated ovum gives exit to an ovoid embryo or "planula," freely locomotive by means of cilia, which are at first diffused over the body, but which soon becomes restricted to transverse bands, or to definite outgrowths of the body ("epaulettes") which are disposed with bilateral symmetry. The larva or "pseudembryo" (fig. 92) next develops an alimentary canal, with a distinct mouth and anus, dividing the embryonic body into two bilaterally symmetrical halves. A mass of actively formative protoplasm now appears on one side of the stomach, within which are developed a circular and radial tubes, the whole being the rudiment of the ambulacral system of the future Echinoderm. A symmetrical calcareous skeleton, not converted into that of the adult, may be developed in the larva (as in the Echinoids and Ophiu-roids), or it may be wanting (as in the Asteroids and Holo-thuroids). The mass of protoplasm, above mentioned as developed on one side of the stomach, rapidly increases in size, envelops the stomach, which it appropriates, and is ultimately converted into the adult Echinoderm; the remainder of the larva being absorbed or cast off as useless.
The essential peculiarity of the development of the typical Echinoderms, as above summarised, is that the larva possesses provisional organs, which may be ultimately absorbed or thrown off, but which are not converted into the corresponding structures of the adult. Thus the larva of an Echinoid (fig. 92) possesses a mouth and alimentary canal, which are not converted into, 'and in no way correspond with, the mouth and alimentary canal of the adult. The larva, or "pseudem-bryo," as it is termed by Sir Wyville Thomson, leads a perfectly independent existence, and the true Echinoderm is produced from it by a process of internal budding or rearrangement.
Sir Wyville Thomson has, further, shown that there are various cases amongst the Echinoidea, Asteroidea, Ophi-uroidea, and Holothuroidea, in which the young are developed directly from the egg, without the intervention of a locomotive pseudembryo. In these cases, the eggs are hatched, and the young are brought up, "within or upon the body of the parent, and are retained in a kind of commensal connection with her until they are sufficiently grown to fend for themselves." There is no sort of organic connection in these cases between the young and the parent; but the young are often brought up in a special receptacle upon the exterior of the mother, to which the appropriate name of the "marsupium" has been given. This viviparous mode of reproduction seems to obtain specially among the Echinoderms of the cold northern and southern seas.
The Echinodermata are divided into seven orders - viz., the Crinoidea, Cystoidea, Blastoidea, Ophiuroidea, Asteroidea, Echinoidea, and Holothuroidea. Of these, the first is to a considerable extent extinct, and the two next are entirely so; they are really the lowest orders; but their structure will be better understood if the higher orders are considered first.
Fig. 92. - Larva of Echinus (after J. Muller). A A, Front arms with their internal skeleton ; F F, Arms of the mouth-process; B, Posterior side-arm; a Mouth; a' OEsopha-gus; b Stomach; b' Intestine; d Ciliated bands; f f Ciliated epaulets ; c Disc of the future Echinus.