ECHINODERMATA, (Neo Lat. nom pl.. from Gk. ixiyar, echinos. hedgehog olpaa, (if rinu, skin). One of the great branches or phyla of invertebrate animals, constituting an independent. well-characterized of or ganisms. The group is of higher rank than the Codenterata, and is lower than the Verna's:, WWI both of which groups it has few features in eommon. The animals of this phylum do not 11 produce by either budding or fission. and therefore they are never found in compact colo nies like those formed by the codenterates. The body of the animal is built. more or less con stantly throughout the phylum. on the pentain era] plan of bilateral symmetry, in accordance with which the number five is a dominant feature in the arrangement of the parts.
ExTeux CovEmNo. The external covering of the echinoderms is remarkable for its diversity, for even within the limits of the same family we find a soft, leathery body-wall at one ex treme. and a firm. almost immovable 'shell' at the other. This diversity is entirely due to the amount of mineral matter deposited in the body wall, and the arrangement of the plates formed from it. In the holothurians the body-wall is chiefly composed of connective tissue, within which are deposited disconnected particles of lime. while in most sea-urchins there is little connective tissue and the earbonate of lime is deposited in firm plates of definite size and shape firmly welded into a hard shell, or 'test,' as it is commonly called. This external cover ing usually functions quite as much as a skele ton for the attachment of muscles as it does as a 'shell' for protection. The body-wall is often covered with more or less evident knobs, tubercles, or spines, and from the frequency of the latter has arisen the name 'spiny-skinned' animals. These knobs and spines are attached to the plates of the skeleton, either iM1/10V ably or by a joint which permits of perfect freedom of movement.
VIscEn.u. ANATOMY. There is well-developed alimentary canal suspended in and distinct from the general body-cavity or aelome. and provided with both mouth and anus, though the latter may be more or less suppressed in some groups.
combined respiratory and locomotory system of complicated nature, called the water-vascular s:%stem, is peculiar to the members of this pity hum only. A nervous system, blood-vascular sys tem, and reproductre system are more or less fully developed.
The nervous system is variously modified in the different classes of echinoderms. In all there is a well-developed epidermal system, with cir eumoral ring and radial branches.
Sense-organs are few and imperfect. We find occasionally pigment eyes. ciliated pits (per
haps olfactory), and positional organs. The sense of touch is 'well developed. The blood-sys tem is rarely developed, though many forms have an 'axial organ and strands.' the anatomy and physiology of which are still little understood. Special respiratory and excretory organs are found in a few eases.
The water-vascular system. which is present in more or less complete form in all members of the group, consists of a ring vessel about the mouth. From this ring vessel a number of tubes. gen erally five, radiate to the ambulaeral areas, or `radii.' These tubes connect with the 'tube feet,' or ambulacra, which are extensible sac-like ten tacles that project from the surface of the body through pores in the plates of the ambulaeral areas. These tube feet function not alone as locomotory organs, bet also as respiratory or gans, and in some classes they assist in convey ing food to the mouth. The circumoral ring vessel has communication with the outside, in most echinoderms, through a tube of variable form, called the stone eanal or madreporie canal, which extends from the ring vessel to a special plate. the madreporie plate, on the alloral side of the holy. This plate is sieve-like. and through it is sucked the water that fills the water vascular system. Connected with the ring ves sel. in all classes except the Crinoidea, are, peculiar bladder-like organs, the Polian vesicles. that serve to regulate the pressure in the radial tubes. Lack of active locomotion obviates the necessity for a well-developed muscular system, although some special muscles of nature are present in connection with particular organs, as the jaws of cehinoids.
The mouth of echinoderms is sometimes, espe cially in the llolothuroidea, surrounded by a crown of tentacles. Usually it is a simple open ing, but in the Echinoidea tsca-urchins) it is provided 11 complex t•ySt1.111 of jaws that form the so-called 'Aristotle's lantern.' Out growths from the alimentary canal. called 'respi ratory trees,' found in the holothurians, and an accessory intestine, attached to the intestMe proper in the eehinoids, and other modifications of va•ious organs, constitute the respiratory organs: there are no strictly special respiratory organs in the members of this branch.
The sexes are separate, and development takes place through metamorphosis. The egg pro duce.; a bilaterally symmetrical free-swiinming larva, Which differs somewhat in form in the different classes. Three types of larva )lave been distinguished, and they have received the names aurieularia. bipinuaria. and plotens.