ECHINODERMATA. 621 ECHINODERMATA. chiefly comiu>si'U of connective tissue, within which are deposilcd disconnected particles of lime, wliile in most sea-urchins there is little connective tissue and the earhonate of lime is deposited in firm plates of definite size and shape lirmly welded into a hard shell, or 'test,' as it is commonly called. This external cover- ing usually functions nuile as nuich 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 immovably or by a joint which permits of perfect freedom of movement. ViscER.vi. An.tojiy. There is well-developed alimentary canal suspended in and distinct from the general body -cavity or ca?lome, and provided with botli mouth and anus, though the latter may be more or less suppressed in some groups. A combined respiratory and locomotory system of complicated nature, called the water-vascular sjstem, is peculiar to the members of this phy- lum only. A nervous system, blood-vascular sys- tem, and reproductve system are more or less fully developed. The nervous system is variously modified in the diti'erent classes of echinoderms. In all there is a well-developed epidermal system, with cir- eimioral 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 cases. The water-vascular system, which is present in more or less complete form in all members oi the group, consists of a ring vessel about the mouth. From this ring vessel a number of tubes, gen- erally five, radiate to the ambulacral 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 ambulacral areas. These tube feet function not alone as locomotory organs, bi-t also as respiratory or- gans, and in some classes they assist in convey- ing food to the mouth. The cireumoral ring vessel has communication with the outside, in most echinoderms. through a tube of variable form, called the stone canal or madreporic canal, which extends from the ring vessel to a special plate, the madreporic plate, on the aboral side of the body. This plate is sieve-like, and through it is sucked the water that fills the water- va.scular 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 complicated nature are present in connection with particular organs, as the jaws of eehinoids. The mouth of echinoderms is sometimes, espe- cially in the Ilolothuroidea, surrounded by a crown of tentacles. Usually it is a simple open- ing, but ill the Echinoidea (sea-urchins) it is provided with a complex system of jaws that form the so-called "Ari-stotle's lantern.' Out- growths from the alimentary canal, called 'respi- ratory trees,' found in the hololhurians, and an accessory intestine, attached to the intestine proper in the eehinoids, and oilier modilicalions of various 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- duces a bilaterally symmetrical free-swimming larva, which difl"ers somewhat in form in the dilTcrcnt classes. Three types of larva have been distinguished, and they have received the names auricularia. bipinnaria. and pluleus. EcoLOCiY. About ;iOOO living echinoderms are known. They are all marine, and the sjjecies are scattered through all seas of the globe and are found at all depths, but they are most abundant iv the warm seas of the tropics. The only fixed members of the branch are those included in the sub-branch Polinatozoa, comprising the crinoids, cystoids, and blastoids. JIany echinoderm^. espe- cially the crinoids, are gregarious, and they are found associated together in vast numbers. Echinoderms are largely bcnthonic organisms, crawling slowly on the bottom of the sea by means of the tube feet or arms, as in the star- fish and sea-urchins, or by the oral tentacles, as in holothurians. The fixed pelmatozoans are cither bcnthonic. when fixed to the bottom, or pseudo-planktonic, when attached to floating EXPLANATION OF THE PLATE OF ECHINODERMATA. 1. Section of the ra.v of starfisli, sliowing structure of the skeleton : a, a, ambulacral ossiclet^, between which lies the anihulacral vessel in the ambulacral grroove. 2. Aboral plates of a starfish: a, madreporite; b. anus: t', one of the five baeal plates: rf, one of the five radial-s- e, circle of secondary radials: f, dorso-centrnl : fr. terminal or ocular plat^' (compare Plate of Crinoids. Fig. 7). 3. .boral view of a sea-urchin, stripped of epidermis: a. ambulacral plates, in five zones; c, plates of interambulacral zone. 4. Tube feet of a starfish, as used in turning the body over. 5. Section of the disk and arm of a starfish: a. mouth, leadin^in to the wide stomach: />, anus: e.heitatic prtdonpra- tion of the stomach into the cavity of the arm: rf. genital organs: e, madreporic plate (oiiening of madreporic or stone canal); f. interradial diverticulum of the rectum; ^, ambqlicral feet. 6. Diagram of the water-vascular system of a starfish: .t. oval circular vessel ; ^. one of the ampullte or Polian vesicles: c, stone canal ; (7. madreporic plate ; e, one of the series of ambulacral feet connect^Ml with the side twigs of the radial canal ; f, one of the anii>nllie of the p;ime. 7. transver.se sections of the ambulacral fields of two blastoi<ls. shf)wing forms of hydrospires: ». Onipho- crinua: /). Granatocrinus. 8. Anatomy of a holothurian : t, tentacles: r, calcareous ring at the base of the tentacles: p, Polian vesicle: s, sand cnnal; i. i, i, alimentary canal ; ^. duct of the reproductive organs: el. cloaca: a. anus: 7, 7, respiratory tree. 9. Vertical section of an echinoid (sea-urchin) : a, ocular plate ; b, anus : e. nbornl ring bIood-ves.sel ; ^7. madreporite: e, radial extension of the stomach, surrounding the radial nerve: f, rectum: g, ovoifl gland: h, intestinal blood-vessel: /.stone canal:./, alimentary canal; *. Polian vesicle; 7, /. muscles imssing from the auricles to Aristotle's lantern ; m, oral ringblood- vessel : n, radial ambulacral vessel; o. siphon: /*. intes- tine; q, ccplome ; r, radial nerve: s. radial ambulacral vessel; t. ampulla> ; w. ambulacral feet: v. radial nerve; ir. mouth : .v. auricle, in. Horizontal section of n sea- urchin, showing organs (letters as in Fig. 9) : also. c. (psophagns; /■/', ovary ; m', .rlstotIe's lnnt**rn. 11. .pieal Hvst<*m of plates and al>ornl extremities of zones of the shell of a sea-urchin: », ambulacral zones; h. genital plates: r. (black) ocular plates ; fi. interambulacral zones: e, nin»lreporite; /. periproct. 12-15 lliagranm (if the de- velopment of the larvtt> of echinoderms; 12, primitive form;!.'*, avicnlarla (holothurians): 14. a pluteus (sea- urchins and brlttlestars) ; 15. a bipiunnrln (starfishes).
