ECHINODERMATA (531 " Aristotle s lantern." The calcined internal arched pro cesses termed auriculae, at the oral end of the ambulacra in the Echinoida (of the interambulacra in Cidaris), regarded as homologous with the internal ambulacral ossicles of the Asteridea and Ophiuridea, are formed each of two pieces. From the top of the auriculae pass retractor muscles to the outer edge of the alveoli. The oral skeleton is provided also with protractors proceeding from the alveoli to the lower interambulacral edge of the corona, besides special muscles connected with the radii. In the Clypeastroida arched (in Cidaris unarched) processes are given off from the ambulacral plates, at the sides of the ambulacral canals ; and in the Clypeastroid genus Scutella the dorsal and ventral walls of the corona are connected by vertical calcareous plates or trabecube. None of the above- mentioned internal calcareous processes is developed in the Spatangoida. The mouth communicates by a tortuous oesophagus with the stomach. The intestine forms a sort of festoon on the inner side of the shell, and is attached to it by a mesentery. Of the internal organs of the Ecliinidea the most import ant and characteristic are those constituting the ambulacral system. These are (1) a circular or slightly pentagonal vessel placed around and traversed by the oesophagus, on the inside of the alveoli, and between the nervous and blood-vascular rings ; (2) ctecal appendages of this vessel, called Polian vesicles (absent in the Clypeastroida and Spatangoida}, answering to the racemose appendages of the Holothuridea ; (3) the membranous or calcareous madreporic canal, termed also the sand-canal, which runs nearly vertically through the axis of the body, and, communicating with the exterior by the madreporic tubercle, supplies water to 1 ; (4) vessels radiating from 1 along the parietes of the body, and opening eventually in f o basal sacs, or ampulla?, proceeding from the canals of the pedicels shortly above their origin. In the vessels of the ambulacral system is contained a watery fluid strained from the perivisceral cavity. The pedicels, which may vary considerably in shape, are tubular structures, usually terminated by a sucking-disk ; they have contractile, muscular walls, and are capable of being protruded beyond the extremities of the spines. They subserve locomotor, tactile, or branchial functions. The corpusculated perivisceral fluid is kept in motion by the cilia clothing the lining membrane of the body and the viscera. Where modified pedicels or ambulacral gills are absent, as in the Echinoida, the Cidaridoe excepted, aeration of that fluid is apparently promoted by branchial develop ments from the peristome, the hollow stems of which com municate with the body-cavity. The nervous system con sists of a slender, pentagonal, red or violet hoop around the gullet, superficial to the circular ambulacral vessel, with five ganglia sending off as many cords, which, passing out between the alveoli, take a course similar to that of the ambulacral radial canals, giving off fine side branches which pass in their course through the ambulacral pores, pro bably supplying the pedicels, spines, and pedicellarise, and terminate eventually in the pigtnented eye-spots. The principal vessels of the pseud-heemal or blood-vascular system of the Echinidea appear to be two trunks, the one on the dorsal, the other on the ventral side of the alimentary canal ; these, according to Hoffmann, com municate either directly or by a distinct trunk with the vater-vascular ring. From an examination of Echinus sphccra, Psamtnecl Linus miliaris, Toxopneu-stes lividus, and Amphidctus cordatus, Perrier deter mined that, as maintained by Hoffmann, the circulatory and aqui ferous systems are identical; that the so-called "heart" is only a gland, which opens by a canal, in to a funnel-shaped space bounded by the lining membrane of the test and the madreporite ; that the artery proceeding from the water-vascular ring is distributed upon the first loop of the intestine, forming there ramifications which unite with, those of the intestinal vein ; and that the vein has no communication with the water- vascular ring, but is connected by ten branches and by its two extremities with a collateral canal, which floats freely in the perivisceral cavity beneath the intestine. Further, he found that the ambulacral vessels and their branches terminate blindly, the circulation consisting simply in a to-and-fro movenient.of their contents. The reproductive organs are large racemose glands, situated beneath the upper termination of the inter- ambulacra, and opening externally by the genital pores. The sexes are distinct. The spermatozoa have vibratile filaments ; the egg is fecundated after leaving the body of the female, and in about eight hours undergoes complete yelk-division. The pseud-embryo or echinopsedium, at first ciliated and spheroidal, becomes after a time wedge-shaped; at its broad end appears the mouth or jiseudostomc, and at the other the anus or pseudoproct. Simultaneously with these the skeletal rods and ciliated bands of the pluteus begin to be produced. The develop ment of the ambulacral system commences with the formation of a sac which lies to the left of the junction of the pseud-embryonic gullet and stomach, and is prolonged into a canal opening by a pore on the dorsal surface of the larva. The blind end of the sac becomes a quinque-petaloid rosette, from which radiate the ambu lacral vessels ; a new mouth is formed in the centre of this, at the bottom of a depression in the integument of the pseud-embryo, and the canal of the sac becomes the madreporic tube. The skele ton of the pluteus separates, as development proceeds, into several pieces, and is by degrees discarded, whilst its processes atrophy, and the body assumes the rounded form of the embryo urchin. This, however, has in many cases to undergo sundry important changes before its resemblance to the adult is complete. Thus in the young of species of Spatangus the peristome is almost central, and is pentagonal in form. Tnthe Echinoida, before the appearance of the anus in the embryo, its place is occupied by a single plate, the sub-anal, and the anus appears near its border, towards the posterior right ambulacrum. It lies within a circle formed by five imperforate pieces, the future genital plates, and these again aie surrounded by five imperforate ocular plates, with intervening ambulacral plates. The central anal plate persists in the family Salenidce among the Echinoida. If it be considered as homologous with the dorso-central plate of Marsupitcs, or the basalia of the calyx in other Crinoids, the genital plates correspond to the parabasalia, and the ocular to the first radialia. The food of the Echinidea consists either of seaweed, and small shell-fish and crustaceans, which are conveyed to the mouth by the pedicels, or, as in the case of the edentulous forms,, of sand and earth containing nutritive material. In the species Anochanus sincn- sis, one of the Cassidulidcc, Grube discovered the presence of an incu batory chamber at the apical pole of the test, containing embryos in various stages of their growth. Certain species, as observed by Cail- liaud, Deshayes, and Lory, have the power at a very early age of drill ing for themselves burrows in the hardest rocks, such as granite and grit- Allusion is made to the echinus in the writings of Aristophanes, Horace, Martial, and other classical authors. By the ancients it was considered a delicacy, and the common species Psammcchinus (Echinus, L.) esculentus, Ag., especially in spring, when the ova are matured, is still eaten in some parts of Europe. Sir Thomas Browne, in his Vulgar Errors, mentions a notion formerly current that the spines of the echinus were a remedy for the stone, and films in horses eyes." They are put to practical use in some countries as slate-pencils. Various systems of classification have been adopted for the Echinidea. In the following scheme the principal groups are arranged chiefly according to the position of the anal opening and the nature of the ambulacra . Sub-order I. TESSKLATA. Echinidea with interambulacra of more than two rows of plates. Sub-order II. TYPICA. Echinidea with interambulacra of but two rows of plates. Group I. REGULARIA or ENDOCYCLICA. Mouth central or sub- central ; anus usually central and opposite the mouth, and never exterior to apical disk. A. Anus central. i. Shell round. Cidaridce. Tubercles smooth, perforate, peristome unnotched; anal plates ten ; auriculae unclosed; buccal branchiae absent. Dicidematidce. Tubercles crenulate, perforate, peri stome notched, spines hollow. Allied forms are
the fossil Hemicidaridce.