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 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).
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 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.
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.
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.