(516). Yet even these are not all the muscles that act upon the masticating apparatus: ten others (h h), arising in pairs from the middle of the interspaces between the arches, are connected with the bifurcated extremities of the slender curved processes (e e), each of these receiving a muscle from two contiguous spaces; and, from the length of the levers upon which these muscles act, we may well conceive the force wherewith they will influence the motions of the whole mass of the jaws.

(517). Such is the complex structure of the mouth of Echinus escu-lentus - a piece of mechanism not less remarkable on account of the singularity of its construction, than as exhibiting an example of the sudden development of a dental system, whereof not a vestige is visible in any of the preceding Echinoderm families. In others of the Echi-nidae having the shell much depressed, the dental lantern is modified in form and proportionately flattened, but the different parts are essentially similar to those we have described.

(518). The oesophagus (fig. 98, d) is continued from the termination of the central canal that traverses the axis of the lantern, and, after a short course, terminates in a much wider portion of the digestive tube, into which it opens on the lateral part of its ca?cal origin, in a manner precisely resembling the communication between the large and small intestines of man.

(519). The dilated alimentary tube (c) presents no separation into stomach and intestine, but is continued in a winding course around the interior of the shell, which it twice encircles, and, becoming slightly constricted, terminates at the anal orifice (i.) The walls of the intestine are extremely delicate, although they may be distinctly seen to contain muscular fibres and are covered with innumerable vascular ramifications. The external tunic of the whole canal is derived from the peritoneum, that lines the entire shell, invests the dental lantern, and forms sundry mesenteric folds as it is reflected upon the other viscera.

(520). The system of vessels provided for the circulation of the blood has been differently described by different authors - a circumstance by no means surprising when we consider the great difficulty of tracing such delicate and extensively-distributed canals. According to Delle Chiaje, the course of the nutritious fluid is as follows: - A large vein runs along the whole length of the intestine, from the anus to the oesophagus, where it terminates in a vascular ring surrounding the mouth, - into which, as in Asterias, the contractile vesicle, which he considers to be a receptacle for the nutrient fluid, and the antagonist to the tubular feet, likewise opens. The intestinal vein he regards as the great agent in absorbing nourishment from the intestine and conveying it to the vascular circle around the oesophagus, from which the arteries are given off to supply the whole body. These arteries are, 1st, a long vessel to the intestine, which runs along its whole length and anastomoses freely with the branches of the intestinal vein; 2ndly, five arteries to the parts connected with the mouth; 3rdly, five dorsal arteries that run along the interior of the shell between the ambulacral rows as far as the anal orifice, at which point each dorsal artery leaves the osseous box through an aperture specially provided for its exit, and, arriving upon the outer surface of the shell, supplies the soft external membrane, and in some species may be traced back again between the rows of ambulacral suckers as far as the mouth.

These dorsal arteries, like the corresponding vessels in Asterias, supply the vascular origins of the innumerable protractile feet.

Alimentary canal of Echinus esculentus: a, interior of shell; b, ambulacral foramina.

Fig. 98. Alimentary canal of Echinus esculentus: a, interior of shell; b, ambulacral foramina; c c, intestinal canal; d, commencement of oesophagus from the base of the "lantern of Aristotle"; e, heart; f, g, vascular trunks following the course of the intestine.

(521). The chylaqueous system of the Echinidae, comprehending a considerable mass of fluid filling the cavity of the spherical shell, has been generally regarded as sea-water poured into the visceral cavity through perforations in certain membranous processes of the shell, which have received the name of branchice, and are distributed in groups around the circumference of the oral membranous disk. The latter, however, according to Dr. Williams, are not connected with the suctorial or water-vascular system, but are distended by injections thrown into the open chamber of the shell, being protruded only by the force of the fluid driven into their interior; they are consequently not perforated.

(522). In addition to the meridional rows of suctorial feet, the shell of Echinus is perforated by numerous hollow membranous processes lined within and without by vibratile cilia, and penetrated exclusively by the fluid of the visceral cavity; they show no traces of blood-vessels, and can only subserve a respiratory purpose on the supposition that the subject of that process is the chylaqueous fluid. There is, therefore, no direct evidence to show that the external element enters through openings in the integuments into the peritoneal cavity of the Echinus.

(523). Nevertheless, besides this diffused respiration, Delle Chiaje regards a series of pinnated tentacula in the neighbourhood of the mouth as being in some degree capable of performing the office of branchiae. These organs, which are protruded through a row of distinct orifices placed around the oral aperture of the shell, are eminently vascular; and as they present a large surface to the action of the water and receive numerous vessels from the circular trunk that surrounds the mouth, they may, no doubt, very well contribute to the complete exposure of the blood to the influence of the surrounding medium.