ECHINODERMATA. 



101 



The following is essentially the arrangement of the whole aquiferous 

 system (see tig. 62). From the madreporiform tubercle on the 

 largest of the genital plates there proceeds a membranous canal, the 

 " sand-canal " («), by which the outer water is conducted to a central 

 tube (r), which forms a ring 

 round the gullet. The tu- 

 bercle is spongy, and is per- 

 forated with little holes, and 

 its function is probably to 

 act as a filter, and prevent 

 foreign particles gaining ac- 

 cess to the Ulterior. The 

 " circular canal " carries five 

 little bladders of unknown 

 function, termed the " Polian 

 vesicles" (p). From the 

 " circular canal " round the 

 gullet pi'oceed also five "ra- 

 diating canals " which take 

 their course towards the 

 summit of the shell, under- 

 neath the ambulacral areas 

 (a). In its course each radi- 

 ating canal gives off numer- 

 ous short lateral tubes [t) — 

 the ambulacral tubes or tube- 

 feet — which gain the exte- 

 rior of the shell by passing 

 through the apertures in the 

 ambulacral plates of the shell, 

 and which terminate in little 

 sucking-discs. The tube-feet 

 can be distended with water 

 by means of a series of little 

 muscular bladders or " am- 

 pullae" (v) placed at their 

 bases, and they can thus be 

 thrust far out beyond the 

 shell, into which they can be 

 again withdrawn at the will of the animal. However long the 

 spines may be, the animal can protrude the tube-feet to a, still 

 greater length ; and by the combined action of the little suckers 

 at their extremities locomotion is effected with moderate rapidity, 

 considering the bulk of the body. 



Fig. 62. — Diagram of the ambulacral system of 

 Echinv^. m Madreporiform tubercle ; s Stone- 

 canal ; r Central cesopliageal ring ; p p Polian 

 vesicles ; a a Radiating ambulacral vessels. 

 Only the bases of four of the radiating vessels 

 are shown ; and a few of the tube-feet (t), 

 with their secondary vesicles or "ampullae" 

 (v), are shown on one side of one of the radiat- 

 ing canals. 



