408 



ZOOLOGY 



open on the exterior, the terminal part, or rectum, projecting as a 

 tubular papilla on the surface. In the living animal the rectal tube 

 is observed to undergo frequent movements of contraction and 

 dilatation, by means of which water is drawn into and expelled 

 from the intestine ; so that here, as in the Sea-urchin, there 

 would appear to be a process of intestinal respiration. 



The ambulacral system consists of a ring-vessel surrounding 

 the mouth, and a series of radial vessels (Fig. 333, ra(^. ami.) which 

 run in the ambulacral grooves, giving off branches to the pinnules. 

 Connected with the radial vessels and their branches are a series 

 of minute tubular appendages, the so-called tentacles (Fig. 334, 

 tent.), which are homologous with the tube-feet of the Starfishes 



teftt 



cent.ca.Tis 



" cz-rr 



Fig. 334. — Antedon. Diagrammatic view of a median vertical section through the disc, jiassing 

 through one radius and one inter-radius, amb. ambulacral vessels ; ax. co. axial nerve-cord 

 passing through the ossicles of the arm ;■ Br.1 Br.'^ brachial ossicles ; CD. centro-dorsal 

 ossicle ; cent. caps, central capsule ; chamh. org. chambered organ ; cirr. cirri ; ecL. ne. 

 ambulacral (epidermal) nerve-ring and radial nerve ; gen. st. genitai stolon ; int. intestine ; 

 vio. mouth; ii.i i;.2 ij.s radials ; ros. rosette; tent, tentacles; wat.p. water -pores. (After 

 Milnes Marshall.) 



and Sea-urchins, but are devoid of terminal suckers. These are 

 not organs of locomotion : they bear numerous sensory papilla, and 

 are therefore to be looked upon as tactile organs, but they probably 

 also have a respiratory function. Connected with the ring-vessel 

 are a number of ciliated, branched, tubular diverticula, the tvater- 

 tubes, which are suspended within the coelome, and may open freely 

 into it at their extremities. A large number of vessels with 

 minute ciliated openings — the water-pores (wat.p) — lead through 

 the actinal wall of the disc : these and the ciliated tubes are to be 

 considered as together representing the madreporic canal and its 

 openings in the Star-fish and Sea-urchin. 



The nervous system consists of three perfectly distinct parts — 

 superficial, deep, and axial or ahoral. A superficial radial nerve- 



