ECHINODERMATA 2 1 3 



is found except that it begins in a complex masticating apparatus 

 of five parts (Aristotle's lantern). In the asteroids the mouth 

 opens by a short esophagus into an expanded stomach which is 

 divided into an oral, or cardiac, and a pyloric portion (Fig. 98). 

 From the pyloric part the narrow intestine passes to the anus. 

 Outpocketings (caeca) may occur in any of these divisions. 

 The most important are the hepatic caeca which are glandular in 

 function. 



FIG. 98. 



FIG. 98. Vertical (sagittal) section through an arm and an interradius of a Starfish (diagram- 

 matic), c, anus; amp., ampulla; c.b.. circular blood vessel; c.w., circular water canal; co., ccelom; 

 CO. e. ccelomic epithelium; d.b., dermal branchiae; e, position of the eyespot; ect., ectoderm; ent. t 

 entoderm; /, ambulacral foot; g, ambulacral groove; h, hepatic caeca or liver; *', intestine; i.e., 

 intestinal caeca; mes, mesoderm; mo., mouth; m.p., madreporic body; n.r., nerve ring; os., ossicles 

 in mesoderm; r.n., radial nerve band; r.b., radial blood vessel; r.p., reproductive pore; r.w., radial 

 water canal; s.c., stone canal; sp., spines; z, lacunar spaces in the mesoderm. (Adapted from 

 various sources.) 



246. The body cavity is well developed in the disc and usu- 

 ally in the arms, is lined with a ciliated epithelium, and contains 

 a fluid with amoeboid corpuscles. It is completely distinct from 

 the digestive cavity. Thin outgrowths of the body- wall (papula 

 or branchid) contain extensions of the ccelom. These assist in 

 respiration. 



247. Ambulacral or Water-vascular System. This system 

 of tubular organs is peculiar to the echinoderms. It originates 

 (see also 254), in common with the body cavity, as an outgrowth 

 from the archenteron and is to be regarded as a specialized por- 



