Introduction to A -nimal MorpJu >.' 127 



calcified* layer of many-jointed plates,f bounding a 

 central, ambulacral groove on the under surface of 

 the arms. The dorsal surface of the body is convex, 

 and called antambulacral. 



The mouth is central, inferior, often surrounded 

 by flat spines or tooth-like processes ; it opens into 

 a central globular or pentagonal stomach in the 

 centre of the disc, held in its place by radial mesen- 

 teries. The intestine is spiral, ending neiir the mouth 

 (Comatula), or short, ending in a dorsal anus, or none 

 (Ophiuridae, Ctenodiscus, Llwydia, Astropecten). The 

 circulatory organs consist of the circum-oral and 

 circum-anal rings, and a pulsating heart. (Jourdain 

 regards this as a gland, and the other pseudhsemal 

 as water-vascular. {) For respiratory purposes, the 

 sea-water not only bathes the surface, but enters the 

 body cavity either by surface pores, or by the genital 

 :-es (Ophiuridae), or by inter-radial lamina* cribrosa 

 11 as by the porous madrepori- 



form plate. When there are several of these plates, 

 as in Kchinaster, there have been several larval invo- 

 lutions of integument, and hence several stone-canals. 



The pharyngeal nerve ring has few nerve cells, 



or none (Asteracanthion), and sends two threads 



with ganglion-cells (ambulacral brains) 



ulonjr -ach ambulacral space; these widen to the 



mide:' nd branches to each joint of the 



calca ular fibres lie- between the 



leton, t<> in locomotion. The 



aml)i (i m consists of a circum-oral ring, 



Containing te, 



. 



