Introduction to Ajiimal AI or photo gy. 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 near the mouth 

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

 (Ophiuridae, Ctenodiscus, Lhvydia, 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 pseudh^mal 

 tubes as water-vascular. J) For respiratory purposes, the 

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

 body cavity either by surface pores, or by the genital 

 fissures (Ophiuridae), or by inter-radial tamiim cn'brosa 

 (Solaster, &c.), as well as by the porous madrepori- 

 form plate. When there are several of these plates, 

 as in Echinaster, 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 

 covered with ganglion-cells (ambulacral brains) 

 along each ambulacral space ; these widen to the 

 middle, and send branches to each joint of the 

 calcareous axis. Muscular fibres lie between the 

 joints of the skeleton, to assist in locomotion. The 

 ambulacral system consists of a circum-oral ring, 



* Containing a trace of Calcium Phosphate, 

 t Corresponding to the Auriculas of Echinidse. 



X The existence of a separate circum-oral vascular ring has been recently 

 •denied. 



