VIII EGHIXODERMATA — WATER VASCULAR SYSTEM 435 



peden, Luidia, etc.), while in very many other genera {e.g. Asterias, 

 Solaster) a slight difference in shape occurs. Only the tube-feet near 

 the ends of the arms retain the primitive shape, while a well-developed 

 disc is found on all the rest. The former then function chiefly as 

 tactile tentacles. 



The wall of the tentacles shows the typical layers. In the tactile tube-feet, the 

 epithelium at the conical end is much thickened, and contains very numerous 

 sensory cells. Within the epithelium, a layer of nerve fibres is developed ; these 

 run from the base to the tip of the foot. The layer is specially strongly developed 



1 



Fig. 367.— Portion of the disc of Hemipholis cordifera, from the oral side (after Lyman). 1 , 

 oral tube-feet ; 2, buccal shields ; 3, jaw = oral-angle plates ; 4, lateral buccal shields ; 5, first ventral 

 shield ; 6, oral integument, lip ; 7, spines on the marginal plates ; S, retracted tentacle ; 9, tentacle 

 scale ; 10, ventral shields ; 11, e.xtended tentacle ; 12, tentacle pore ; 13, torus angularis ; 14, teeth. 



within the terminal sensory epithelium. A similar deep sensory epithelium, consist- 

 ing of sensor}', supporting, and glandular cells, also covers the sucking discs of the 

 other tube-foot ; these discs have a depression at their centres, while, round their 

 edges the nerve tissue lying within the epithelium becomes thickened into a nerve 

 ring. 



From the centre of each sucker, radial muscle fibres run out towards the peripliery 

 and are attached round the ambulacral canal which ends below the sucker. These 

 muscles, by their contraction, caixse the sucker to adhere. They are entirely dis- 

 tinct from the longitudinal muscles of the appendage, which e.xj^lains the fact that 

 a tube-foot sticking to an object may be cut off without becoming detached. 



This applies almost equally well to the sucking discs of the tube-feet of the 

 Echinoids. 



