OPHiUEOIDEA 



199 



infrabasals, beyond this system are the ten radial shields (rs) 

 and the five second interradials (ir), which do not belong to the 

 primary apical system. 



The completeness of the apical system varies much in the group. In 

 some forms, the embryonic condition is retained, but even here it may 

 be reduced to the central and five radials, or central and five basals. 

 Not infrequently it ( 



happens that the 

 integument of the 

 disc is soft and the 

 plates small, scat- 

 tered and incon- 

 spicuous. The 

 radial shields are 

 perhaps the most 

 constant and con- 

 spicuous of the 

 upper skeletal 

 plates of the disc 

 (including the 

 apical system). 

 They sometimes 

 reach from the base 

 of the arm to near 

 the centre of the 

 disc. 



FIG. 143. Plates of the upper surface of the disc of Ophiomusium 

 validum (from Lang, after P. H. Carpenter), ce central plate ; 

 r radial, ba basal plates ; the dotted plates betweeu the basals 

 and the central are the infrabasals ; ir interradial plates ; 

 rs radial shields ; br arms. 



At the end of 

 each arm there is 

 a median termi- 

 nal plate which differs from the terminal plate of Asteroids in 

 the fact that it not only lies over the terminal unpaired ten- 

 tacle of the water-vascular system, but completely surrounds 

 it. 



The alimentary canal, the mouth of which has already been 

 described, leads by a short oesophagus into a large stomach 

 (Fig. 144). The stomach is without any special glandular 

 appendages and there is no anus. 



The nervous system is arranged very much as in Asteroids, 

 except that the ectoneural plexus is absent in those forms in 

 which the epithelium of the ectoderm is reduced, and that the 

 ventral system (circumoral ring and ambulacral nerves) is re- 

 moved from the surface and lies in the wall of an epineural 

 canal (Fig. 140, 7). This canal, which is lined by ectoderm, being 

 developed as an ectodermal groove, is covered ventrally by 

 calcareous plates. It is found in connexion with the circumoral 



