ECHINODERMA GENERAL DESCRIPTION 



The epiblast develops into an ectoderm, ciliated in whole or 

 part. In the adult this often becomes merged in the mesoderm 

 so as to be indistinguishable ; in Ophiuroids it is for the most part 

 calcified by the immigration of calcigenous cells ; in Asteroids and 

 Echinoids it remains with its cilia ; in Crinoids it remains on the 

 tegmen of some forms, and in the ventral grooves, where it is 

 ciliated ; in Holothurioidea it is very variable, being best preserved 

 in Synaptidae. From the epithelium is derived the superficial 

 " oral nervous system," composed of the circumoral ring and 

 radial nerves. In Asteroidea and Crinoidea this remains on the 

 surface, but in the other classes it sinks below, while the grooves in 



4 



cor 



FIG. XXV. 



Eyes of Bchinoderms. 1, the end of a ray in young Asteroid, t, terminal tentacle ; p, podia ; 

 e, eye-spot. 2, section across the eye-spot in Asterias, showing seven eye-cups, en, endo- 

 theliuin of perradial water-vessel ; c/, connective tissue fibres ; tin, nerve below epithelium. 

 3, section across an eye-cup (after Cuenot). oil, cilia; cut, transparent cutis, below which 

 are the pigmented and retinal cells. 4, diagram of eye-spot of Diadema setosum, modified 

 from Sarasin. The eye-spot, with its hexagonal elements, is surrounded by the pigmented 

 integument, composed of a glandular, columnar epithelium (cp) which merges into the cornea 

 (cor) above, and the eye-cups below ; each cup is coated at its base with pigment (pg), and rests 

 on a nervous layer (tin), below which is again pigment. Connective tissue fibrils (cf) pass 

 through this in places. 



which the nerves originally lay are closed over them, forming 

 "epineural canals " (Fig. XIII.). 



Two other nervous systems are formed in Echinoderma : (a) 

 the " deeper oral nervous system " from the mesoderm, underlying 

 and roughly following the course of the superficial system ; said 

 to be absent in Crinoids, but is probably represented by their 

 " sub-epithelial system " ; present in all other Echinoderms except 

 those Echinoids that have no masticatory apparatus ; it chiefly 

 innervates the muscles in the oral side of the body wall ; (b) the 

 " apical nervous system," most pronounced in Crinoids, and derived 

 from the endothelium of the axial sinus ; it is believed to occur in 

 all other classes except Holothurioidea ; it innervates the dorsal 

 musculature of the test and appendages. 



