273 



ZCIII^ODEEMATA. 



have been found in the Echinoidea and Asteridea. In the former 

 (Cidaridea] there are, on special plates (ocular plates), at the apical 

 pole, five tentacle-like prominences, in each of which a nerve ends. 

 The eyes of the Asteridea are most accurately known. According to 

 Ehrenberg's discovery, they have the form of red pigment spots, and 

 lie on the ventral side of the rays at the distal end of the ambulacral 

 groove. They are spherical pedunculated prominences, and the 

 convex surface is covered by a simple membrane, which hides a 

 number of conical simple eyes (fig. 221). The simple eyes appear to 

 have their axes directed towards a common point. They each con- 

 sist of a red mass of pigment surrounding a refractive body, and a 



nervous apparatus. ^^ 

 Reproduction is mainly 

 sexual, and separate sexes 

 are the rule. Only Sy- 

 napta and Amphiftira 

 are hermaphrodite. The 

 organs of reproduction, 

 of the two sexes are ex- 

 tremely alike, so that if 

 it were not that the colour 

 of the generative products 

 is different, the seminal 

 fluid is mostly white and 

 the ova red or yellow, 

 a microscopical examina- 

 tion of the contents of 

 the generative glands 

 would be the only means of distinguishing between them. Sexual 

 differences of the external form or of definite parts of the body are 

 only very rarely present, since as there is no copulation the sexual 

 functions are usually confined to the secretion and preparation of the 

 generative material. Ova and spermatozoa, with some rare excep- 

 tions, first come in contact in the sea water outside tho body of the 

 mother. Internal fertilization, which is very rare, occurs in several 

 viviparous species of Ainpliiura and Pliyllopltorus. The number and 

 position of the generative organs are generally in strict correspondence 

 with the radial structure; nevertheless there are numerous excep- 

 tions to this. In the regular Echinoidea, five-lobed ovaries or testes, 

 which are composed of branched blind tubes, lie in the interradii on 

 the internal surface of the dorsal part of the shell (fig. 222). The 



FIG. 222. Genital organs of Echinus. A ', R .c'irn ; G, 

 genital glands lying on the iiiterarnbulacral plates ; a, 

 rows of ampullee. 



