ECHINODERMA. 117 



slender chord surrounding the mouth (^), from which three delicate 

 filaments are sent off opposite the base of each ray : the middle one 

 is continued along the middle of the ambulacral groove, and swells, 

 according to Ehrenberg, into a small terminal ganglion, immediately 

 behind that bright coloured speck at the extremity of the ray which 

 the same acute observer regards as a rudimental organ of vision. 



The organs of generation consist of groups of ramified tubes 

 (^fi9' 64'. d), arranged in pairs in each ray, and opening upon the 

 calcareous circle which surrounds the mouth. In the males these 

 sacculi are distended with a white fluid abounding in spermatozoa : 

 in the females they are laden with ova of a bright yellow or orange 

 colour, which distend the rays during the breeding season. 



The five pairs of generative organs are restricted to the central 

 disc in the Ophiurce, which part in the breeding season is distended 

 with the milky fluid of the testis in the male, and with the round 

 yellow eggs in the female. They are discharged by orifices on the 

 ventral surface. In the Comatula the ovarian receptacles are much 

 more numerous, and are of smaller size : they occupy the inner side 

 of each of the pinnae or articulate processes sent ofi" from the rays. 



Echinidce. The calcareous pieces entering into the composition of 

 the complex skeleton of the Echinus are those of the shell, of the 

 buccal apparatus called the " lantern," of the ambulacral tubes, and 

 of the pedicellariae. 



All the Echini are admirable for the regular and beautiful pattern 

 in which, as in a tesselated pavement, the numerous calcareous pieces 

 composing their globular crust are arranged ; many of the species are 

 formidable from the size and form of the sjDines with which the shell 

 is beset. The component plates of the shell are divided into several 

 series, called oral, anal, genital, ocular, ambulacral, and interambulacral 

 plates. The proper shell, one half of which is exposed by removal 

 of the spines in figure 65, is built up of the two latter kinds, which 

 constitute a hollow spheroid, having a large aperture at each pole, 



where the first four kinds of plates 

 are situated. The ambulacral 

 plates {a, fig, 65.) are perforated 

 for the passage of the tubular 

 feet, the parallel rows of which 

 ^ intercept and overshadow spaces 

 compared by Linnaeus to avenues 

 or ambulacra; these plates like- 

 wise support spines. The inter- 

 ambulacral plates (z, fig. 65.'), 

 which support a greater number 

 I 3 



