ECHIXODEBMATA. 199 



Dr. AVilliams.* The water of the chylaqaeous cavity of the abdomen 

 is introduced by the numerous contractile slender tubes which project 

 from the dorsal aspect of the body, and are perforated at their free 

 end : these tubes are covered both within and without by a ciliated 

 epithelium. 



In the true star-fishes the organs of generation consist of groups 

 of ramified tubes {Jig. 93, d\ arranged in pairs in each ray, and 

 opening upon the calcareous circle which surrounds the mouth. In 

 the males these sacculi {fig. 95, o, o,) 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. They are discharged by groups at dis- 

 tinct intervals of time, and are found in very different stages of de- 

 velopment in the same ovarium. 



The generative products in the ventless star-fishes (e. g. Astero- 

 pecten) dehisce into the chylaqueous cavity and escape by the re- 

 spiratory tubes ; but in some of the species with a vent {Astera- 

 canthioH rubens and Solaster papposus) there are two so-called 

 " laminae cribrosae " at each angle of the inter-radial space, where the 

 excretoi-y ducts of the ovaria or testes open and expel their contents. 

 The five pairs of generative organs are restricted to the central disc 

 in the Ophiuridce, 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 Ophiothrix fragilis each testis or ovarium 

 is coiled like a ram's horn, and is deeply cleft into many lobes. In 

 the ComatultB and Peyitacrini, the seminal or ovarian receptacles are 

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

 side of each of the pinnules, are covered by the soft perisorae, and 

 discharge their contents by dehiscence. 



Echinoidea. — 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. 



AU 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 spines with which the 

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

 several series, called oral, anal, genital, ocular, ambulacral, and inter- 

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

 by removal of the spines in^^. 96, is built up of the two latter kinds, 



♦ cLxm. 



O 4 



