Il6 ANNULOIDA. 



but the stomach terminates blindly ; and there is, therefore, no 

 anal aperture. 



Each arm is furnished with an internal and an external 

 skeleton. The internal skeleton consists, as in the star-fishes, 

 of a chain of " ambulacral ossicles," placed along the centre 

 of the arm. The ossicles are, however, now amalgamated in 

 pairs, each coalescing with its fellow on the opposite side ; so 

 that in place of a double row of movably articulated ossicles, 

 we have a single row of bilaterally symmetrical pieces. 



Fig. 69. Recent Ophiuroids. a Ophiura texturata, the common Sand-star; 

 b Opkiocoma neglecta, the grey Brittle-star (after Forbes). 



The external skeleton of the arms is composed of four rows 

 of plates, one on the dorsal surface, one on the ventral, and 

 one on each lateral surface. The lateral plates generally carry 

 more less developed spines. In the extinct genus Protaster 

 it is said that the plates of the ventral row are double ; and 

 Ptilonaster is stated to possess four ventral rows of plates. 

 The disc, as before said, has a well-developed external skeleton 

 of scales, granules, and spines ; but there are never any of 

 those modified spines which are known as " pedicellariae," and 

 which occur in the Asteroids and Echinoids. 



