170 SYNOPTIC COLLECTION. 



the homologies of which are a subject of much discus- 

 sion. 



It is generally considered that the ambulacral plates are 

 inside the arms in the form of an axis of jointed sections 

 or arm-bones. If this is the case, then the ventral plates 

 are additional ones and are not homologous with any other 

 plates of the Asteroids. 1 They may be developed for the 

 purpose of protecting the delicate water-tube, blood-tube, 

 and nerves which run through the arms and which are ex- 

 posed in Asterias. The lateral plates bear lateral spines 

 which are probably helpful in locomotion. Each arm-bone 

 is pierced by a water-tube or tube foot which is without 

 ampulla or sucking disc, and therefore of no use as a lo- 

 comotive organ. These tube feet come out between the 

 ventral and lateral shields. Above, the base of each arm 

 is protected on either side by the so called radial shields, 

 while below near the mouth are the oral shields. 



The internal organs are all concentrated within the disc. 

 The genital organs open by slits on the low r er side at the 

 base of the arms. The madreporic body is also on the 

 lower side in one of the mouth plates. Ophiopholis de- 

 velops without a metamorphosis. The disc of Ophiura 

 (No. 310, O. panamteri Lutk.) is granulated, and the 

 arms are well protected by the numerous short flattened 

 spines. 



Ophioplocus (No. 311) resembles Ophiura in having a 

 granulated disc. The radial shields are small. Ophio- 

 coma (zthiops Liitk. (No. 312) has wide upper arm-plates 

 and large spines. 



The young Astrophyton resembles the typical Ophiuran 

 in having a flat disc covered by plates. In the process 

 of growth, these become covered by a granulation and 

 later both granulation and plates, except those at the mar- 

 gin, disappear. 2 



1 Bull. Mus. Comp. Zool., loc. a'f., p. 144. 

 8 Chall. Rep., Zool., V, part 14, 1882, p. 253. 



