ECHINODERMA TA 



233 



Class II. OPHIUROIDEA (Brittle Stars). 



Chaeactekistics. — Echinodermata with a central disk hearing 

 long slender arms, into the cavity of which no part of the 

 alimentary canal is prolonged. There is no anus. The 

 madreporic plate is ventral, and usually is an oral plate. 

 There is no amhulacral groove, and the tube-feet are lateral in 

 position. 



This class is allied to the Asteroidea, and is sometimes 

 included with the latter in a single class. The Ophiuroids, 

 however, differ from the 

 Asteroids in the sharp dis- 

 tinction between disk and 

 arms, a condition approached 

 by Brisinga, in the absence 

 of any digestive diverticula 

 in the arms, in the ventral 

 position of the madreporic 

 plate, and in the almost 

 universal absence of pedi- 

 cellariae. In the adult also 

 the ectoderm is absent ex- 

 cept on the tube-feet. 



The arms are long and 

 slender, in most cases they ^^^_ us,-OpUophoii^ um (upper surface), 

 are protected by four rows 



of plates, a ventral, a dorsal, and two lateral, the tube-feet 

 protrude between the ventral and lateral; they have no 

 ampullae. The nervous system has sunk under the skin, 

 and is protected by the ventral plates. Dorsal to it is the 

 radial blood-vessel, and dorsal to that the water-vascular 

 vessel. In a transverse section of the arm, the greater part 

 of the space is occupied by the ambulacral ossicles. Originally 

 paired, these have fused and become single; they are grooved 

 dorsally and ventrally. The dorsal groove lodges part of the 

 coelom, the ventral the above-mentioned vessels and nerve 



cord. 



The mouth is armed with certain modified ossicles ; it is 

 central in position, and leads into a spacious stomach, which 



