BRITTLE-STARS 299 



the central disc (cf. Brisinga). These arms are muscular, 

 and useful in wriggling and clambering ; they do not con- 

 tain outgrowths of the gut, nor reproductive organs. 

 Moreover, there is no ambulacral groove, and the tube-feet 

 which project on the sides are usually very small. They 

 are often of locomotor service, adhering even to vertical 

 surfaces, but in many cases they seem to be only sensory. 

 Each segment of the arm includes a central " vertebral 



Fig. 162. — Ventral surface of disc of an Ophiuroid 

 {Ophiothrix fragilis). — After Gegenbaur. 



g., Openings of genital pockets or bursae ; m., mouth ; v., ventral 

 plates of arms ; sp., spines of arms ; t.f., tube-feet — at the right side 

 these are represented as retracted ; 0., the openings through which 

 they are protruded ; p., plates around mouth bearing the so-called 

 teeth ; one of these plates is perforated, and functions as the 

 madreporite. 



ossicle," with four plates forming a tube round about it. 

 There is a complex oral skeleton. -The madreporic plate 

 is situated on the ventral surface, usually on one of the 

 plates around the mouth. The food canal ends blindly. 



Some brittle-stars have small luminescent glands, e.g. 

 Amphiura squamata. The reproductive organs lie in 

 pairs between the arms, and open into pockets or bursae 

 formed from inturnings of the skin, which communicate 

 with the exterior by slits opening at the bases of the arms. 



