ZYGOPHIURAE 



497 



being represented by 0. ciliaris and 0. albida ; the former is the 

 commoner. An allied species dredged by H.M.S. " Challenger " 

 is represented in Figs. 217 and 218. 



Ophiomusium (Fig. 219) is a very peculiar genus. The 

 mouth-papillae on each side of each mouth-angle are confluent, 

 forming a razor-like projection on each side of each mouth- 

 angle (Fig. 220). 

 The arms are 

 short, and the 

 podia are only 

 developed at the 

 bases of the 

 arms. Ophio- 

 pyrgus has the 

 dorsal surface 

 raised into a 

 conical elevation 

 protected by a 

 central plate 

 surrounded by 

 five large plates. 



In the re- 

 maining four 



Fig. 219. Aboral view of Ophiomusium pulchellum. 

 (From Wyville Thomson.) 



x 7. 



families the arms 



are inserted on 



the under surface of the disc ; in other words, the interradial lobes 



which make up the disc have completely coalesced dorsally ; and 



the spines stand out at right angles to the arm. 



Fam. 2. Amphiuridae. Mouth -papillae present, but no 

 tooth-papillae ; radial shields naked ; small scuta buccalia. 



The most interesting Brittle Star belonging to this family is 

 Amphiura squamata(elegans), a small form, with a disc about \ inch 

 in diameter covered with naked plates. It is hermaphrodite 

 and viviparous, the young completing their development inside 

 the bursae of the mother. Occasionally the whole disc, with the 

 exception of the mouth -frame, is thrown off and regenerated. 

 This appears to be a device to enable the young to escape. Three 

 other species of Amphiura are found in British waters. 



Ophiactis is another genus belonging to this family, dis- 

 vol. 1 2 k 



