494 ECHINODERMATA OPHIUROIDEA chap. 



The number of genital organs varies very much. In the 

 small Amphiura squamata there are two gonads, an ovary 

 and a testis, attached to each bursa, but in the larger species 

 there may be very many more. 



We follow Bell's classification, 1 according to which the 

 Ophiuroidea are divided, according to the manner in which 

 the vertebrae move on one another (cf. Fig. 210), into three 

 main orders, since these movements are of prime importance in 

 their lives. 



(1) Streptophiurae, in which the faces of the vertebrae have 

 rudimentary knobs and corresponding depressions, so that the 

 arms can be coiled in the vertical plane. These are regarded 

 as the most primitive of Ophiuroidea. 



(2) Zycophiurae, in which the vertebral faces have knobs 

 and pits which prevent their coiling in a vertical plane. 



(3) Cladophiurae, in which the arms can be coiled as in (1) 

 and are in most cases forked. No teeth ; the arm-spines are 

 papillae, the covering plates of the arms are reduced to granules. 



Order I. Streptophiurae. 



This is not a very well denned order ; it includes a few genera 

 intermediate in character between the Cladophiurae and the 

 Zygophiurae, and believed to be the most primitive Ophiuroids 

 living. It is not divided into families. The vertebrae have 

 rudimentary articulating surfaces, there being two low bosses and 

 corresponding hollows on each side, and so they are capable of 

 being moved in a vertical plane, as in the Cladophiurae ; the 

 arms never branch, and further, they always bear arm -spines 

 and lateral arm -plates at least. No species of this order are 

 found on the British coast, but Ophiomyxa pentagona, in which 

 the dorsal part of the disc is represented only by soft skin, is 

 common in the Mediterranean. 



Ophioteresis is devoid of ventral plates on the arms, and 

 appears to possess an open ambulacral groove, though this point 

 has not been tested in sections. Ophiohelus and Ophiogeron 

 have vertebrae in which traces of the double origin persist 

 (see p. 491). 



1 Bell, "Contribution to the Classification of Ophiuroids," Proc. Zool. Soc. 

 1892, p. 175. 



