xvii CLASSIFICATION 



491 





one such organ on each side of each bursa, but in other genera 

 (of. Ophiarachna) a large number of small ones. The genital 

 products are shed into the water through the bursae. 



Classification of Ophiuroidea. 



Before proceeding to study the classification of Brittle Stars, 

 it is necessary to give some account of the range of structure 

 met with in the group. 



Number of Radii. The number of arms is rarely increased, 

 and hardly ever exceeds six ; a few species (each an isolated one 

 in its genus) have six arms, and in one case (Ophiactis virens), at 

 any rate, this is associated with the power of transverse fission. In 

 many Cladophiurae the arms fork repeatedly, so that although there 

 are only five radii, there is quite a crowd of terminal branches. 



Vertebrae. The vertebrae differ in the manner in which 

 they articulate with one another. In Ophiothrix fragilis taken 

 as the type, which in this respect resembles the vast majority 

 of species (Zygophiurae), the knobs and pits on the faces of the 

 vertebrae prevent the arms from being coiled in the vertical 

 plane. In Ophioteresis (Fig. 210, A) and some allied genera 

 (Streptophiurae) the knobs are almost obsolete, and the arms are 

 free to coil in the vertical plane ; whilst in Gorgonocephalus and 

 Astrophyton (Cladophiurae) the arms are repeatedly branched 

 and the vertebrae have saddle-shaped articulating surfaces, so 

 that they have quite a snake-like capacity for coiling themselves 

 round external objects. In Ophiohelus (Fig. 216) each vertebra 

 consists of two rod-like plates placed parallel with the long axis 

 of the arm and fused at both ends, but divergent in the middle, 

 leaving a hole between them. 



Covering Plates of the Arms. The upper arm-plates are 

 the most variable. They may be surrounded by small supple- 

 mentary plates (Ophiopholis) or double (Ophioteresis). In all (?) 

 Cladophiurae and most Streptophiurae they are absent, being 

 replaced by minute calcareous granules. Under arm-plates are 

 absent in Ophioteresis and in the distal portion of the arms in 

 many Cladophiurae. Side arm-plates are constantly present, and 

 in most Cladophiurae meet in the middle line below. 



Arm-Spines. The spines borne by the lateral covering plates 

 of the arms vary greatly in character. In Ophiura and its 



