402 ECHIN0Z0A. 



surface of the body and its connection with the oral plates, with one 

 of which it is generally confluent. 



As regards the digestive system, the Ophiuroids differ from the 

 majority of the Asteroids in the fact that the alimentary canal ter- 

 minates blindly, and there is, therefore, no anal aperture upon the 

 dorsal surface of the disc. 



Lastly, as regards the reproductive system, the generative glands 

 in the Ophiuroids are placed interradially, and their ducts open into 

 singular folded pouches or " bursse," which in turn communicate 

 with the exterior by means of slit-like openings (the "genital 

 fissures "), which are placed, singly, or rarely in pairs, on the sides 

 of the arms inferiorly, at their junction with the disc (fig. 276, c). 

 Calcareous plates are sometimes developed in the walls of these bur- 

 sas, and currents of sea-water flow in and out of the genital fissures. 

 It is probable, therefore, that these " genital bursae " are partly res- 

 piratory in function, and that they correspond with the so-called 

 " hydrospires " of the Cystoids and Blastoids. 



The living Ophiuroids are all inhabitants of the sea, and the 

 marine deposits of almost all the great geological periods have 

 yielded examples of the group, the oldest known forms occurring in 

 Ordovician strata. Most of the remains of Ophiuroids, and par- 

 ticularly those of the more ancient formations, are, however, more 

 or less imperfect, while fossils of this group are, as a rule, exceedingly 

 rare. It will be sufficient here, therefore, to deal with the fossil 

 forms of Ophiuroids very briefly. 



The existing Ophiuroids fall naturally into two sections — the 

 Euryalida and Ophiurida — the characters of which are sufficiently 

 distinct. In the Euryalida, the arms may 

 be simple, but are more usually branched, 

 and in either case their integument is 

 leathery, the arms being devoid of the rows 

 of plates so characteristic of the typical 

 Brittle-stars. The type of this group is 

 the genus Astrophyton (Euryak), com- 

 prising the well-known "Medusa -head 

 Stars," characterised by their much- 

 branched arms. The only two extinct 

 genera which appear to be referable to the 

 Fig. ^.-onychaster flexiiis, Euryalida are Eudadia and Oiiychaster. 

 viewed sideways, of the natural The former of these is found in the Silu- 



size, with the arms rolled up. . . . 



From the Carboniferous Lime- nan rocks of Britain, and is characterised 



stone of Indiana. (After Meek i ,i_ • r i , i j • i 



.and worthen-copied from zittei.) by the possession of a granulated disc and 



of five bifurcating arms. The genus Ony- 



xhaster (fig. 278) is based upon forms found in the Carboniferous 



Limestone of North America, and is characterised by the possession 



