TYPE EOHINODERMA. 561 



III. Class Ophiuroidea. 



The Ophiuroidea, or Brittle-stars, resemble the starfishes 

 closely in their general form, consisting of a central disk from 

 which five arms radiate (Fig. 257). The arms, however, are 

 in all cases slender and distinctly marked off from the disk, 

 and in Astrophyton branch dichotomously. Closer examina- 

 tion reveals, however, considerable differences from the Star- 

 fishes ; there is no anus, the madreporite is on the oral sur- 



FiG. 257. — Opliioglyplia aculeata from thb Aboeal Surface to show the 



Pkksistekt Apical System of Plates. 



(The arms are cut ofE close to tbe disk). 



1 = centrodorsal plate. 3 = basals. 



2 = under basals. 4 = radials. 



face, there are no visible ambulacral grooves on the arms, 

 which are more or less circular in section and do not con- 

 tain csecal processes of the digestive tract. Furthermore, on 

 each side of each of the five radii there is upon the edge of 

 the oral surface of the disk a slitlike opening, divided into 

 two parts in Ophioderma, and leading into a thin-walled cili- 

 ated sac, which is to be regarded as an invagination of the 

 wall of the body. There are thus ten of these genital hursce 

 as they are termed, two being situated in each interradius. 

 They seem to have a respiratory function, and serve also for 

 the exit of the reproductive elements, in some forms, e.g. Am- 



