568 



IN VEHTEBBA TE MORPHOLOG Y. 



aborally by the deTelopment of the oral angle-pieces. The 

 radial nerves are, however, contained in the wall of the sinus, 

 coming to the surface of the body at the tips of the arms, 

 where they terminate by fusing with the general ectoderm. 

 The muscular nervous system is, as in the Asteroidea, closely 

 associated with the oral ring and radial nerves, lying on their 

 aboral surface and separated from them only by a thin layer 

 of connective tissue. The aboral system consists of a ring 

 situated beneath the aboral surface of the body, from which 

 branches pass off towards the reproductive organs. Indeed 

 the entire system is intimately associated with the genital 



Fig. 360.— Section through 



A = ambulacral ossicles. 

 br = scbizocoelic siuus. 

 bt — buccal tentacles. 

 G = ccjeiom. 

 M = muscle. 

 nr = radial nerve. 



T bt 

 AN Ophiukan showing Stbucttjke (after 



LUDWIG). 



= moutb. 

 p = pala angularis. 

 ps = peripharyngeal space. 

 PV = Polian vesicle. 

 S = peripharyngeal septum 

 T = torus angularis. 



wr = hydroccel- vessel. 



cords, and its course can be understood from a description 

 of these structures. No special sense-organs other than the 

 terminal and buccal tentacles and the tube-feet, already de- 

 scribed, occur in the Ophiuroidea. 



As already stated, the genital cord arises from a group of 

 cells in the wall of the ovoid gland (Fig. 259, gr) and passes 

 in an interradius towards the aboral surface of the body, 

 carrying with it a portion of the axial siuus. Arrived at this 

 point the sinus and cord form rings, the aboral nerve-ring 

 lying in the wall of the sinus, while the genital cord lies in its 

 interior, attached to its wall by a lamella of connective tissue. 

 From the genital-cord ring ten short branches are given off 



