490 



ECHINODERMATA OPHIUROIDEA 



CHAP. 



constitute the main mass of the ovoid gland ; those of the outer 

 side remain thin. The cavity of the ingrowth is shut off from 

 the general coelom, but persists throughout life. In Asteroids a 

 similar ingrowth takes place, but both walls thicken and become 

 converted into germ cells, and the cavity disappears, and, as in 

 Asteroidea, a considerable number of the germ-cells in the stolon 

 degenerate. 



The genital rachis (Fig. 213, gen.r) is an outgrowth of the 

 distal end of the genital stolon, which extends in a complete 

 circle round the disc. The rachis does not, however, lie every- 

 where in the same plane, but by its undulating course bears 

 witness to the distortion which the disc has undergone. In the 

 radii it is, as in the Asteroid, dorsal ; but in the interradii it is 



Fig. 214. Diagram of a tangential section through the edge of the disc of an Ophiuroid 

 to show the relations of the disc, arm, and genital bursae. ep, Epineural canal ; muse, 

 longitudinal muscle of the arm ; nerv.rad, radial nerve cord ; ov, ovary ; perih, 

 radial perihaemal canal ; w.v.r, radial water- vessel. 



ventral, this ventral portion having, like stone-canal and axial 

 sinus, been carried down by the preponderant growth of the 

 dorsal parts of the disc. It is everywhere ensheathed by the 

 aboral sinus, w T hich, as in Asteroids, is an outgrowth of the coelom. 

 The rachis is embedded in a strand of modified connective tissue, 

 to which we may (as in the case of Asterias) apply the name 

 " aboral blood-ring." Both on the central and peripheral sides of 

 this sinus are vertical muscles connecting the genital and the 

 radial plates, which bring about the respiratory movements 

 already referred to. Just above the madreporite, at the end of 

 the genital stolon, is a small, completely closed space, which by 

 its position corresponds with the madreporic vesicle of Asteroids 

 and represents the right hydrocoel (Fig. 213, mv). As the 

 rachis passes over the genital bursa it gives off branches, which 

 swell up to form the genital organs. In Ophiothrix there is 



