498 COMPARATIVE ANATOMY chap. 



five pairs of adradial genital strands recall the five pairs of genital 

 strands of the Asteroidea. 



The gonadial tubes are sessile upon the genital strands of the 

 bursfe, and project freely into the body cavity of the disc. These 

 gonadial tubes are either single pear-shaped tubes, great numbers of 

 Avhich are arranged in rows along the genital strands, or they are 

 collected into tufts, and then there is one tuft on the adradial and one 

 on the abradial wall of the bursa. 



In the former case {e.g. Ophiogli/2>Jia, Ophiomyxa, Ophiocoma) the two rows of 

 genital tubes (the adradial and the abradial) stand rather low down on the wall of 

 the bursa, near its aperture, almost jjarallel with the edges of the latter. Each 

 genital tube has its special aperture into the bursa. 



In the second case, the points of insertion of the two tufts of gonads within the 

 ventral region of the bursal wall, i.e. the position of the bases of the gonads, 

 seems to vary greatly, and each tuft appears to have only one aperture into the 

 bursa {OphiopJiolis, OphiothrLr). 



It is still an open question whether the genital apertures are constant in adult 

 Ophiurids, or whether they only break through into the bursal cavity at the time of 

 maturity. 



The gonadial tufts arise as originally solid buds from the genital strands, and, 

 while forming, bulge out the wall of the sinus which contains the strand ; the tubes 

 are thus here also surrounded by a genital sinus, which communicates with the ring 

 sinus, and through it with the axial sinus (Fig. 393). 



The ring-like strand is attached by a thick band to the wall of the ring sinus. 

 It is solid, and consists of two kinds of cells : (1) cells which entirely resemble 

 those of the axial organ, of which the ring-like strand is a prolongation ; (2) 

 enclosed in these cells, there is a strand of cells proved to be genital germ cells 

 (rachis genitalis). The cells of the former kind progressively decrease in number, 

 and those of the second kind increase in number the nearer the ring-like strand 

 approaches the gonads. The former are not even continued into the gonadial 

 tubes, while the latter kind yield tlie germinal cell material of the gonads. It is 

 very probable that, after tlie sexual products have been ejected, a new formation 

 of germinal cell material takes place, by some kind of forward movement, from the 

 rachis genitalis. 



The development of the genital system from the axial organ and the axial sinus 

 proceeds in the same manner as in the Asteroidea. 



Ophiadis virens, a form distinguished by reproduction by means of 

 fission, and by the peculiar arrangement of the appendages of the 

 Avater vascular system, is the only Ophiurid in which the bursa3 are 

 altogether wanting. The gonads open direct outward on the oral side. 



E. Eehinoidea (Figs. 358 and 370, pp. 419 and 443). 



Although the genital system of the Eehinoidea appears to resemble 

 in its development that of the Asteroidea and the Ophiuroidea (a point 

 on which, however, further research is desirable), marked deviation 

 takes place in the adults. 



The gonads, at least in regular Echinoids, are five in number, and 



