500 



COMPARATIVE ANATOMY 



CHAP. 



F. Crinoidea (Fig. 395). 



In the Crinoids, a genital strand runs through the arms, branching 

 with them, and entering into their last ramifications the pinnulse. 

 While this genital strand, which is to be found even below the food 

 grooves of the tegmen calycis, remains as a rule infertile in the calyx 



and in the arms, in the 

 pinnules the germinal 

 cells which it contains 

 give rise to the genital 

 cells. The genital strand 

 in the pinnulse becomes 

 a gonadial tube. 



On the position of the 

 genital strand cf. p. 414 

 and Fig. 356. It runs 

 between the three brachial 

 sinuses of the body cavity 

 (between the dorsal canal 

 and the two ventral 

 canals), below the food 

 grooves of the arms. 



It is contained in a 

 special coelomie sinus 

 (like the ring-like strand 

 and the genital strands 



of the Afteroidea and the 



Ophiuroidea), to the wall 



FIG. 305. Transverse section through an ovarial 

 pinnule of a Crinoid, diagrammatic. 1, nerve trunk of the 

 apical nervous system in the joint of the pinuula ; 2, genital 

 sinus ; 3, germinal epithelium of the gonadial rachis (genital 

 strand or tube); 4 and 9, sinuses of the brachial ca-lom ; 

 5, deeper longitudinal nerves of the pinnula ; (>, tentacle canal ; of which it is attached by 

 7, radial canal of the water vascular system ; 8, nerve ridge of ni . r 



the superficial oral system ; 10, sacculus, see p. 490. 



tissue. 



The ccelomic sinus is continued on to the gonadial tubes of the 

 pinnulse and there becomes the genital sinus. 



The genital strand is at first solid, but at a later stage becomes a 

 hollow genital tube. This genital tube widens in the pinnules into 

 the gonadial tube, which, in mature pinnules, is filled either with eggs 

 or spermatozoa, these having their origin in the cells of the wall of the 

 gonadial tubes (the germinal epithelium). 



In a transverse section of the genital tubes of the arms, the wall 

 appears thickened at one part. This thickening is the section of a 

 ridge whose cells seem to correspond with those of the germinal epi- 

 thelium of the gonadial tubes. 



It is very probable also, that after the ejection of the sexual pro- 

 ducts from the pinnules in Crinoids, the new formations of these pro- 

 ducts proceed from the germinal cells, which are pushed forward out 

 of the ridge of the genital tube into the pinnules. 



