ECHINODERMATA GENITAL ORGANS 501 



That the cells of the genital ridge (and, indeed, originally all the cells of the 

 genital strand) are germinal cells is further proved by the fact that, in exceptional 

 cases, gonads may develop in the arms also, and even under the ambulacral furrows 

 of the calyx (e.g. in individuals of the species Antedon and Actinometra, and in one 

 species not specified). 



The gonadial tubes are sometimes long, sometimes egg-shaped. They run 

 through a larger or smaller number of joints of the pinnule. At the time of 

 maturity they swell and often bulge out the wall of the pinnule in such a way as 

 to show at a glance which pinnules contain ripe sexual products. 



The manner in which the ripe products are ejected from the pinnules is not yet 

 satisfactorily explained. There seem to be no constant genital apertures in the 

 adult. It appears that the ejection takes places through two merely temporary 

 apertures (one on each lateral wall of the pinnule). 



Round the mouth, finally, there are five genital strands with the 

 sinuses in which they lie, running from the periphery, i.e. from the 

 bases of the arms below the food grooves of the tegmen calycis. It is 

 not certainly known what becomes of these genital strands ; according 

 to some accounts, they are continued round the mouth into the strands 

 of the axial organ. They are said : also to develop ontogenetically as 

 outgrowths of that organ (cf. p. 446). 



If the axial organ of the Crinoids is homologous with that of the Ophiuroids, 

 Asteroids, and Echinoids (which homology cannot be considered as certainly estab- 

 lished), then we should have the same relations subsisting between the axial organ 

 and the genital organ in the Crinoids as in the other groups above mentioned. But 

 in the Crinoids the genital strands, which only become fruitful as gonadial tubes in the 

 pinnulae, are oral outgrowths of the axial organ, whereas in other Echinoderms (apart 

 from the Holotliurioidca, which are quite isolated) they are apical outgrowths. 



G. Origin of the Sexual Products. 



The first origin of the sexual products has been accurately described 

 for the Ophiurid Amphium squamata. They, and the cells of the axial 

 organ, arise out of one and the same rudiment, which consists of the 

 endothelial cells of the eoelom. Tie Echinoderms would thus, as 

 far as the origin of the sexual products is concerned, agree with the 

 Annulata, the Mollusca, and the Vertebrata. 



The specific cells of the axial organ seem incapable of becoming 

 germinal cells. 



H. Hermaphroditism in Eehinoderms. 



Hermaphroditism is an altogether exceptional phenomenon in 

 Echinoderms, and is only of frequent occurrence in one order of the 

 Holothurians, the Paractinopoda (Synaptidce). Apart from this order, 

 it is only certainly established in one Asteroid (Asterina gibbosa) and 

 one Ophiurid (Amphium squamata). 



(a) Paractinopoda. All species of the genera Synapta and Anapta, examined 

 with regard to this point, and a few species of the genus Chirodota, are herma- 

 phrodite. 



