viii ECHINODERMATA GENITAL ORGANS 493 



reaches the bases of the gonads it is further continued along all the 

 individual tubes to their tips. The gonadial tubes thus have a double 

 wall first, their own wall, which is a continuation of that of the out- 

 growths from the axial organ ; and secondly, an outer wall, which is 

 a continuation of the wall of the axial sinus. Between these two 

 walls lies the narrow ccelomic sinus, which is in open communication 

 by means of the sinuses of the genital strands, with the ring sinus, 

 and through this latter with tlie axial sinus. 



The relations existing between the gonads, the axial organ, and 

 the system of sinuses, is clearly elucidated by the ontogeny of the 

 Asteroidea, which shows that in quite young animals the axial organ 

 grows out apically, and first forms the ring strand. Out of this the 

 genital strands bud, and from these latter again the bundles of 

 gonadial tubes arise, which are at first solid outgrowths, and only 

 become hollow secondarily. During this whole process the growing 

 axial strand, which finally produces the rudiments of the gonads, 

 continually carries the axial sinus along with it, so that the ring-like 

 strand, the genital strand, and the genital tubes are entirely sur- 

 rounded by a sinus, which constantly remains in open communication 

 with the axial sinus. 



At those points of the genital strands from which the gonadial 

 bundles bud, i.e. at the future bases of the gonads, the duct which 

 perforates the body wall is formed from within at a later stage. 



The form of the individual gonadial bundles requires little notice. The genital 

 tubes of which each bundle is composed are usually not long, sometimes they 

 resemble short sacs and are vesicular, they are rarely branched. 



Of much greater interest are the number and arrangement of these bundles. 



In the simplest cases, five pairs of gonadial bundles are present ; this is the case, 

 as far as examination of the various species on this point has taken place, in the 

 following families : the Aster inidcc, Solastcridce, Echinasteridce, Linckiidw, Asteri- 

 ii.lr. In these, the five pairs either lie in the disc, one bundle at each side of each 

 interradius, or have shifted into the bases of the arms (Echinasteridce, Linckiidaz). 

 More than five pairs of gonads are found in the families of the Astropedinidai, 

 Pcntaccrotida', and Gymnasteriidcc. They either lie in the disc in rows at the sides 

 of the interradii, or the five pairs of rows stretch into the arms. This last arrange- 

 ment is found in the most extreme form in Luidia, where, on each side of each arm, 

 a row of nine runs to near its tip, one or two pairs occurring on each skeletal 

 segment. 



In all cases, each bundle has its separate genital aperture. 



As a rule, each bundle has only one aperture, but it sometimes happens (Asterias, 

 Solaster] that the duct which traverses the apical body wall branches, and opens 

 through several genital pores lying near one another. 



Asterina gibbosa is an exception to the rule that the genital apertures lie on the 

 apical side of the disc or arms. The apertures here lie on the oral side, a peculiarity 

 connected with the fact that these Asteroids do not simply eject their eggs into the 

 water, but attach them in combs or plates to stones, etc. 



It must, finally, be noted that the aboral ring sinus is not always simple, but 

 may break up into a circular network of sinuses (e.g. Echinaster sepositus). 



