GENITAL ORGANS. 681 



wanting, the lateral posterior genital organs are more fully developed 

 than the anterior ones (PL XXXII. f. 2, 12), and the right anterior one 

 is the least developed (PL XXXII f. 2, 12). In those genera in which 

 there are only three genital openings it is usually the right genital opening 

 which disappears first, and when there are only two genital openings, 

 the posterior pair alone is left. Something analogous can also he traced in 

 the Desmosticha (PL XXVII. f. 2), in which two of the genital organs, the 

 posterior ones, are more fully developed than the others. The same is the 

 case in the Clypeastroids ; the posterior genital organs are usually the 

 largest. The genital organs in the Petalosticha are attached to the test by 

 delicate membranes extending from the abactinal part of the genital organs; 

 in PI XXXIII f. 2, the scars left by the delicate mesenteries, can be seen 

 in the interambulacral spaces. For the structure of the egg see the chapter 

 on the Embryology of the Echini. The spermatozoa consist, according to 

 Kblliker and Peters, who have described them, of a minute head with a long 

 slender tail. 



