ORGANS OF THE SPAT 75 



underneath the adductor muscle, where one on each side can be traced to 

 an opening in the supra-branchial chamber (Plate VII, fig. 18). 



Reproductive Organs become conspicuous in spat of 32 mm. (Plate 

 VII, fig. 20), appearing first in sections that fall between the posterior 

 tips of the palps and the anterior tips of the gills, and occur nearly as far 

 back as the end of the descending loop of the intestine below the adductor 

 muscle. In the most anterior of these sections the follicles are confined 

 to small areas in the angles of the body, each side from the palps and close 

 under the ectoderm of the surface. In going backwards the areas extend 

 upwards until they form a deeply staining band completely surrounding 

 the liver with its enclosed stomach, and in front of the heart there is a 

 tranverse band separating the part which follows the rectum for a short 

 distance from that which follows the descending loop of the intestine. 

 The follicles present the appearance of portions of coiled tubes distended 

 with innumerable spermatozoa, having a fine, parallel, fibrous arrange- 

 ment in which, under high powers, the heads are recognizable as dots. 

 The longest of the already mentioned nephridial tubes can be pursued 

 far forwards on each side, in the extreme angles, outside of and below the 

 the lateral areas referred to, and very close to, but not touching, the testis 

 follicles. 



In a 21 mm. spat the beginnings of follicles can be recognized in the 

 median portion^of the above defined region (Plate VII, fig. 19), although 

 there is not a sufficient histological specialization to determine what they 

 are going to be without comparison with sections of an older individual. 

 Judging from these, it is pretty safe to assume that males become sexually 

 mature at about 1 inch ( = 25 mm.) in length. 



