LIGIA. 93 



sheath. The anterior end of this is flagellate, and by 

 contractions of the slightly muscular walls of the testis it 

 is "forced into the vas deferens. Here the sperm colonies 

 are found bound together in masses (PL IV., fig. 5). The 

 substance which causes this cohesion is probably secreted 

 by a number of large cells which are situated in the 

 anterior end of the vas deferens near the openings of the 

 testes. 



In the female the ovaries are very conspicuous in the 

 breeding season, entirely filling up the dorsal part of the 

 body cavity. They lie at each side of, and beneath, the 

 heart, and extend from the first thoracic segment to about 

 the fourth abdominal segment. They are usually filled 

 with eggs of approximately the same size. A short 

 distance behind the middle of the ovary a thin walled 

 oviduct is given off. This opens to the exterior by a small 

 longitudinal slit at the base of the fifth pair of pereiopods, 

 immediately at the base of the brood pouch lamellae, these 

 being the last pair of brood pouch lamellae. 



The ova are large, oval in shape, and contain a large 

 amount of yolk. In copulating, the male walks on to the 

 back of the female and grasps the anterior thoracic 

 segments with the first three pairs of pereiopods ; 

 copulation may last one or more days. After the eggs 

 are extruded, they are carried about by the female in the 

 brood pouch, where they develop; the young remain for 

 a short time in the brood pouch. 



Development. 



The development of Ligia oceanica has been studied by 

 Nusbaum. According to jNusbaum, the early cleavage 

 is discoidal, although McMurrich has found superficial 

 or centrolecithal segmentation in the Isopods which he 

 has investigated. The first cleavage cell becomes separated 



