XII.] THE FRESH-WATER CRAYFISH. 141 



wards side by side in the middle line: the 

 third lobe is directed backwards. 



@. The two vasa defer entia arise just where the 

 posterior lobe of the testis meets the two ante- 

 rior. Each is narrow near the gland, but widens 

 as it proceeds back from it, and becoming ex- 

 tremely convoluted, finally ends at the genital 

 opening on its own side (1. c. 7.). Trace the 

 course of the vas deferens on that side from 

 which the thoracic wall has been removed (12). 



7. Tease out a bit of the testis in water, and ex- 

 amine with J obj : it will be seen to be com- 

 posed of sacculated tubes. In it or in the vas 

 deferens some of the spermatozoa may be found : 

 they are motionless and have the form of nucle- 

 ated cells provided with radiating processes. 



8. The ovary is a gland in shape and colour 

 very similar to the testis of the male. From it 

 two short oviducts arise and pass almost directly 

 downwards to the genital openings (1. c. 7.). 



I. Of the Lobster. 



a. The testes are two long tubes which lie partly 

 in the thorax and partly in the abdomen. Their 

 posterior portions meet in the middle line, but 

 in front they diverge, and about one fourth the 

 length of each from its anterior end a short 

 transverse branch unites the two. 



/3. The vas deferens arises a little in front of the 

 middle of each testis and passes without convo- 

 lutions towards the genital opening. Its distal 

 half is dilated. 



