i 4 6 ELEMENTARY BIOLOGY. [CHAP. 



laterally in the intervals between each pair of 

 somites. In the sixth abdominal somite it termi- 

 nates by splitting up into three or four large 

 branches which pass in a radiating manner into 

 the telson. On account of the small size of the 

 crayfish this artery is difficult to dissect in it. 



c. The sternal artery presents an enlargement at its 

 commencement just where the above branch 

 arises from it. It then passes vertically down- 

 wards towards the ventral surface, passing on 

 one side of the intestine. Its subsequent course 

 must be followed later (15). 



13. Reproductive organs. 



These differ considerably in the crayfish and the lob- 

 ster. They lie partly beneath the heart, which must 

 therefore be removed or pushed on one side in order 

 to see them. Both animals are unisexual. 



a. Of the Crayfish. 



a. The testis. A trilobed yellowish mass : two of 

 its lobes are larger than the third and pass 

 forwards side by side in the middle line: the 

 third lobe is directed backwards. 



j3. 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 be- 

 coming extremely convoluted, finally ends at 

 the genital opening on its own side (i. c. y.). 

 Trace the course of the vas deferens on that 

 side from which the thoracic wall has been 

 removed (12). 



