140 ELEMENTARY BIOLOGY. [XII. 



in pairs; one, the sternal, the largest of all, given 

 off from the posterior end. 



b. Cut away the terga of the abdominal somites and 

 follow back the superior abdominal branch of the 

 sternal artery, removing carefully the muscles 

 which lie over it in the abdominal region. It 

 will be seen as a transparent tube lying in the 

 middle line on the intestine (14. &.), or in the 

 female lobster separated from it anteriorly by 

 the posterior ends of the two ovaries. It gives 

 off branches from its upper side to the muscles 

 over it, and also a pair of branches which run out 

 laterally in the intervals between each pair of so- 

 mites. 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 downwards to- 

 wards the ventral surface, passing on one side of 

 the intestine. Its subsequent course must be fol- 

 lowed later (15). 



13. Keproductive 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 for- 



