92 LABORArORY STUDIES IN ZOOLOGY ' 



jxurs situated Ijctwccn segments 9 and 10 and 10 and 11, 

 respectively. 



The most conspicuous structures of the male reproductive 

 organs are three pairs of seminal vesicles located in segments 

 9, 11, and 12, respectively, connected with two central reserviors 

 lying in the midventral portions of somites 10 and 11. These 

 sperm sacs are associated with the testes of the male which lie 

 encased within the central reservoirs. By carefully peeling away 

 the tops of the reservoirs on the right side of the animal the glove- 

 shaped testes of that side may be exposed, one each at the anterior 

 ends of the tenth and eleventh segments near the midventral 

 line. Behind each testis is the enlarged ciliated funnel which 

 forms the opening of the sperm duct or vas deferens. Trace a vas 

 deferens back through the septum, noticing the several convolu- 

 tions, and then the straight caudal extension along the floor of the 

 body cavity to its opening, through the sperfniducal pore, in 

 the fifteenth segment. Note that the two vasa deferentia on each 

 side unite in the twelfth somite. 



If any difficulties were experienced in dissecting this system 

 on the right side of the body repeat the dissection upon the left. 



Label the parts of the Reproductive system on the plate of the 

 anterior end of the earthworm. 



A Cross-section of an Earthworm. — Study the cross-section 

 first under a dissecting microscope for the plan of structure of a 

 coelomate animal, that is, one which possesses a coelom. Note 

 the relative thickness of the body wall at different levels and 

 compare the contour of its outer and inner surfaces. Recall 

 structures previously studied which may account for the division 

 of the body wall into definite, well-marked areas. How many of 

 these areas are there? 



If the section studied happens to be in the proper plane the 

 slender, curved, rodlike setoi already noted upon the surface may 

 be seen. They are of the same composition as the cuticle and 

 like it secreted by the epidermis. They may be seen projecting 

 through the body wall. Between the body wall and the intestine 

 is the coelom within which fragmentary portions of nephridia 

 may be made out. Why do the sections not show complete 

 nephridia? Note the intestine with its dorsally invaginated 

 typhlosole. Identify the dorsal Mood vessel lying above the 

 typhlosole and embedded in its large cells, the ventral blood 

 vessel below the intestine, and the ventral nerve cord just below 

 the ventral blood vessel. 



