EARTHWORM 89 



II. Internal Anatomy. 



With the scissors cut through the body wall in the mid- 

 dorsal line from about the middle of the body to the third 

 somite, taking care not to cut the viscera lying beneath. 

 Carefully cut the membranous partitions, the septa, at the 

 points where they join the body wall and pin back the flaps 

 of the latter. Notice that the body wall forms a tube in 

 whose cavity, the body cavity or coelom, lies another tube, 

 the alimentary or digestive tube: also that the septa divide 

 the body cavity into smaller chambers. What relation is 

 there between the septa and the external segmentation? 

 Observe first the alimentary canal which extends through 

 the animal; also several pairs of conspicuous white bodies 

 near the anterior end. The latter are the sperm sacs. 



1. Circulatory System. This system is a series of closed 

 tubes consisting of several longitudinal vessels and many 

 circular vessels connecting them. The largest of the longitu- 

 dinal vessels, the dorsal vessel, is in the middorsal line 

 against the alimentary canal. Beneath the intestine is 

 the ventral vessel which will be seen later when the alimentary 

 canal is removed, and ventral to the nerve cord is a third 

 longitudinal vessel, the subneural vessel. The circular 

 vessels extend between the dorsal and the ventral vessels 

 and occur in pairs. Five pairs of these circular vessels in 

 the anterior region (somites seven to eleven) are very large, 

 and being contractile are called hearts; the dorsal vessel is 

 also contractile. Carefully dissect away the septa and 

 expose the hearts. 



2. Reproductive System. The earthworm is a hermaph- 

 rodite animal containing both the male and the female 

 organs in a single individual. 



