AN EARTHWORM 57 



The circular or commissural blood vessels connect the dorsal 

 and the ventral vessels and have a paired and segmental arrange- 

 ment. They are not all of equal size. Observe the four or five 

 large pairs near the forward end of the animal, which pass directly 

 between the dorsal and the ventral vessels. They are, like the 

 dorsal vessel, contractile and are sometimes called the hearts. 

 In which somites are they ? Find the commissural vessels poste- 

 rior to them. These are much smaller and do not pass directly 

 between the longitudinal vessels, but break into capillaries be-* 

 tween them. 



The Digestive System. The pharynx is an oval, muscular pouch 

 occupying four or five somites directly back of the mouth ; radi- 

 ating muscle fibers join it with the body wall. The oesophagus is 

 a slender tube following the pharynx and running between the 

 conspicuous sperm sacs to the crop in about the fifteenth somite. 

 Press aside these sacs and notice beneath them three pairs of 

 white glands ; these are lateral diverticula of the oesophagus and 

 contain calcareous crystals which have a digestive function. The 

 crop is a thin-walled dilation of the oesophagus which lies about 

 in somites 15 and 16, close to the gizzard, a muscular, thick-walled 

 chamber of the same size as the crop and lying in somites 17 to 19. 

 This is followed by the stomach intestine, a large tube with lateral 

 segmental pouches, which passes to the hinder end of the body ; 

 covering the surface of the stomach intestine is a loose mass of 

 yellowish-brown cells, the chloragogue cells, whose function is 

 unknown. 



Exercise 3. Make a drawing of the opened animal on a scale of 3, 

 showing the segmentation and representing the organs above de- 

 scribed in their proper somites ; label all carefully. 



Sever the alimentary tract just back of the pharynx and re- 

 move it from the body. 



The Reproductive System. The earthworm is hermaphroditic 

 and possesses the following genital organs : 



The Male Organs, (i) The sperm sacs. These have already 

 been noticed. They are large, white, irregularly lobed sacs occu- 

 pying somites 9 to 13 ; they vary in size with the sexual condi- 



