146 



GENERAL ZOOLOGY 



means that its body is made up of a linear series of more or less 

 similar structural units called segments. The annulated form 

 of the body is an external indication of segmentation. As a 

 result, the body cavity instead of being a continuous space is 

 subdivided by transverse septa, called dissepimenta, extending 

 from the body wall to the intestine, in the planes of the annular 



Fig. 92. — Stereogram showing the relations of organs in posterior segments 

 of earthworm. The anterior end is to the right. A piece of the intestine has 

 been removed and two dissepiments have been cut. Blood vessels are shown 

 in black. The blood moves toward the anterior end (right) in the dorsal vessel 

 and posteriorly in the subintestinal one. d, dissepiment; n, nerve cord; nt, 

 nephridial tubule; t, typhlosole, a median longitudinal fold hanging from the dor- 

 sal wall of the intestine. (After McGregor and Calkins.) 



grooves (Fig. 92). Thus each body segment has its own body 

 cavity. In each of these cavities, except the first three and the 

 last one, are two nephridial tubules, one on each side of the body. 

 Five regions can be distinguished in each tubule, as follows: (1) a 

 ciliated nephrostome, (2) a ciliated neck, (3) a coiled narrow tubular 

 part, (4) a wide glandular portion, and (5) an ejaculatory duct 

 opening to the outside by a nephridiopore (Fig. 93). The 

 nephrostome, provided with a slit-shaped opening, projects 

 through the anterior wall of the compartment in which the 



