346 ESSENTIALS OF ZOOLOGY 



remains constant, while the number posterior to this varies. This 

 is because growth is accomplished by the addition of segments pos- 

 terior to the clitellum. 



The peristomium is a sort of knoblike lobe at the anterior end, 

 projecting out over the mouth. It is not considered a true meta- 

 mere. The first segment is incomplete due to the opening of the 

 mouth through its ventral side. The openings of the oviducts 

 through which the eggs pass to the outside are seen as minute pores, 

 one on each side of segment XIV. The pores of the seminal receptacles 

 occur in pairs, one pair in the groove between segments IX and X, 

 and one between X and XI. The openings of the vasa deferentia 

 (sperm ducts), which convey sperms to the outside, are located, one 

 on each side, in the anterior part of segment XV. In sexually mature 

 worms, segments XXX, XXXI, or XXXII to segment XXXVII are 

 swollen to form the clitellum, a sort of saddle-shaped structure, the 

 function of which is to secrete the cocoon in which eggs are de- 

 posited during reproduction. 



Each segment, except the first and last, bears four pairs of chitin- 

 ous setae, which are fine, stiff bristles. They are moved by pro- 

 tractor and retractor muscles and serve to help the worm move 

 through the soil. A pair of nephridiopores (the external openings 

 of nephridia) is situated on the posterior ventral side of each seg- 

 ment except the first two or three. 



The body of the earthworm is covered by a thin, transparent 

 cuticle which is secreted by the epidermal cells just beneath it. It 

 serves as a protection against physical and chemical injury to the 

 animal's body and as a respiratory membrane. 



Internal Anatomy 



The body of the earthworm is in the form of a tube within a tube, 

 the digestive tube being the inner one and the body wall the outer 

 one. The space between them is the coelom. The constricted regions 

 dividing the segments on the outside correspond to the positions 

 of the septae which divide the coelom into separate segmental com- 

 partments. These coelomic divisions communicate with each other 

 by means of pores in the septae so that the clear fluid which fills 

 the coelom can circulate freely. The septae are absent between 

 segments I and II and incomplete between segments III and IV, 

 and XVII and XVIII. The walls of the coelom are lined by a thin 

 layer of cells known as peritoneum (mesothelium). 



