PHYLUM ANNELIDA 201 



ments. 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 deposited during 

 reproduction. 



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

 ous setae. They are fine, stiff bristles which may be located by 

 passing the hand lightly over the worm. They are moved by 

 protractor and retractor muscles and serve to help the worm move 

 through the soil. A pair of nepliridiopores (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 

 contains numerous minute pores through which secretions of the 

 unicellular glands beneath are poured and through which gaseous 

 exchanges between the blood and moist soil can take place. It 

 serves also as a protection against physical and chemical injury 

 to the animal's body. 



Internal Anatomy 



The body of the earthworm, if cut open along the mid-dorsal 

 line, gives the general appearance 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). 



