172 AN INTRODUCTION TO ZOOLOGY 



about half-way. In the worm the ventral surfaces of segments 

 8-12 are swollen by the so-called capsulogen glands, and on the 

 dorso-lateral aspect of segments 32-37 inclusive in the adult there 

 is a saddle-shaped thickening, the clitellum or cingulum. The 

 position and appearance of this structure is a useful character 

 for determining the species of the worm, for it varies consider- 

 ably in different forms, but is constant in the one species. It 

 is functional in secreting a cocoon in which the eggs are laid, 

 and also in furnishing them with a nutritive fluid. The end 

 somite is small and bears terminally the anus, and so is called 

 the anal segment. The outermost layer of cells, the epidermis, 

 does not remain exposed freely, but secretes a very delicate 

 membrane, the cuticle, which adheres closely to it. If a dead 

 worm be left some time in water this cuticle can be readily 

 stripped off, and appears as a thin transparent irridiscent sheath. 

 In spite of this covering of smooth cuticle a certain roughness is 

 felt if the ventral surface of the worm is drawn across a finger. 

 Closer inspection reveals the fact that this is due to the presence in 

 the skin of a number of very tiny bristles, the setae or chsetse. 

 These are not irregularly distributed, but arranged in four pairs in 

 every segment, save the first and last ; two pairs lie on the ventral 

 surface, and the other two are lateral to these. Each seta has the 

 form of an f-shaped rod of chitin pointed at its ends and thickened in 

 the middle, and is embedded in an invagination of the epidermis, 

 known as the setigerous sac, to which are attached certain muscles. 

 They are generally directed backwards, acting as a series of points 

 that help the worm in its crawling, but, when pointed forward, as they 

 can be, make it very hard to remove the worm from its burrow. 

 In the fifteenth segment the two pairs of ventral setae lying close to 

 the male external aperture are modified to form the penial setse. 



Not only do we find a mouth and anus, but a number of other 

 apertures opening to the outside, some unpaired and the others 

 paired. The unpaired openings consist of a number of minute 

 holes in the mid-dorsal line, the dorsal pores, lying in the grooves 

 between the somites and leading into the ccelem of the segment in 

 front. They are present in all the grooves save the first seven or 

 eight. The remaining apertures are paired and connected with the 

 excretory or reproductive systems. On every segment of the body, 

 except the first three and the last, are a pair of apertures, the 

 nephridiopores, the openings of the excretory organs. They are 

 situated just in front of the outermost seta of the ventral pairs. 

 On the ventral surface of segment 15 are a pair of conspicuous 

 slits with swollen glandular lips ; these are the male apertures, the 

 openings of the vasa deferentia or the spermiducal pores. In a 



