THE CH^TOPODA 19 



as the first somite. The prostomium itself, for reasons which 

 will appear more clearly when we study the development of 

 the earthworm, is not reckoned as a somite, but as a part 

 anterior to the segmented body equivalent to the better 

 developed head of other worms. It should "be noticed that 

 on the dorsal surface the prostomium is produced into a 

 backward projection which is, as it were, dovetailed into the 

 first somite. The most anterior somites are rather larger, 

 and the grooves separating them are deeper than in the more 

 posterior somites. 



At a distance of about one-third of the whole length of the 

 body from the anterior end is a thickened band of glandular 

 tissue, nearly encircling the body, and extending from somites 

 32 — 37 inclusive. This band, known as the clitellum, 

 varies in position in different species, but is always constant 

 in position in the same species, and therefore of some im- 

 portance in enabling us to recognise different kinds of earth- 

 worms. In some it forms a complete band round the 

 body, but in L. herculeiis it is saddle-shaped, not reaching 

 to the ventral surface. The clitellum enables us to divide 

 the body of the earthworm into regions. There are 31 

 pre-clitellar somites, 5 clitellar somites, and about 117 post- 

 clitellar somites, the terminal one bearing the anus. 



The general surface of the body is moist and smooth, and 

 displays iridescent hues, due to the peculiar structure of a 

 definite cuticle which uniformly covers it. But on passing a 

 worm through the fingers it feels rough to the touch. This 

 roughness is due to the presence of a number of bristles or 

 cliaetse, of which there are four pairs or couples in every 

 segment except the first and last. The chsetae are easily 

 seen with a magnifying glass, and their arrangement should be 

 carefully noted. In each somite, with the exceptions above 

 mentioned, there is a pair of lateral couples placed on the 

 right and left sides of the body, rather nearer the dorsal than 

 the ventral middle line, and a pair of ventro-lateral couples, 

 placed between the lateral couples and the mid-ventral line. 

 The chsetse are/^shaped chitinous rods embedded in epidermic 

 sacs, and movable by special muscles. They are the chief 

 organs of locomotion of the earthworm, and from them the 

 name " Chjetopoda " is given to the group to which the earth- 

 worms belong. The ch^tje of the clitellar region differ from 



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