46 THE BIOLOGY OF AN ANIMAL. 



The somites from the 1st to the 27th are rather broad, 

 and gradually increase in size. A variable number cf the 

 somites lying between the 7th and 19th are often swollen on 

 the ventral side, forming the so-called capsulogenous glands. 

 Between the 28th and 35th (the number and position vary- 

 ing slightly in different specimens) the somites are swollen 

 above and on the sides, and the folds between them are 

 scarcely defined except on the ventral aspect. Taken together, 

 they form a broad, conspicuous girdle called the clitelluin 

 (Fig. 21, c), whose function is to secrete the capsule in which 

 the eggs are laid, and also a nutritive milk-like fluid for the use 

 of the developing embryos. (The clitellum is not present in 

 immature specimens.) Behind the clitelluin the somites are 

 narrower, somewhat four-sided in cross-section, and ilattened 

 from above downwards. This flattening sometimes becomes 

 very conspicuous towards the posterior end. Towards the very 

 last they decrease in size rather abruptly, and they end in the 

 anal somite, which is perforated by a vertical slit, the anus 

 (Fig. 21, an). All the somites are perforated by small openings 

 leading into the interior of the body, and forming the outlets of 

 numerous organs ; the position of these openings will be de- 

 scribed in treating of the organs. Each somite, excepting the 

 anterior two or three and the last, 

 gives insertion to four groups of 

 short and minute bristles or setce^ 

 which are arranged in four longi- 

 tudinal rows along the body. T\vo 

 s -- j ^ - of these rows run along the ventral 



FIG. 22. Diagram to illustrate the aspect, two are more Upon the 



. 



the seta and its muscles when from the interior of the body, 



J^tSSS SEEL" 1 where th *y are supplied with small 



muscles by which they can be 



turned somewhat either forwards or backwards, and can also be 

 protruded or withdrawn (Fig. 22). The setse are of great use 

 in locomotion. When pointed backwards they support the worm 

 as it crawls forwards ; when they are turned forwards the worm 

 can creep backwards. They are of interest, therefore, as repre- 

 senting an extremely simple and primitive limb-like organ. 



