46 



THE BIOLOGY OF AN ANIMAL. 



The somites from the 1st to the 2Tth 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 2Sth and 35th (the number and position vary- 

 ing shghtly in different specimens) the somites are swollen 

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

 scarcely defined except on the ventral aspect. Taken together, 

 they form a broad, conspicuous girdle called the clitelluiiv 

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

 the eo;2;s are laid, and also a nutritive milk-like lluid for the use 

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

 immature specimens.) Behind the clitellum the somites are 

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

 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 amis 

 (Fig. 21, ail). 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., 

 wliich are arrano^ed in four lono^i- 

 tudinal rows aloncr the bodv. Two 

 of these rows run along the ventral 

 aspect, tw^o are more upon the 

 sides. The setse extend outwards 

 from the interior of the bodv, 

 where they are supplied with small 

 muscles by which they can be 

 turned somewhat either forwards or backwards, and can also be 

 protruded or ^vithdrawn (Fig. 22). The setsB 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. : 



Fig. 22.— Diagram to iUustrate the 

 action of the setae. The dotted 

 outline represents the position of 

 the seta and its muscles when 

 bent in the opposite direction, m, 

 muscles ; s, seta ; %i\ body- wall. 



