122 



TEE BIOLOGY OF AN ANIMAL. 



capsule in which the eggs are laid, and also a nutritive milk-like 

 fluid for the use of the developing embryos. Behind the clitel- 

 lum the somites are less broad, somewhat four-sided in cross-sec- 

 tion, and flattened from above downwards. This flattening some- 

 times becomes very conspicuous towards the posterior end. To- 

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

 in the anal somite, which is perforated by a vertical slit, the ami* 

 (Fig. 72, ail). All the somites are perforated by small openings 

 leading into the interior of the body, and forming the outlet for 

 numerous organs ; their position will be described in treating of 

 these organs (see also the practical study, p. 168). Each somite, 

 excepting the anterior two or three and the last, gives insertion to 

 four groups of short and minute bristles or sctce, which are ar- 

 ranged in four longitudinal rows along the body. Two of these 

 rows run along the ventral aspect, two are near the sides. The 

 setse extend outwards from the interior of the body, where 



they are supplied with small mus- 

 cles by which they can be turned 

 somewhat either forwards or back- 

 wards, and can also be protruded 

 or withdrawn (Fig. 73). The setse 

 are of great use in locomotion. 

 "When pointed backwards they hold 

 the worm fast as it crawls forwards ; 

 when turned forwards the worm 

 creeps backwards. They are of in- 

 terest therefore as representing an 

 extremely simple and primitive form of limb. 



Plan of the Body. The body of the earthworm (Fig. 74), like 



A 



-s- 



FIG. 73. Diagram to illustrate the ac- 

 tion of the setae. The dotted outline 

 represents the position of the seta 

 and its muscles when bent in the op- 

 posite direction, m, muscles; s, seta; 

 iv, body-wall. 



FIG. 74. A, diagram of the earthworm as seen in a longitudinal section of the body, show- 

 Ing the two tubes, the coplom, and the dissepiments. B, diagram of cross-section, al, 

 alimentary tube; cm, anus; co>, ccelom; m, mouth. 



that of all higher animals, consists of two tubes, one (al) within 

 the other and separated from it by a considerable space or cav- 



