VI ANNELIDA 73 



segment is the mouth (Fig. 37, B, in), a small crescent- 

 shaped aperture, whilst the anus is an oval aperture in the 

 last segment. 



A little way behind the head (segments 32 to 37, usually) 

 the top and sides of the body are somewhat thickened and 

 swollen, forming the structure known as the saddle or clitellum 

 (Fig. 37, cl). This swelling is due to the presence, in this 

 region, of glands in the skin which secrete the mucous sub- 

 stance of which the egg capsule is made (see p. 77). The 

 size and appearance of the clitellum vary at different times 

 of the year, and in worms of diiferent ages. 



Although at first sight the body of the worm appears 

 smooth all over, if carefully examined the presence of four 

 double rows of little bristles can easily be discovered, two 

 rows placed ventrally and two laterally (Fig. 37, JD). These 

 can be seen if the worm is held up to the light and turned 

 round so that the light catches the projecting bristles, or they 

 can even more readily be felt, if the worm is gently stroked 

 from tail to head. These bristles, or " chaetae " as they 

 are technically called, are of great use in locomotion ; if a 

 worm is placed on a piece of rough paper its 

 method of movement can be easily observed. It is 

 seen to stretch out the front part of its body, making it 

 longer and thinner, the bristles are retracted and disappear 

 from sight; next it contracts the front end, at the same 

 time pressing the front bristles into the rough surface of the 

 paper so that the body cannot slip back ; as this takes place 

 the hinder part lengthens, becomes free from the substratum, 

 and is drawn up nearer to the head, and so alternate waves 

 of elongation and contraction pass down the body, resulting 

 in a forward movement of the whole body. The retraction 

 and protrusion of the bristles can be seen, and the slight 

 scratching made as they move over the paper is clearly 

 audible. 



The power to move the body in this way is due to the 

 development of special layers of muscles which lie just within 

 the body-wall and are attached to it. One layer, that next 

 below the skin, consists of muscles which run round the body, 

 the circular muscles ; these by their contraction, at any 

 part of the body, cause that part to become longer and 

 thinner. Within this layer is another of muscle fibres 



