LUMBRICUS TERRESTRIS 



159 



pointed in front and flattened behind. It reaches a length of seven 

 inches. There is no distinct head, but a lobe known as the pro- 

 stomium overhangs the mouth, which is a crescentic opening 

 on the lower side of the front end. The body is divided into a 

 series of rings, the segments, and at the hinder end is the terminal 

 anus. The first segment is the peristomium and the mouth lies 

 between it and the prostomium. On the dorsal side, the latter 

 projects across the peristomium. 

 There are about 150 segments, 

 but the number probably increases 

 slightly throughout life. (In the 

 related Allolohophora the pros- 

 tomium reaches only half-way 

 across the peristomium, and the 

 common A. chlorotica has about 

 no segments.) At about one-third 

 of the length of the body from its 

 front end, in segments 32-37 in- 

 clusive, a glandular thickening of 

 the epidermis lies athwart the back 

 like a saddle ; it is known as the 

 clitellum. The colour of the worm 

 ranges from brown to purplish, 

 but is somewhat paler below. The 

 skin is covered with an iridescent 

 cuticle of collagen secreted by the 

 underlying cells. In every somite 

 except the first and the last there 

 are eight bristles, the chaetas or 

 setae (Fig. no) in two pairs on each 

 side, a lateral pair, slightly above 



the middle of the side, and a ventral pair between the lateral 

 and the mid-ventral line. The chaetae can be felt with the 

 fingers ; they are made of chitin, which is effectively an amino- 

 cellulose, and are embedded in sacs of the epidermis, by which 

 they are secreted, and to these sacs are attached muscles, by which 

 they can be moved. The chaetae, as we shall see later, assist in 

 locomotion. The ventral chaetae of the clitellum, of the twenty- 

 sixth, and of the tenth to the fifteenth segments, are straighter 

 and more slender than those of other segments, which are stout 

 and somewhat bent. This modification is in connexion with the use 



Fig. 1 10. — A diagram of a chaeta of 

 the earthworm and the struc- 

 tures connected with it. — From 

 Potts, after Stephenson. 



cm.. Circular muscle of body-wall ; ch. 

 chaeta; cm., cuticle; ect., ectoderm; 

 foL, follicle; fm.c, formative cell of 

 chaeta ; per., peritoneum ; pr.nt., pro- 

 tractor, and rt.m., retractor muscles of 

 chaeta. 



