CHAPTER XXV. THE EARTHWORM. 



(Lumbricus terrestris.) 



1. Form and Mode of Life. This familiar animal 

 (fig. 174) has a cylindrical body varying in length up to 

 six inches or more, pointed at the anterior end, somewhat 

 swollen for the anterior third of its length, and rather 

 flattened dorsoventrally in the hinder two-thirds. The 

 mouth is almost terminal, ventral, and overhung by a 

 prse-oral lobe (an outgrowth from the first somite); the 

 anus is quite terminal. The most striking feature is the 

 obvious external metamerism, the whole body being divided 

 into a very large number (about a hundred and fifty) of 



Fig. 174. FRONT PORTION OF EARTHWORM. 

 Side View. (After Howes.) 



segments (somites or metameres) all almost exactly alike. 

 No other of our types (unless it be Amphioxus) exhibits 

 metameric segmentation so fully and plainly. With the 

 exception of the first thirty-five or so of the somites, where 

 the reproductive organs and certain special parts of the 

 alimentary canal are found, each somite is an exact repeti- 

 tion of any other. There is no distinct head. In a mature 

 worm the region of somites 32 to 37 is enlarged, forming 

 the clitellum ; here the skin is very glandular (see 2). 



Though devoid of limbs, the earthworm has a highly 

 muscular body-wall, and there project from its surface a 



336 



