90 Descriptive Zoology. 



The Skin. This consists of two layers. Outside is the 

 cuticle, a thin layer, usually showing a beautiful iridescence. 

 The cuticle often peels off in specimens that have been in 

 alcohol. Underneath the cuticle is the epidermis (often 1 

 called the hypodermis). 



The Muscular Coat. The muscular coat is very much 

 thicker than the skin. It consists of two layers, an outer 

 layer of circular muscle fibers, and an inner layer of fibers 

 running lengthwise. The inner layer is much thicker than 

 the outer. 



The Bristles. The bristles, or setce, are short, stiff, chi- 

 tinous spines, in four rows along the ventral surface and 

 lower part of the sides. They are outgrowths of the skin, 

 and are lodged in infoldings, or pockets, of the cuticle, which 

 are called setigerous glands. As the bristles are worn out 

 and become useless, they are replaced by others ; and in 

 the same sac may be found bristles in various stages of 

 development. Each row of bristles is double ; and each 

 segment, except the first and last, has four pairs of them. 

 Special muscles are attached to the base of the sac hold- 

 ing the bristles, so that the bristles can be turned and held 

 in various directions. The bristles can also be protruded 

 and withdrawn. 



How the Earthworm Crawls. When the worm wishes to 

 crawl forward, the spines are turned backward. Then the 

 longitudinal muscles shorten, and the posterior end of the 

 body is pulled forward, the whole body becoming shorter 

 and thicker. Next the circular muscular fibers are short- 

 ened ; this narrows and elongates the body ; but as the 

 spines prevent any part from being pushed backward, the 

 result is a forward movement. By a repetition of these 

 acts the worm effects its slow locomotion. If it wishes to 



