THE EARTHWORM 



167 



the epidermis, ep. Some of the cells are sensory cells ; others 

 have the power of secreting a slimy material which keeps the 

 surface of the animal moist, and these are called gland cells; 

 Fig. 82. Under the epidermis is a layer of circular muscles, 

 cm, extending around the body, each muscle in the form of 

 a very long, slender fiber, tapering at both ends. Extending 

 around the body as they do in a 

 circular direction, their contraction 

 will tend to constrict the body and 

 reduce its diameter. Under this is 

 a thicker layer of muscles, running 

 lengthwise, the longitudinal mus- 

 cles, Im. These are arranged in 

 bundles and in a cross section they 

 appear to radiate like a feather, but 

 each longitudinal muscle fiber has 

 the same structure as the circular 

 muscles. By their contraction the 

 animal's body is shortened. Under 

 the longitudinal muscles is an ex- 

 tremely delicate layer of flat cells 



forming a thin membrane bound- (Modified fromDahlgren and Kepner.) 



ing the body wall on the surface lying next to the body 

 cavity. This is the peritoneal (Gr. peri around + teinein = 

 to stretch) epithelium, per. 



Eight delicate bristles, called setae, extend through the mus- 

 cle layers of the body wall and protrude through the skin, 

 Fig. 81 s. They are arranged in four groups, two in each 

 segment, and are attached by several minute muscles on the 

 inner end. By means of these the setae may be slightly ex- 

 truded, or moved to and fro so that the tips may be directed 

 forward and backward. If the earthworm is pulled gently 

 through the fingers, the projecting setae may be felt as a slight 

 roughness on the skin. 



Motion. The motor system of the earthworm is extremely 



FIG. 82. HIGHLY MAGNIFIED 

 SECTION OF THE SKIN OF 



THE EARTHWORM 



gc, gland cells; 



ra, muscles; 



n, nerve; 



so, sensory nerve cells. 



