CELETOPODA 67 



segment are dilated and pulsate ryhthmically, hence are some- 

 times called hearts. The blood is red, the color being due to 

 the presence of hemoglobin (the same substance which makes 

 our blood red) in the liquid itself, though the blood contains 

 colorless corpuscles. 



The nervous system consists of a double cerebral ganglion 

 connected with a double ventral chain of ganglia by a pair of 

 commissures which pass around the esophagus. 



The earthworm has no eyes, yet it can distinguish not only 

 light, but the direction from which it comes, and it will crawl 

 " away from the light of high intensity and toward a light of low 

 intensity." This tendency, and the fact that the moisture of 

 the skin would be rapidly evaporated in daytime, and the ab- 

 sence of enemies, induce the earthworms to feed at night. 



The earthworm has no organs of hearing, but its general sense 

 of touch is so delicate that it detects the approach of danger 

 by the jarring of the earth about its burrow. 



It can distinguish and choose between different kinds of food, 

 so it must have a sense akin to smell or taste. It is thought that 

 the " goblet-shaped bodies " on the prostomium and on the 

 anterior segments are the seat of this sense. 



The body wall is composed of, first (on the outside), the cuticle, 

 then the epidermis, the dermis, a muscular layer of circular 

 fibers, a layer of longitudinal muscle-fibers, and underneath this 

 the ccelomic epithelium which lines the body cavity. 



Respiration takes place through the thin moist skin which 

 is everywhere underlaid by a network of blood-vessels. These 

 absorb the oxygen from the air and give off the carbonic acid 

 gas through the skin. 



Locomotion. Each segment, except the one at each end of the 

 worm, is furnished with four pairs of setae, or short, stiff, chitin- 

 ous bristles. They arise from the setigerous glands or sacs 

 made by the infolding of the cuticle. By special muscles, at- 

 tached to the base of each of these sacs, the setae can be turned 

 in different directions. In locomotion the earthworm uses 

 these setae as levers. When it moves forward the setae are 

 turned backward and stuck into the soil, the longitudinal 

 muscles contract, pulling the body together, then the circular 

 muscles contract, making the body smaller and longer and fore- 



