172 PRACTICAL ZOOLOGY 



vessels carries the blood about the body and with it the digested 

 food (Fig. 96, dv, vv, pv, Inv, snv). Waste products pass 

 into the blood as it circulates and are excreted by organs 

 called nephridia (Fig. 96, ). Almost every segment contains 

 a pair of these organs. They open into the cavity surrounding 

 the alimentary canal by a ciliated funnel (Fig. 97, nephrost) 

 which draws waste matter out of the fluid within the body cavity. 

 The nephridia are well supplied with blood vessels, and in some 

 way extract from the blood circulating through them the ex- 

 cretory substances. These are then expelled from the body 

 through pores, one to each nephridium, which open on either 

 side near the ventral surface of the body (Fig. 97, ext. neph}. 



Respiration. Oxygen is as necessary for the vital processes 

 of the earthworm as it is for those of higher animals, but there 

 are no well-defined respiratory organs. The oxygen passes 

 through the outer membrane of the body wall into the blood, 

 and carbon dioxide passes out of the blood in the same way. 



Sensations. As might be expected, the earthworm has no 

 well-developed sense organs, like eyes or ears, but nevertheless 

 it exhibits many of the ordinary sensory reactions characteristic 

 of more complex creatures. Thus if a light is thrown upon it 

 at night or if the earth is disturbed near by, it will retreat at 

 once into its burrow. This proves that its sensitiveness to light 

 and to tactile stimuli is sufficient to cause it to seek safety in 

 flight. The senses of taste and smell are probably also present, 

 since a preference for certain kinds of food, such as cabbage 

 leaves, and carrots, is often shown. Minute sense organs have 

 been found at the surface of the body. These seem more abun- 

 dant at the anterior and posterior ends. 



Nervous System. The nervous system which connects 

 with these sense organs consists of a brain lying above the 

 pharynx and a ventral nerve cord (Fig. 96, nc) which is situ- 

 ated between the body wall and the alimentary canal in the 

 median central portion of the body, and extends almost the 

 entire length of the animal. This nerve cord becomes enlarged 



