82 



EARTHWORMS, LEECHES, AND SEAWORMS 



of the waste materials of the body are excreted through the 

 skin in the form of carbon dioxide. But the principal 

 organs of excretion are convoluted tubes, known as ne- 

 phridia, a pair of which is foxind in each segment of the body 

 except the first three or four and the last one. Each ne- 

 phridium opens at one end to the exterior by a minute pore 

 in the body walls between the upper and lower rows of setae. 

 The inner end of each nephridium has a funnel-shaped 

 orifice lined with cilia that opens freely into the body cavity. 

 These organs act as simple kidneys and carry off the waste 

 matters of the body. 



The nervous system. — This animal has a brain composed 

 of two ganglia on the dorsal side of the anterior part of 

 the pharynx. From each gangUon of 

 the brain a nerve cord passes down- 

 ward on the corresponding side of the 

 pharynx. These cords meet below. 

 Thus the pharynx is completely en- 

 I, circled by what is called the " nerve 

 ring," or "esophageal collar" (Fig. 

 42). After the nerve cords meet they 

 7"*'' /' . — H — '[~\~ff pass throughout the length of the 

 g worm on the floor of the body cavity 

 and become so closely fused that they 

 Fig. 42. — ABterior end of appear as a single cord. In each seg- 

 earthworm : b, brain ; g, ment of the body both cords become 



ganglia. After Leuckart. , , , „ n i , t 



enlarged and form a double ganguon 

 which appears as one. Smaller nerves pass from these 

 ganglia to different parts of the body (Fig. 42). 



Senses of the earthworm. — It has been shown that all 

 parts of the skin of the earthworm contain cells each of 

 which gives off a nerve fiber that runs directly to the large 



