82 



EARTHWORMS, LEECHES, AND SEAWORMS 



of the waste materials of the body are excreted through the 

 skin in the form of carl^on dioxide. But the principal 

 organs of excretion are convoluted tubes, known as ne- 

 phridia, a pair of which is found 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 setce. 

 The inner end of each nephridium has a funnel-shaped 

 orifice fined witli cilia that opens freely into the body cavity. 

 These organs act as simple kidneys and carry off the waste 

 matters rf the body. 



The r.ervous system. — This animal has a brain composed 

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

 the pharynx. From each ganglion 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- 

 circled by what is called the "nerve 

 ring," or "esophageal collar" (Fig. 

 42). After the nerve cords meet they 

 y^ / _-^)— ' \ -g pass throughout the length of the 

 worm on the floor of the body cavity 

 and become so closely fused that they 

 appear as a single cord. In each seg- 

 ment of the b(5dy both cords become 

 enlarged and form a double ganglion 

 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 



Fig. 42. — Anterior end of 

 earthworm : b, brain ; g, 

 ganglia. After Leuclcart. 



