Q6 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



and straw. ' Its cylindrical body consists of numerous 

 segments, the wall being very muscular and covered 

 by a tough, smooth, transparent cuticle. The body 

 cavity is subdivided by numerous transverse membra- 

 nous partitions. The digestive system extends through- 

 out the body and there are well-developed nervous and 

 circulatory systems. The former lies 

 mainly below the digestive organs and 

 consists of a nerve cord running the 

 length of the body. This cord is 

 composed of pairs of ganglia con- 

 nected by longitudinal and transverse 

 branches. Above the mouth opening 

 is a pair of ganglia forming the 

 " brain." The larger blood vessels 

 surround the esophagus and one, the 

 dorsal vein, lies above the digestive 

 system and may be seen through the 

 FIG. 52. - Earthworm, in- skin on the back. There are no well- 



ternal anatomy of the ir-i tiii 



anterior region. The body defined sense organs, although, judg- 

 has been opened along the m g f rom experiments, the earthworm 



dorsal line and the inter- 

 nal organs turned to the is sensitive to touch, is affected by 



left; ph, pharynx; h, ... . ,. 



"heart"; /, muscular changes in the intensity of light, and 

 partitions; ,, waii of ^^ evidence of having the sense of 



body; r, reproductive or- 

 gans (in part) ; dv, dorsal 

 blood vessel ; i, intestine ; 

 g, " brain " ; vn, ventral 

 nerve cord; si, subintes- 

 tinal blood vessel; sn, 

 subneural blood vessel. 



taste. Respiration is carried on by 

 the vascular skin, there being no 

 lungs or gills. Earthworms feed 

 upon decaying vegetable matter and 

 upon organic particles contained in the earth, swallowed 

 in the process of making the burrow, or for the sake of 

 the contained food. The refuse from the body is piled 

 up around the mouth of the burrow in the form of pel- 

 lets. The amount of earth annually brought up from 

 the deeper layers of the soil is sufficient to be of consid- 

 erable geological and economic importance. 



