174 PRACTICAL ZOOLOGY 



gallop over the compact turf from one end of the field to the 

 other, and not strike a single stone with its shoes" (Fig. 98). 



The continuous honeycombing of the soil by earthworms 

 also makes the land more porous and insures the better pene- 

 tration of air and moisture. Furthermore the thorough working 

 over of the surface layers of earth helps to make the soil more 

 fertile. 



Segmentation. - - Before leaving our study of the earthworm 

 several characteristics should be emphasized. The first of 

 these is segmentation. The most successful animals on the 

 earth, the Arthropoda, Vertebrata, and Annelida, have their 

 bodies built on the segmented plan. The linear row of seg- 

 ments, the somites, or metameres, as they are often called, are 

 very clearly visible in the earthworm, the centipede, and in the 

 abdomen of insects, crayfishes, and scorpions, but are not so 

 obvious in vertebrates. All of these animals, however, have 

 their internal organs segmentally arranged; this is most evident 

 in the case of the nervous system of arthropods and annelids, 

 in the nephridia of the earthworm, and in the backbone of man 

 and other vertebrates which consists of a row of similar bones, 

 the vertebrae. 



Of all these animals the earthworm is the best for the demon- 

 stration of both external and internal segmentation, since each 

 external ring indicates a single segment and corresponds to a 

 set of internal parts that are repeated in almost every segment. 

 These internal parts are a pair of nephridia, a ganglion of the 

 nerve cord, series of muscle bands, a part of the alimentary 

 canal, and a section of the body cavity separated from the cavity 

 in other segments by transverse partitions, the septa (Fig. 96, s). 



Body Cavity. The body cavity is another characteristic 

 of the annelids and higher animals that is worthy of mention; 

 and it is best discussed in connection with the earthworm, where 

 it is very simple. The body-cavity, or ccelom, is filled with a 

 liquid which aids in the distribution of nutritive substances, 

 and bears waste materials for the nephridia to carry from the 



