404 GENERAL ZOOLOGY. 



one, the body is bilaterally symmetrical. In it we can 

 recognize anterior and posterior; dorsal and ventral; 

 light and left. Under the other we place those forms in 

 which these features do not exist; there is no right and 

 left, but the parts are radially arranged around an axis, 

 like the spokes around the axle of a wheel. To this latter 

 group belong the coalenterates ; to the first, all other 

 divisions reviewed in this volume. Even the Echino- 

 derms belong to the bilateral type, for their development 

 (fig. 90) shows that in the early stages they have not a 

 trace of radial symmetry, but only acquire it later in 

 life. 



In the bilateral animals, in turn, two types can be 

 recognized: the segmented and the unsegmented. The 

 segmented forms show their peculiarities in the most 

 striking manner in some of the Annelids, like the earth- 

 worm. In these the body is made up of a series of rings 

 or segments, each essentially like its fellow, and each 

 containing a portion of all systems of organs muscular, 

 nervous, circulatory, digestive, excretory, etc. In the 

 arthropods this segmentation again appears, but here 

 there are tendencies in two directions: towards a fusion 

 of segments, and towards an increase of one segment at 

 the expense of another. In annelids and arthropods this 

 segmentation is visible externally; in the vertebrates it 

 is not so plainly shown, but it nevertheless exists. The 

 trunk muscles (see p. 17) are thus arranged; the spinal 

 nerves and the vertebrae correspond to the muscle seg- 

 ments, as do also certain blood-vessels (intercostals), 

 while in their early history the kidneys are segmentally 

 arranged. 



On the other hand, the lower worms show no traces of 

 segmentation, while the molluscs show it to a very slight 



