MORPHOLOGY OF ANIMALS. 



We are now in position to review some of the facts we 

 nave already discovered, and to draw some general conclu- 

 sions. 



Excepting some Protozoa, each and every animal can be 

 placed under one of two heads. In the one, the body is 

 bilaterally symmetrical. In it we can recognize anterior 

 and posterior; dorsal and ventral; right 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 

 sponges and coelenterates ; to the first, all other divisions 

 reviewed in this volume. Even the Echinoderms belong 

 to the bilateral type, for their development 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 (p. 235). 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 muscu- 

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

 arthropods this segmentation again appears, but here there 

 are tendencies in two directions : towards a fusion of seg- 

 ments, and towards an increase of one segment at the 

 expense of another. In annelids and arthropods this 



331 



