SEGMENTATION 141 



systems of organs more than others, but usually there is a 

 tendency for the various systems to be repeated in the same 

 degree. In this way the body is divided into segments, each one 

 of which contains a segment of each system of organs. In 

 elongated animals the body segments are arranged in a linear 

 series, and are called metameres. Within the metamere each 

 system of organs is represented by a single segment if the system 

 is median in position, or by a symmetrical pair if they are lateral. 



335. The segments of radial animals are called antimeres 

 and they are arranged radially about the principal axis of 

 symmetry. The antimeres are also bilaterally symmetrical. 

 (Why?) 



336. Metameric segmentation introduces a new type of 

 differentiation, the differentiation of segments. In the phylum 

 Vermes there is little differentiation of segments and hence the 

 segments are said to be homonymous. When the segments are 

 differentiated they are heteronymous. This occurs in progres- 

 sive stages through the phyla Arthropoda and Vertebrata, so 

 that in the higher forms the segmentation is considerably 

 obscured. Of course, differentiation of segments greatly in- 

 creases the complexity of organization. 



337. Segmentation of the body is a means by which its 

 flexibility may be provided for. This is of special importance 

 in animals having a skeleton. 



INTEGUMENT 



338. The amoeba is said to be a naked cell, i. e., it has no cell 

 wall, and therefore can scarcely be said to have an integument. 

 The surface layer of the protoplasm, the pellicle, is slightly 

 denser than that lying deeper, and its consistency is such as 

 to maintain a well-defined boundary between the organism and 

 the surrounding water. The protoplasm is so nearly the density 

 of water and the animal so minute that little force is required 



