9° 



COLLEGE ZOOLOGY 



5. The Forms of Animals 



Although most animals pass through similar stages in their 

 development from the egg, the adult organisms differ widely 

 in the form of their bodies. This is a result of two factors: (1) 

 the initial structure of the germ, and (2) the influence of the 

 environment. Differences in the form of animals are due 

 principally to symmetry, metamerism, and the character of the 

 appendages. 



Symmetry. — Animals are either symmetrical or asymmetrical. 

 The symmetrical animals may be divided into two types: (1) 

 radially symmetrical, and (2) bilaterally symmetrical. 



A radially symmetrical animal possesses a number of similar 

 parts, called antimeres, which radiate out from a central axis. 

 The adult starfish (Fig. 131) is a good example; its arms are 

 similar and radiate out from the central disc. Some simple 

 sponges (Fig. 55), the majority of the Ccelenterata (Fig. 79), 

 and most adult EchinodeRmata are radially symmetrical. 

 Radial symmetry is best suited to sessile animals, since the 

 similarity of the antimeres enables them to obtain food or repel 

 enemies from all sides. 



The bodies of bilaterally symmetrical animals are so constructed 

 that the chief organs are arranged in pairs on either side of an 

 axis passing from the head or anterior end to the tail or posterior 

 end. There is only one plane through which their bodies can 

 be divided into two similar parts. An upper or dorsal surface 

 and a lower or ventral surface are recognizable, as well as right 

 and left sides. Bilateral symmetry is characteristic of the most 

 successful animals living at the present time, including all of 

 the vertebrates and most of the invertebrates. 



Metamerism. — Metameric animals have bodies composed 

 of more or less similar parts or organs arranged in a linear series 

 along the main axis. Each part is called a metamere, somite, 

 or segment. In many animals metamerism is not shown by the 

 external structures, but is exhibited by the internal organs; this 



