244 CLASSIFICATION AND ORGANIZATION OF ANIMALS 



probably come from ancestors that were attached and hence 

 radially symmetrical, if they do not have an attached stage 

 in the life cycle. In many cases, animals that are essentially 

 bilateral in type but are attached, like some of the segmented 

 worms, Annulata, show a modification of bilateral organs in the 

 direction of radial symmetry. 



The bilateral sijmmetry that is characteristic of all the more 

 highly developed animal types is a symmetry on either side of a 

 plane, in contrast to radial symmetry which is through a line. A 

 radially symmetrical body may become bilateral by modification in 

 one of its radii, as when an umbrella has a curved handle or a 

 glass cyhnder is modified to form a beaker, and can then be 

 divided into symmetrical halves by only one plane. Another way 

 to define bilateral symmetry is to say that each of the halves sep- 

 arated by the median plane is a mirrored image of the other, or 

 that the halves are " rights " and " lefts," hke hands and feet. 

 In general, bilateral symmetry is related to going " head-end 

 first," and to the antero-posterior differentiation described in a 

 subsequent paragraph. Some organisms are asijmmetrical; that is, 

 there is no plane that will divide them into symmetrical halves. 

 The most familiar examples are Protozoa like paramoecium. 



Other Modes of Differentiation. — In addition to being organ- 

 ized according to the foregoing types of symmetry, animals may 

 show 'proximal and distal differentiations, as in attached forms, 

 where the base or proximal end differs from the distal end. An 

 example is hydra, with its " foot " and tentacles (Fig. 121, p. 250). 

 In other cases there is an oral side, which is differentiated from the 

 ahoral, as in a jellyfish (Fig. 127, p. 265) or a starfish. Such 

 proximo-distal and oral-aboral modifications are most typical of 

 radially symmetrical animals. 



Bilateral animals, on the other hand, characteristically exhibit 

 dor so-ventral differentiation into a "back" and a "belly" side; 

 and antero-posterior differentiation into " head " and " tail " ends. 

 Metamerism, or the segmental condition that appears so clearly in 

 the earthworm (Fig. 137 A, p. 284) and other Annulata, is also 

 characteristic of the Arthropoda and Chordata, as evidenced by 

 the abdomen of the crayfish (Fig. 155, p. 323) and the vertebral 

 segmentation of a vertebrate (Fig. 40, p. 69). 



T3rpes of Internal Organization. — The internal structure of 

 many-celled animals may in turn be broadly classified as belonging 



