132 ANIMALS 



also affects the organism, impinges more strongly from above, 

 it will also operate to differentiate the upper and lower sides. 

 These two sides are distinguished as dorsal and ventral, 

 respectively. 



313. Fishes which do not rest on the bottom but always 

 float suspended in the water do not present the same degree 

 of dorso-ventral differentiation. In this case the action of 

 gravity is practically eliminated by the buoyancy of the water. 



314. Right and Left.^ — With the differentiation of anterior 

 and posterior and of dorsal and ventral the animal comes to 

 have a right side and a left side. These two sides are so related 

 to the external world that every force which acts on one side 

 affects the other side also in a symmetrical way. In con- 

 sequence, the two sides are also symmetrical in form in every 

 way. 



315. Bilateral Sjmimetry. — An organism Hke the one just 

 described is divided into right and left symmetrical halves by 

 the vertical plane in w^hich the principal axis lies. No other 

 symmetrical division of such a form is possible. A body 

 having such a form is said to be bilaterally symmetrical, and 

 this is the type of symmetry found in most animals and gener- 

 ally in those having marked freedom of locomotion. 



316. Radial Symmetry. — Those animals which are very slug- 

 gish in movement or actually fixed, show little or no evidence of 

 bilateral symmetry. The principal axis is perpendicular to the 

 substratum, and its two poles are differentiated; the mouth and 

 associated organs for ingestion are at the free, oral, pole, while 

 the opposite, aboral, pole is modified for attachment. If the 

 animal is not actually fixed, the oral pole may be toward the 

 substratum. In either case, the organization of the animal is 

 more or less perfectly radial with respect to the principal axis, 

 the number of rays being 2, 4, 6 or 5 or a multiple of one of 

 these numbers. The oral-aboral differentiation in part cor- 

 responds to the dorso-ventral differentiation of bilateral forms. 



