MORPHOLOGY OF HIGHER TYPES 107 



degree of polarity in that the germ cells tend to develop at one pole or 

 on one side of the sphere. 



Asymmetry. — Animals whose bodies cannot be cut by a plane into 

 two mirrored halves are asymmetrical. Examples of asymmetrical 

 Protozoa are Paramecium, Amoeba (in most states of motion), and 

 Stylonychia. The snail with its coiled shell is asymmetrical but the body 

 is built on a bilaterally symmetrical plan which has been subsequently 

 modified by twisting. 



Many animals which are externally symmetrical may have their 

 internal structures arranged on an asymmetrical plan or on a plan of 

 symmetry different from the external plan. Cases in point are found in 

 the heart, stomach, and in fact the major portion of the alimentary 

 tract and the lobes of the liver in man, which are arranged asymmetrically. 



Symmetry and Locomotion. — An examination of a large number of 

 animals in regard to their symmetry and a consideration of the mode of 

 life employed by each animal, whether sessile, or capable of slow move- 

 ment or of rapid movement, reveals a certain relationship between the 

 type of symmetry and the character of locomotion. It is found that 

 most animals which move rapidly or are capable of well coordinated 

 direct movement exhibit bilateral symmetry. Examples in support 

 of this statement are such mammals as the dog and horse, the fishes, 

 amphibians, reptiles, the ai'thropods, most mollusks and most worms. 

 A large proportion of animals of very slow movement, and especially 

 those which are sessile, show radial symmetry. Examples are the coelen- 

 terates, sponges, and some echinoderms.^ Universally symmetrical 

 animals are aquatic, and progress by a rolling motion. 



Asymmetrical animals which have retained their powers of locomo- 

 tion are relatively rare except among the Protozoa. In many species 

 whose individuals are apparently as3'mmetrical the asymmetry is due to 

 modification of a fundamental plan of bilateral sj^mmetry. The flat- 

 fishes, such as the hahbut, flounder and sole, which have two eyes placed 

 on one side of the head, are in their early stages bilaterally symmetrical, 

 but one eye migrates through the head to its new position. Asymmetry 

 in adult parasitic Crustacea is fairlj^ common and is the result of de- 

 generacy. Those species of Crustacea, including some parasitic species 

 as well as free-living ones, which retain their powers of locomotion 

 are bilaterally symmetrical. The individuals of many parasitic species 

 which are asymmetrical in either the adult or larval periods are always 

 bilaterally symmetrical in their free-swimming stages. 



^ Frequent mention must be made, in this chapter, of the groups of animals. It 

 is not important that the student be able to recognize these groups at the present 

 time, l)ut reference should be made to the synoptic table in Chapter XII to determine 

 whether the groups stand low or high in the animal scale. Common names of mem- 

 bers of the group may usually be found in the glossary. 



