106 



PRINCIPLES OF ANIMAL BIOLOGY 



ainplcs of nearly cylindrical animals are to be found among the Coelente- 

 rata, such as Hydra, jellyfishes, sea-anemones and most hydrozoan and 

 coral polyps. A few of these are illustrated in Fig. 71. In all these 

 animals, whether sessile or motile, the ends differ from each other. Hence 

 a plane which divides the body into two mirrored halves must pass 

 through the long axis of the body; but since the body is circular in cross- 

 section and not differentiated into dorsal and ventral sides almost any 

 plane which passes through the long axis of the body will cut it into mir- 

 rored halves. This type of symmetrj- is called radial. Ideally if 

 an animal is to exhibit radial symmetry it must be possible to pass an 

 infinite number of planes through the long axis, such that each plane 

 cuts the body into mirrored halves. Actual examples of such perfect 

 radial symmetry are unknown and in practice an animal is regarded as 

 radially symmetrical if two or more planes of symmetry are present. 

 Thus in jellyfishes, because of the radial canals and some otlu^r structures, 



Fig. 71. — Various ocelenterates, showing their radial symmetry. .1, sea anemone; 

 B, group of coral polyps; C, the medusa, Mitrocoma cirrata, ventral view. D, polyp of the 

 hydroid, Perirjonimus serpens. (A and B after Jordan, Kellogg and Heath; C after Mayer; 

 D after Alhnan.) 



there are only four planes of symmetry, and in Hydra (leaving out of 

 account certain inconstant features) there are as many planes of sym- 

 metry as there are tentacles, yet these animals are regarded as radial. 

 The starfish is sometimes used to illustrate external radial symmetry, 

 the number of dividing planes being five if five arms are present. How- 

 ever, the starfish falls short of radial symmetry in that one of its organs, 

 the madreporite, is not placed in the center but to one side. 



Universal Symmetry. — A few animals possess an approximate 

 universal symmetry, but none show it perfectly. To exhibit universal 

 symmetry it must be possible to pass planes through the body in any 

 direction. Such planes must necessarily pass through a central point 

 and the only geometrical solid which may be so cut is a sphere. Very 

 few animals or aggregations of cells or individuals are truly spherical 

 and most of those which are spherical exhibit polarity, that is, opposite 

 sides are in some way differentiated, which makes their symmetry radial 

 and not universal. Volvox comes as near universal sj'mnietry as any 

 animal which can be named, yet some species of Volvox exhibit a certain 



