94 



ZOOLOGY 



but the structure is such that no plane will separate the animal into symmetrical 

 parts (Paramecium and many other active Protozoa; see Figs. 68-71). 



3. As a variation of the universally symmetrical condition seen in I, a limited 

 number of axes may become distinguishable from the others by some specialization 

 of structure (Fig. 48). These special axes are similar and their two poles are alike. 



4. Starting again from the undifferentiated spherical form, one of its numerous 

 similar axes may come to differ from all those perpendicular to it by increased or 

 diminished length, or by a difEerence in construction. This special axis is to be 

 known as the principal axis. The poles of the principal axis do not usually remain 

 alike. Perpendicular to this principal axis one may select an indefinite number of 

 subordinate axes which are essentially similar to one another. The poles of each 

 subordinate axis are alike. Such a condition is realized in the simplest gastrulae 

 (Fig. 13; i4 4, 5 3, C 2). Any plane which includes the principal axis will divide 

 such an organism into two equal halves. In general external appearance a hen's 

 egg would illustrate the type. This is the least differentiated form of what is 

 known as radial symmetry. 



Two important variations from this simple condition of radial symmetry are 

 found among animals. 



Fig. 49. 



ab. 0. 



a' 



Fig. 49. Diagram of Medusa, illustrating radial symmetry. A, viewed from the oral end of 

 the principal axis; B, a section along the principal axis and through one of the subordinate axes aa^i 

 o, ab, o, the oral and aboral poles of the principal axis; o, o>, and 6, 4>, the similar poles of the two 

 chief subordinate axes. 



Questions on the figures. — Are the poles of the oral-aboral a,xis alike or unlike? 

 How many clearly differentiated secondary axes are there? What would be the 

 appearance of a section midway between aa^ and ii'? Would the resulting halves 

 be symmetrical? Compare this condition with the definition of radial symmetry 

 in the text. Find other illustrations of radial symmetry in the figures of this 

 book. 



(o) Special organs, such as those of locomotion and the like, may be developed 

 about the principal axis. These usually come to be arranged in a limited number 

 of the planes which may be passed through the principal axis. Considered from 

 the point of view of the subordinate axes this means that there are a limited number 

 of special axes (Fig. 49, aa^ and 66') perpendicular to the principal axis (Fig. 49, 



