PROMORPHOLOGY. 



II ; A 4, B 3, C 2). Any plane including 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. 



FIG. 46. 



P' 



FIG. 46. Actinomma astcracanthion, a Radiolarian with a limited number of special- 

 ized radii (axes), symmetrically arranged about the centre. A, whole animal with 

 portion of two spheres of shell removed. B, section, showing relation of the proto- 

 plasm to the skeleton, n, nucleus; p, protoplasm; sk., skeleton. From Parker and 

 Haswell. 



Questions on the figure. In what way does this species differ in 

 symmetry from Fig. 44? How many specially developed axes appear to be 

 present? By how many planes may the organism be divided into essen- 

 tially equal portions? 



Two important variations from this simple condition of radial sym- 

 metry are found in the animal kingdom : 



(a) 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 prin- 

 cipal axis. Considered from the point of view of the subordinate axes 

 this means that there are a limited number of special axes (Fig. 47, aa* 

 and bb l ) perpendicular to the principal axis (Fig. 47, o-ab.o) instead of 

 an indefinite number as in the former case. These special subordinate 

 axes are usually 3, 4, or 5, or some multiple of these numbers. The num- 

 ber however may be reduced to two in which the four poles are all alike. 

 Many of the medusae, coral polyps, and some echinoderms illustrate this 

 type of symmetry. 



(&) A further variation of (a) is seen in the fact that in some ani- 

 mals, otherwise similar to those described in (a), one of these special 

 axes perpendicular to the principal axis comes to differ from the other. 



