THE SHAPES AND SIZES OF ANIMALS 123 



blastula however is absolutely spherical in the 

 sense in which I have used the word above, 

 for the boundary lines between the individual cells 

 must be of different value to the cells themselves, 

 so that all parts of the surface cannot be identical. 

 Moreover, the component cells very usually, perhaps 

 always, present differences of size or structure by 

 which upper and lower hemispheres may be 

 marked off from each other, and by which the true 

 spherical symmetry becomes still further disturbed. 

 Volvox and Pandorina may be quoted as examples 

 of permanent blastulse in which the component 

 cells present no such differences, but they are 

 forms the animal nature of which is still extremely 

 doubtful. 



Leaving the spherical forms, the next character- 

 istic shape we meet with among animals is that 

 known as radially symmetrical, of which the most 

 typical instances are met with in the group of 

 Ccelenterates ; an ordinary jelly-fish affording as 

 excellent an example as one could wish to find. 

 A sphere is said to have an infinite number of 

 axes, all equal to one another ; but a jelly-fish may 

 be described, if the mathematicians will pardon the 

 phrase, as having of axes a number that can only 

 be expressed as being one more than infinity, for 

 its bell-shaped body, besides having an infinite 

 number of transverse equal axes, has one definite 

 longitudinal axis round which all the parts are 

 symmetrically arranged. Watch a jelly-fish swim- 

 ming in still water, and you will note that while 

 locomotion is always effected in the direction of the 



