6 HlCKSON, Animal Symmetry. 



tentacles, but they vary in number from 5 to 10 in 

 specimens of the same species. In the Sea-anemone too, 

 there are similar variations both in the number of the 

 tentacles and in the number of the mesenteries. In the 

 jelly-fish Aurelia aurita Ehrenberg had recorded extra- 

 ordinary variations in the number of rays, of the gastric 

 pouches and of other organs. 



All of these variations are what Bateson has called 

 meristic, and do not affect the general plan of radial 

 symmetry. 



With bilaterally symmetrical animals on the other 

 hand the variations are far less frequent, and are usually 

 confined to organs of secondary importance It is not 

 necessary to enlarge upon this point in this place because 

 the truth of it must be apparent to any one who has had 

 long experience in a zoological laboratory, but mention 

 may be made of the rarity of any variation from the 

 normal that we find in any of the important organs of the 

 Earthworm, Crayfish, Frog, Rabbit, and other animals 

 that are frequently dissected in the zoological classes. 



But this difference as regards variability is just what 

 would be expected. Any variation from the normal in a 

 bilaterally symmetrical animal unless paired or exactly 

 median would interfere with the balance of the body on 

 the antero-posterior axis and impede the rapidity and 

 accuracy of movement. We can understand therefore 

 that in evolution any tendency to variations on an im- 

 portant scale are checked unless such variations are equi- 

 poised on the median line, but even bilateral or median 

 variations become rarer as the rapidity and co-ordination 

 of movement are increased. For example, in the slow 

 crawling Annelida variations in the actual number of 

 somites of a species are common but in the more rapid 

 Arthropoda and the higher Vertebrata variations in the 



