8 



ON CLASSIFICATION. 



membrane. I do not wish to strain the analogy too far, but it 

 is, at any rate, interesting to observe this close morphological 

 resemblance between one of the lowest of animals and that form 

 in which all the higher animals commence their existence. It 

 is a very remarkable characteristic of this group, that there is 

 no separation of the body into distinct layers, or into cellular 

 elements. The Gregarinida are devoid of mouths and of 

 digestive apparatus, living entirely by imbibition of the juices 



Fig. 1. — A, Gregarina of the earthworm (after Lieberkuhn) ; B, encysted; C, D, with 

 the contents divided into pseudo-navicellae ; E, F, free pseudo-navicellae ; G, H, free 

 amcebiform contents of the latter. 



of the animal in whose intestine, or body cavity, they are con- 

 tained. The most conspicuous of those phenomena, which we 

 ordinarily regard as signs of life, which they exhibit, is a certain 

 contraction and expansion along different diameters, the body 

 slowly narrowing, and then lengthening, in various directions. 

 Under certain circumstances (though the conditions of the 

 change are not thoroughly understood), it is observed that one of 

 these Gregarinida, whatever its form may be, will convert itself 

 into a well-rounded sac, the outer membrane ceasing to exhibit 



