8 



INTRODUCTION TO CLASSIFICATION. 



worms. They are all microscopic, and any one of them, leaving 

 minor modifications aside, may be said to consist of a sac, com- 

 posed of a more or less structureless, not very well-defined 

 membrane, containing a soft semi-fluid substance, in the midst, 

 or at one end, of which lies a delicate vesicle ; in the centre of 

 the latter is a more solid particle. (Fig. 1, A.) No doubt 

 many persons will be struck with the close resemblance of the 

 structure of this body to that which is possessed by an ovum. 



Vig. 1. A, Gregarina of the earthworm (after Lieberkilhn) ; B, encysted; C, D, with 

 the contents divided into psuedo-navieellae ; E, F, free pseudo-navicellse ; G, H, free 

 amcebifbrm contents of the latter. 



You might take the more solid particle to be the representative 

 of the germinal spot, and the vesicle to be that of the 

 germinal vesicle ; while the semi-fluid sarcodic contents might 

 be regarded as the yelk, and the outer membrane as the 

 vitelline 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, 



