THE GKEGAl'JNIDA. 9 



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

 contained. 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 any longer those movements of which I spoke, and becom- 

 ing coated by a structureless investment, or "cyst" (Fig. 1, B). 

 The substance of the body contained within the cyst next 

 undergoes a singular change. The central nucleus and the 

 vesicle disappear ; after a time, the mass breaks up into a series 

 of rounded portions and, then, each of those rounded portions 

 elongates, and, becoming slightly pointed at each end, consti- 

 tutes a little body which has been called a " Pseudo-navicella" 

 from its resemblance to the Diatomaceous Navicula or Navicella 

 (Fig. 1, C, D). Next, the capsule bursts and the Pseudo- 

 navicellse (Fig. 1, E, F) are scattered and passed out of the 

 body of the animal which they inhabit. Though, of course, a 

 great number of them are destroyed, some, at any rate, are 

 devoured by other animals ; and, when that is the case, the 

 little particle of protein substance which is enclosed within the 

 Pseudo-navicella is set free from its shell, and exhibits much 

 more lively movements than before, thrusting out processes in 

 various directions, and drawing them in again, and, in fact, 

 closely resembling one of those animalcules which have been 

 called Amcebtc (Fig. 1, H). The young Amoebiform Gregarina 

 grows, increases in size, and at length assumes the structure 

 which it had at first. That, in substance, is all that we know of 

 this lowest division of animal life. But it will be observed, 

 there is a hiatus in our knowledge. We cannot say that we 

 know the whole nature and mode of existence of this, or any 

 other animal, until we have traced it to its sexual state ; but, at 



