530 ANNULOSA, OK WOEMS. 



nature of the bodies formerly denominated cystic Entozoa, which 

 have always ranked until recently as a distinct group. These 

 are not found, like the preceding, in the cavity of the alimentary 

 canal of the animals they infest ; but always occur in the sub- 

 stance of solid organs, such as the glands, muscles, &c. They 

 present themselves to the eye as bags or vesicles of various sizes, 

 sometimes occurring singly, sometimes in groups ; but upon 

 careful examination,, each vesicle is found to bear upon some 

 part a "head" furnished with booklets and suckers; and thus 

 may be either single, as in Cysticercus (the entozoon whose pre- 

 sence gives to pork what is known as the " measly"disorder), or 

 multiple, as in Caenurus, which is developed in the brain, chiefly 

 of sheep, giving rise to the disorder known as "the staggers." 

 Now in none of these " cystic" forms has any generative appa- 

 ratus ever been discovered ; and hence they are obviously to be 

 considered as imperfect animals. The close resemblance between 

 the "head" of certain Cysticerci and that of certain Tcenice, first 

 suggested that the two might be diflerent states of the same ani- 

 mal ; and experiments recently made by those who have devoted 

 themselves to the working out of this curious subject, have led 

 to the assured conclusion, that the " cystic" Entozoa are nothing 

 else than " cestoid" worms, whose development has been modi- 

 fied by the peculiarity of their position, — the large bag being 

 formed by a sort of dropsical accumulation of fluid when the 

 young are evolved in the midst of solid tissues, whilst the very- 

 same bodies, conveyed into the alimentary canal of some carni- 

 vorous animal which has fed upon the flesh infested with them, 

 begin to bud forth the generative segments, the long succession 

 of which, united end to end, gives to the entire series a worm- 

 like aspect. 



358. In the intestinal canal of Insects, Centipedes, &c., a very 

 curious kind of animal parasite is often to be met with, the sim- 

 plicity of whose structure seems to carry us back to the Protozoa 

 (Chap. IX). It is not yet by any means certain, however, that 

 we know the entire life-history of this parasite, the Gregarina; 

 and it may be only a phase in the existence of some higher kind 

 of Entozoon. Each individual essentially consists of a single 

 cell, usually more or less ovate in form, and sometimes con- 

 siderably elongated ; a sort of beak or proboscis frequently pro- 

 jects from one extremity; and in some instances this is furnished 

 with a circular row of booklets, closely resembling that which 

 is seen on the head of Taenia. Within the cavity of the cell, 

 whose contents are usually milk white and minutely granular, 

 there is generally seen a pellucid nucleus ; and this becomes 

 first constricted and then cleft, when, as often happens, the cell 

 subdivides into two, by a process exactly analogous to that which 

 takes place in the simplest Protophytes (§ 150). The membrane 

 and its contents, except the nucleus, are soluble in acetic acid. 

 Cilia have been detected both upon the outer and the inner sur- 



