12 PEOTOZOA. 



Class II. GREGARINIDA. 



One- or two-celled organisms, with a nucleus, and sometimes a 

 nucleolus, but no contractile vesicle. No pseudopodia. 



Mostly small organisms which resemble, and have been mis- 

 taken for, ova. They differ from single-celled plants in that 

 their tissues are contractile and soluble in acetic acid. Some- 

 times one extremity is furnished with hooks, serving as organs 

 of prehension. They are all internal parasites ; they differ from 

 Amoabae by having an external cell-membrane. 



A conjugative process, so-called, sometimes occurs : two Gre- 

 garinida come together, a cyst forms around them, and certain 

 globular vessels are formed in it, which ultimately become pecu- 

 liar bodies known as " pseudo-navicellas," or "pseudo-naviculse." 

 After a time these escape, and, bursting, give rise to amcebiform 

 bodies which develop into Gregarinida. 



In the perfect form these parasites occur principally in insects, 

 crabs, and worms. They have been also found in the human 

 heart, kidney, &c. They vary in size ; some attain the length of 

 half an inch. 



Monocystidce. Stylorhynchus. 



Monocystis. Actinocephalus. 



Hoplorhynchus. 

 Dicystidce. 



Gregarina. DidymopJiyidcs. 



Pixinia. Didymophyes. 



Class III. INFUSORIA. 



POLYGASTRICA. MlCKOZOA. 



Minute, aquatic animals, or occasionally internal parasites of 

 definite form, swimming by the vibration of cilia. A mouth and 

 rudimentary stomach. No pseudopodia. A contractile vesicle 

 and nucleus. 



An oral aperture or mouth is mostly confined to the ciliated 

 Infusoria ; and an aboral aperture is sometimes present. The 

 other Infusoria are somewhat doubtful organisms. The Acine- 

 tidae have been regarded as the ancestral forms of the true 

 Infusoria. 



