THE GREGARINES AND COCCIDIA 341 



not likely to be of classificatory value. The classification of the 

 future will probably be one which divides all gregarines into Cepha- 

 lina and Acephalina, and distributes the *chizogregarines amongst 

 these two divisions. 



At present the following families of schizogregarines are recog- 

 nized : Ophryocystidce. Schizocystidce , Selenidiidce, Merogregarinidce, 

 and Porosporidce. For the family Aggregatidce, see p. 353. 



ORDER II. COCCIDIA. 



The chief characteristics of the Coccidia are that, with very few 

 exceptions, the parasites are of intracellular habitat during the 

 trophic phase, and that a number of spores or sporozoites are 

 produced within a cyst, all of which are the offspring of a single 

 zygote. Further, there is always an alternation of generations, 

 non-sexual multiplicative schizogony alternating with sexual 

 propagative sporogony. As a general rule the entire life-cycle is 

 confined to a single host, but in one family (Aggregatidce) an alterna- 

 tion of hosts occurs, corresponding with the alternation of genera- 

 tions ; that is to say, the schizogony takes place in one host, the 

 sporogony in another. 



Coccidia are found as parasites of various groups of the animal 

 kingdom. In contrast to gregarines, they are found sparingly in 

 Insects, and, indeed, in Arthropods generally with the exception 

 of Myriopods ; but they occur commonly in Molluscs, and especially 

 in Vertebrates of all classes. They are found also in Annelids, but 

 not abundantly, and in Flat- Worms (Turbellaria) and Nemertines. 

 A parasite of the gregarine Cystobia chiridotce has been identified 

 by Dogiel (602) as a coccidian, and given the name Hyalosphcera 

 gregarinicola. 



The intracellular trophozoite is typically a motionless body, 

 spherical, ovoid, or bean-shaped, often with a considerable resem- 

 blance to an ovum ; hence these parasites were formerly spoken of 

 as egg-like psorosperms (" eiformige Psorospermien "), and the 

 same idea is expressed in such a name as Coccidium oviforme, given 

 by Leuckart to the familiar parasite of the rabbit now generally 

 known as C. cuniculi (or C. stiedce) . The same deceptive resemblance 

 extends to the propagative phases, and the eggs of parasitic worms 

 have before now been mistaken for coccidian cysts, or vice versa. 



The infection of the host takes place in every case, so far as is 

 known at present, by the casual or contaminative method. Resis- 

 tant spores or cysts of the parasite are swallowed accidentally with 

 the food, and germinate in the digestive tract. The sporozoites 

 escape and are actively motile ; in the majority of cases they pene- 

 trate into cells of the intestinal epithelium, but they may under- 



