2 PRACTICAL PABASITOLOGY 



convey infection. They take the form of cysts or shells, the latter 

 occurring in the cnidospores of the Myxosporides and their congeners. 

 During the process of encysting, the formation of the true hard 

 cyst is frequently preceded by the secretion of a casing of mucous 

 or colloid matter. Propagation within the cyst may follow, but it 

 is by no means the rule (compare, for instance, Balantidium coli). 



2. Organelles of Movement. 



(i.) Pseudopodia. Variously shaped processes thrown out by proto- 

 plasm, which are capable of slowly changing their form. They are 

 projected and withdrawn, giving rise to the characteristic " amoeboid 

 movement," a kind of flowing or rolling progression. 



(ii.) Flagella (Whips). Long, fine, plasmic processes occurring 

 in small numbers only. Like the tail of the spermatozoon, these 

 flagella are furnished with an elastic axis-filament which serves as 

 a support, and by means of which they are able to perform rapid 

 waving movements, generally in a spiral direction. 



(iii.) Cilia (Filaments). Hair-like projections from the plasm, 

 shorter than the flagella, and generally present in larger numbers. 

 They are arranged in regular rows and move successively in a definite 

 direction in waves. They are characteristic of the Ciliata. The 

 amalgamation of several cilia in a cross-row gives rise to ciliated 

 lamellae or membranelles. 



(iv.) Myonemes. Contractile fibrillae not unlike the muscle fibrillse 

 of the higher animals. They are arranged either lengthwise or across 

 the body, and occur in the Flagellates, Gregarines, and Ciliates. 



3. Organelles of Metabolism. 



(i.) Special organs for the ingestion of food are not present in those 

 Protozoa which live in a medium containing nutrient matter, and 

 which obtain food solely by endosmosis. Such are the Trypanosomes, 

 Coccidia, and Gregarines. The greater number of the Protozoa, 

 however, ingest solid food, and in these cases the pseudopodia> flagella, 

 and cilia are employed to encircle the food-substance and whirl it 

 within reach of the organism. Nearly all the Ciliates and a large 

 number of the Flagellates possess, for purposes of ingestion, a special 

 constant oral part (cytostome). In its simplest form it is an opening 

 in the ectosarc, frequently continuing as a canal (cytopharynx) which 

 penetrates a greater or less distance into the interior of the organism. 



(ii.) The food-vacuole plays an important part in the digestion of 

 solid material. The bolus of food is surrounded by a bubble-shaped 

 liquid agglomeration, into which acids and ferments from the surround- 



