228 CLASSIFICATION OF ANIMALS 



a spore case but this may be wanting, as, e. g., when there is 

 an alternation of hosts. The number of spores in a case is 

 usually numerous but sometimes they are few or only one. All 

 Sporozoa are parasitic. They are very commonly cell parasites, 

 either in the young stages or permanently. They absorb fluid 

 food by osmose. The Sporozoa are widely distributed and 

 infect all groups of the higher animals, especially worms, arthro- 

 pods, tunica tes, molluscs and vertebrates. Most species are 

 limited to one or a few host species. The passage from one 

 host to another similar (not alternate) host is effected in the 

 spore stage. There is frequently another method of repro- 

 duction which takes place wholly within a single host. This 

 may alternate with the spore-producing generation thus giving 

 rise to a regular alternation of generations. Many Sporozoa 

 are comparatively harmless parasites but among them are also 

 some of the most dangerous. Several examples are described 

 under the head of parasitism. 



514. Class IV. Ciliata. The Ciliata all possess as loco- 

 motor organs, numerous minute vibratile processes called cilia. 

 They are widely distributed. Very few are parasitic, some are 

 saprozoic, but most are holozoic. A few even are carnivorous. 

 They are generally free-swimming but some attach themselves 

 temporarily to other objects and some are permanently fixed 

 by a stalk. The cilia serve for driving currents of water 

 containing food particles to the mouth and in the fixed forms 

 this is the chief function of the cilia. The form of the body is 

 definite but the animal often has the power of considerably 

 changing the form by contraction. The cell is covered by a 

 dense protoplasmic layer called a pellicula. There are usually 

 two nuclei, a large macronucleus and a small micronucleus. 

 Multiplication is effected by division or by budding. Some- 

 times this is accompanied by the formation of a protecting mem- 

 brane or cyst. Cysts are also formed when conditions are un- 

 favorable and represent a resistant condition. 



