676 



NATURE OF REPRODUCTION. 



dispersed throughout the body. They thus reach the muscular tissue, 

 where they become encysted, and remain quiescent until again intro- 

 duced into the intestine of another animal or of man. In this way the 

 existence of sexless and encysted parasites is seen to be entirely analo- 

 gous to that of the caterpillar or the maggot. They are sexless, 

 because they are still in the embryonic or incomplete stage of develop- 

 ment. But they have been produced by the regular mode of generation 

 from parents ; and they will, at a subsequent period, themselves produce 

 young by the same process. 



II. Infusoria. These are microscopic organisms, first discovered by 

 Leeuwenhoek, in 1675, in rain-water which had been kept in standing 

 vases. On account of their active movement and minute size he called 

 them ''animalcules ;" but as they were soon afterward discovered to 



Fig. 217. 



INFUSORIA, of various kinds. 1. Urostyla grandis, from decaying sedge-grass 2. 

 Paramecium aurelia, from vegetable infusions. 3. Chlamydodon mnemosyne, Baltic Sea 

 water. 4. Kerona polyporum, on the fresh-water polype. 6. Oxytricha caudata, open 

 stagnant waters. 6. Ervilia fluviatilis, clear brook water. 7. Heteromita ovata, on aquatic 

 river-plants. Magnified 325 diameters. (Ehrenberg and Stein.) 



make their appearance in great numbers and with remarkable rapidity 

 in watery infusions of organic matter exposed to the air, they received 

 the general name of "infusoria." They present themselves in great 

 variety, and under rapidly changing forms; so much so that Ehrenberg 



