MOVEMENTS AND MODE OF LIFE. 229 



organisms. In the economy of Nature this fact is of pre-eminent 

 importance, for by the invisibly active life of these organisms a large 

 quantity of organic substance, which would otherwise be lost to the 

 animal world, is appropriated and utilised, and the more fully inasmuch 

 as in wealth of form the Infusoria are second to no other group, and 

 in number of individuals largely exceed all others. The latter state- 

 ment is specially true of the putrefactive Infusoria, for these are 

 found along with fungi and Schizomycetes, in a truly astounding 

 multitude, wherever there is any abundant putrefying substance in 

 the water. It is therefore not surprising to learn that numerous 

 forms have found a fit environment in the intestine, and especially 

 in the rectum of living animals. 



The Infusoria are most conspicuously separated from other Pro- 

 tozoa by the possession of cilia and an oral aperture, two structures 

 in this case naturally associated together since the body has not only 

 a more or less firm cuticle which prevents the formation of pseudo- 

 podia and amoeboid ingestion, but has also very generally a firmer 

 cortical layer, which encloses the less dense protoplasm and the 

 abundant remains of food. 



By means of their cilia, the Infusoria have the power of swimming 

 somewhat rapidly, though there are species which, after a longer or 

 shorter free life, become attached to some foreign object. Some of 

 these the so-called Acinetce even lose their cilia after becoming fixed, 

 so that their true nature is easily overlooked, especially since, instead 

 of a mouth, they have a large number of openings mounted on long 

 thin tubular stalks, which enable their bearers to catch and suck the 

 juices of other small animals, mostly Infusoria. 



In this world of invisible life, moreover, these predaceous forms 

 lead on to others, which are parasitic. Besides the free Acinetce, 

 there are also other Suctoria, which, instead of killing their prey 

 usually larger ciliated Infusoria only bore their way into them, and 

 live there as parasites. They grow at the expense of their host, and 

 by repeated division produce a numerous progeny, which, on reaching 

 maturity, break through the body-wall, swim about for some time by 

 means of their cilia, and finally return again to an Infusorian host. 1 



The Cilia exhibit such great and characteristic differences in 

 number and arrangement, that these have been used, not without good 

 results, for systematic purposes. Sometimes the whole surface of the 

 body, sometimes only certain parts, are provided with cilia, and their 



1 These parasitic forms have attained a somewhat unhappy prominence in the history 

 of our knowledge of the Infusoria, since they have given rise to the idea that the ciliated 

 Infusorians were viviparous, and produced ".4 cine to -like embryos" (Stein). The para- 

 sitic nature of these so-called embryos has, however, been experimentally established ; see 

 Butsohli, Zcitschr. f. wiss. Zoo/., Bd. xxv., p. 426, 1875. 



