Glass 2. Infusoria. 



93; 



(and aftei-) the union, a series of changes occurs in the nucleus,, 

 with a partial exchange of nuclear material.* 



The details of the process ai-e as follows : the macronvicleus loecomes m-egular, 

 breaks into several fi-agments, and is completely absorbed into the protoplasm. 

 At the same time, the mioronucleus divides into several pieces (Fig. 51, 2), 

 which with one exception, disappear (Fig. 51, 3), the remaining portion divides- 

 into two parts (Fig. 51 4), one of which migrates into the other animal (Fig. 51, 

 ^; 6), 80 that a miitual exchange of micronuclei occurs. The organisms then 



Fig. 51. Two Infusoria in various stages of conjugation. Diagrammatic, n 

 mioronucleus, JTmacronuoleua. The mioronucleus of the one individual is left clear, that- 

 of the other, dark. See the text. — Orig. 



sepai-ate. The two micronuclei, one of which is derived fi-om each of the 

 conjugates, fuse, and by the division of the nucleus thus fonned, a new macro- 

 nucleus, and a new mici-onucleus arise. 



The food of the Infusoria consists principally of unicellular plants 

 and animals — Bacteria, Diatoms, Flagellata, other Infusoria, etc. 

 In many the food is brought to the mouth by ciliary currents, as 

 described above ; in others, the edges of the mouth (which are often 

 supported by tiny rod-like structures) effect the prehension of prey. 

 The Infusoria are for the most part extremely active little organisms, 

 swi mmi ng by means of their covering of cilia, or by the contractions 

 of the body : or gliding about over foreign bodies. Not a few are 

 temporarily or permanently fixed, and several of these form colonies 

 by incomplete fission or budding. Both as regards individuals 

 and species, they are abundantly represented in freshwater- 

 they congregate principally about decaying plants or animals. A 

 proportionately smaller number occur in the sea; one division of 

 shell-bearing Infusoria is pelagic, its members swimming in the open 

 sea together with Eadiolaria and pelagic Ehizopods. Some live as 

 parasites on the skin of fish or other aquatic animals ; and it is by 

 no means rare to find them in the alimentary canal of different 

 Vertebrata. When the ponds in which they live become dried up,, 

 great numbers in the encysted condition may be carried away by 



* Conjugation usually occurs after the animals have been for some time rapidly 

 increasing by division, and have exhausted the food supply. When there is a constant 

 and abimdant supply of nutrition, conjugation does not take place, and fission con- 

 tinues ; the consequence of this, however, is the degeneration and finally the death of 

 the individual. 



