INFUSORIA 481 



finely powdered carmine or indigo into the water, we shall be able to 

 observe how such bodies are conveyed into the interior of the animal- 

 cule. We shall see that some of the pigment granules are swept by the 

 cilia of the "vestibule" (G.) to the bottom of the latter, and thence 

 through a small opening, the mouth (M.), into the gullet (S.). The 

 particles are next seen to surround themselves with a drop of water and 

 to be absorbed by the soft protoplasm. The food particles, surrounded 

 by fluid in the so-called food vacuoles (Nb.), circulate through the cell 

 and are digested, whilst the indigestible materials are expelled at another 

 spot in the denser superficial protoplasmic layer, through the so-called 

 anus (A.). 



In the interior of the cell are found two other larger vesicles. These 

 are usually connected with a number of radiating canals, giving them 

 a starlike appearance, and are filled with fluid entering by the latter, 

 and discharged periodically to the exterior. As these vesicles are also 

 constantly dilating and contracting, they have been termed contractile 

 vacuoles (p.B.). They probably discharge the same functions as the 

 kidneys of higher animals. 



C. Habitat. 



On account of their delicate organization, Infusoria, like all other 

 Protozoa, can only exist in damp surroundings. (Compare with vine- 

 yard snail and fresh-water polyp.) Hence, with the exception of a few, 

 which live parasitically in the bodies of other animals, all are inhabitants 

 of the water. When food becomes scarce, and if the ponds and streams 

 freeze or dry up, the Protozoa inhabiting the latter surround themselves 

 with a thick capsule-like envelope. (Compare with fresh-water polyp and 

 sponge.) They are carried away with the dust, to be again moistened 

 by water and reawakened to a new life. This also explains their appear- 

 ance in the infusions above referred to. (Why do not marine Protozoa 

 require such a protective envelope ?) It further explains the wide 



D. Distribution of the Fresh-Water Protozoa. 



These animals may sometimes be carried over many hundreds of 

 miles by storms ; accordingly, they are cosmopolitan in their occurrence. 



E. Reproduction. 



The rapidity with which Infusoria multiply in infusions may be easily 

 recognised. In a few weeks the descendants of a single individual will 

 amount to millions. Eeproduction takes place by fission. A constric- 

 tion appears over the middle of the body, which becomes gradually 

 deeper, until the animal finally divides into two. Previously, however, 



