26 THE PROTOZOA THE DAWN OF LIFE 



the swarming bacteria present in such media. While the majority 

 are inhabitants of fresh water, a considerable number are marine ; 

 others are entozoic in their habits, living in the intestines, blad- 

 der, and blood of higher animals. Although these are sometimes 

 spoken of as ''Parasitic Infusoria," it is doubtful if the term 

 is accurately applied, for, so far from gaining their living at the 

 expense of their host, they do not appear to feed upon anything 

 except such substances as would otherwise be ejected from the 

 body ; therefore it would seem highly probable that their presence 

 is really beneficial to their host. 



They are grouped into two sub-classes : (i) the Ciliata, the 

 members of which are either free or fixed forms, with cilia disposed 

 in tracts or bands upon the body for purposes of locomotion and 

 the capture of food; and (2) the Acinetaria, which are fixed 

 sedentary forms possessing suckers or tentacles, and bearing cilia 

 only during their active youth. Reproduction in the simplest forms 

 takes place by transverse fission, while in the higher forms it is 

 accomplished by longitudinal fission and by encystment and 

 spore-formation. Encystment may also be purely a resting 

 stage, from which only one individual escapes. Such resting 

 cysts are capable of resisting the effects of dry air for a consider- 

 able time, and in this way periods of long drought are tided over, 

 the cyst remaining dormant in the dried mud at the bottom of the 

 pond or pool, until the rains once more restore the supply of water. 

 In the same way such resting cysts may be carried away by the 

 wind from their dried-up homes and transported for considerable 

 distances, always with the possibility of falling into other waters. 



We can very readily become familiar with the appearance of 

 some of the Ciliata, by examining under the microscope a drop 

 of water taken from any stagnant pool. This will be almost cer- 

 tain to contain a number of the so-called " Slipper-animalcules" 

 (Paramcecium) , moving about in the most active manner. Their 

 bodies are long, narrowish, bluntly-pointed at one end, and more 

 sharply at the other, so that the general outline of the body some- 

 what resembles the sole of a slipper. They are flat, and with a 

 little attention we shall be able to make out on the ventral side 

 of the body a large oblique depression, the buccal groove, lead- 

 ing into a short gullet which in turn ends in the soft internal 

 protoplasm. The body is covered by small, delicate lashers, or 



