158 .PROTOZOA. 



ranged; in some entirely covering the body, irregularly 

 or in regular rows, in others being situated at definite 

 parts only. By the action of the cilia they are en- 

 abled to swim freely in the water, also to obtain their 

 food, which consists of minute Algse or fragments of 

 animal matter. In many of them there is a special 

 row or set of cilia, which, by the currents it produces, 

 urges the particles of food suspended in the water 

 towards the mouth. The cilia also act as respiratory 

 organs, by changing the water with which their bodies 

 are in contact. In some of the species there are 

 stout bristles or setae, by which they are enabled to 

 crawl upon water-plants. 



On carefully examining the bodies of the Infusoria, 

 rounded granular spots will be seen, frequently con- 

 taining minute Algae, &c. (fig. 28 b). These spots are 

 the digestive cavities, and have been called gastric 

 sac culi ; but whether they are definite sacs or mere 

 excavations, formed by the particles of food having 

 been forced into the softer internal substance of the 

 body, has not been positively determined. The sac- 

 culi may be filled artificially by mixing very fine 

 indigo, or carmine, on a slide with the water in which 

 the Infusoria are contained. A definite food-tube or 

 alimentary canal has been detected in a few of the In- 

 fusoria ; but it cannot be shown to exist in the ma- 

 jority of them. 



A mouth exists in most of them, and is sometimes 

 indicated by a row or set of cilia somewhat larger 

 than those existing upon other parts of the body, 

 and leading to or placed near it. The particles of 

 food which have entered the body are often seen to 

 pass round it, as if circulating, descending on one 

 side and ascending on the other. 



In addition to the gastric sacculi, certain clear 

 transparent spots may also be seen within the body, 

 appearing light or dark according to the adjustment 

 of the focus. If these are attentively watched, they 



