PROTOZOA 155 



ton, in delicate intracellular fibres, and in stalks for attachment 

 to objects (see Figs. 68 and 70). By means of these differenti- 

 ations all the functions necessary to life are performed. There 

 are many colonial Protozoa. In such (as Volvox) there may be 

 some division of labor among the cells, — as between reproduc- 

 tive cells and body cells (Figs. 72, 73). 



196. Nutrition. — The parasites absorb food, already di- 

 gested and fitted for absorption, directly from their hosts. 

 Most of the free forms take solid particles directly into the 

 endosarc through permanent or temporary openings in the 

 ectosarc. In some shelled forms, in which there is no mouth, 

 the food is digested outside the body proper (Fig. 74) by the 

 pseudopodia. These envelop the food and gradually transfer 

 it to the main . body of protoplasm through openings in the 

 shell. In the other instances the digestion takes place in the 

 body of the protoplasm. The enzymes found in the proto- 

 plasm are doubtless responsible for the digestive changes and 

 act in much the same way as the special enzymes secreted 

 from the cells of the digestive glands in the higher animals. 

 Circulation is effected by the general protoplasmic motion. 

 Respiration, whereby the protoplasm gets rid of CO2 and 

 receives O, occurs largely through the cell surface without spec- 

 ial structures. All projections of the cell-body assist in this 

 exchange by increasing the area of the surface. Excretion may 

 take place from the surface of the cell. It seems probable that 

 the contractile vacuoles have both a respiratory and an excre- 

 tory function. 



197. Moveiiient. — The majority of Protozoa move freely 

 in their medium. In Amoeba the motion is of a gliding character 

 and is effected, by putting forth processes into which the proto- 

 plasm streams. The process or pseudopodium thus enlarges at 

 the expense of the body of the cell and progress is had in the 

 direction of the growing pseudopodium. The direction of 

 motion is changed by the formation of new processes in a new 

 direction. In those Protozoa which have a cell-wall special 

 devices enable the animals to move. Most of the free-swim- 

 ming forms possess cilia or flagella, which act as oars on the 



