154 ZOOLOGY 



in the pulsating and food vacuoles, in temporary projections 

 of protoplasm called pseudopodia, in more permanent vibratile 

 projections of the ectosarc known as cilia or flagella, in the 

 mouth found in many forms, in cell- wall and secreted skele- 

 ton, in delicate contractile fibres in the ectosarc, and in stalks 

 for attachment to objects (see Figs. 68 and 70). By means 

 of these differentiations 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 reproductive 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 ferments found in the proto- 

 plasm are doubtless responsible for the digestive changes and 

 act in much the same way as the special ferments 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 C0 2 and 

 receives O, occurs through the cell surface without special 

 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, and it seems probable 

 that the contractile vacuole has an excretory function. 



197. Movement. 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 breaking out of new processes in a new 



