PROTOZOA. 147 



190. Organization. We cannot say that Protozoa have 

 organs in the sense in which we have defined that term hitherto, 

 yet they are certainly organized. The organization shows 

 itself in the nucleus, in the distinction of ectosarc and endosarc, 

 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. 66 and 68). 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. 

 70, 



191. 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. 72) by 

 the pseudopodia. These envelop the food and gradually trans- 

 fer it to the main body of protoplasm. In the other instances 

 the digestion takes place in the body of the protoplasm. The 

 ferments found in the protoplasm 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 CO 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. 



