CYTOPLASM 27 



substance between the globules has the appearance of a network of fibres. 

 Embedded in these apparent fibres or septa are granules of various kinds 

 and sizes. The cytoplasm commonly contains vacuoles, which are 

 spherical spaces containing a material which is more fluid than the con- 

 stituents of the cytoplasm itself. Very frequently within the vacuoles 

 are food particles which the organism has ingested. In such cases the 

 vacuoles are known as food vacuoles or digestive vacuoles, and into them 

 are secreted acid ferments capable of transforming the food into substances 

 suitable for assimilation by the cytoplasm. The products of digestion are 

 gradually absorbed into the cytoplasm, and any residue is got rid of by the 

 vacuole approaching the surface of the body and discharging its contents 

 into the medium in which the organism is living. The vacuole is then 

 no longer visible. In the majority of free-living Protozoa there are one 

 or more vacuoles, which are known as contractile vacuoles or pulsating 

 vacuoles. Such a vacuole is near the surface of the body, and when fully 

 formed contains a clear fluid. By a sudden contraction the contents of 

 the vacuole are discharged through the surface of the body, and the vacuole 

 disappears. Very soon, however, a minute vacuole reappears at the same 

 spot. It gradually increases in size owing to the flow of liquid into it, 

 sometimes along definite channels. When it has attained its full size, 

 expulsion of the contents again takes place. These vacuoles appear to be of 

 an excretory nature, and the intervals between the contractions vary with 

 the temperature and other conditions. For some reason not clearly under- 

 stood, contractile vacuoles are frequently absent in parasitic Protozoa. 



Within the cytoplasm of many Protozoa there occur various structures 

 which are to be regarded as secretions of a skeletal nature. In the 

 Heliozoa, for instance, radially arranged rod-like supports for the pseudo- 

 podia are formed (Fig. 75), while in many of the Radiolaria complicated 

 fenestrated shells of a spherical or other shape are secreted in the cyto- 

 plasm (Fig. 78). These internal structures are not to be regarded as part 

 of the cytoplasm itself, but bear the same relation to it as the external 

 shells and coverings, which are sometimes formed around the organism 

 for protective purposes of a permanent or temporary nature. 



A very noticeable feature of the cytoplasm of Protozoa is its differ- 

 entiation into an ectoplasm and an endoplasm. The former is of tougher 

 consistency and more hyaline than the endoplasm, and forms a superficial 

 layer of varying thickness enclosing the more liquid and granular endo- 

 plasm. The endoplasm, even when the organism is at rest, appears to be 

 constantly streaming in various directions. The different vacuoles and 

 bodies, and even the nucleus itself, are constantly changing position as a 

 result of the currents in the endoplasm. It is in the endoplasm that the 

 various vacuoles and internal skeletal structures occur, while the ectoplasm 



