l6 THE INVERTEBRATA 



Protein reserves are common in holozoic species. Nucleic acid 

 ("volutin") is widespread, probably as a reserve for the nucleus. Oil 

 reserves also occur in practically all groups — a rather remarkable fact, 

 in view of the apparent inability of the Protozoa to digest fats. In phos- 

 phorescent forms (dinoflagellates, radiolarians), the oxidation of fats is 

 the source of the emission of light. 



The excreta of the Protozoa are doubtless often shed from the 

 general surface of the body. Sometimes they are recognizable in the 

 cytoplasm as granules or crystals of urates or phosphates, which may 

 be expelled with the faeces but appear in other cases to be redissolved. 

 Their material is then perhaps passed into the contractile vacuoles. 

 The latter are spaces filled with water which periodically undergo 

 collapse with expulsion of their contents to the exterior. In the 

 simplest cases, as in the familiar laboratory types Amoeba^ Chlamy- 

 domonas and Actinosphaerium (Figs. 23, 33 cyflc), the contractile 

 vacuoles are solitary, spherical cavities, one or more in number ac- 

 cording to the organism; over these in pelliculate genera there is 

 a soft patch in the pellicle through which discharge takes place. 

 Sometimes, as in Euglena and Paramecium^ they are accompanied 

 by accessory vacuoles by whose contents they are reconstituted 

 (Figs. 39 D, 16, 17) and which in some ciHates (Fig. 88 B, C) 

 extend as long canals through the cytoplasm. Another complication 

 sometimes exists in the presence of a *' reservoir" through which 

 the vacuole communicates with the exterior, either directly, as in 

 Peranema (Fig. 39 E), or by way of the gullet, as in Euglena and 

 Vorticella (Figs. 39 D, 2). At least some contractile vacuoles appear 

 to have a lining membrane, and it is probable that they are not 

 entirely abolished at systole. The fact that these organs are commoner 

 in freshwater protozoa than in marine or parasitic species suggests 

 that their primary function may be the discharge of water, which 

 must enter the body when the surrounding medium has a lower 

 osmotic pressure than the protoplasm. Possibly, however, they serve 

 also as organs of excretion. 



Respiration no doubt takes place upon the whole surface of the 

 body. It has been supposed that the contractile vacuoles subserve 

 this function, but, while they no doubt remove carbon dioxide in 

 solution, it is difficult to see how their activity could cause the 

 entry of oxygen. 



Many protozoa either regularly or occasionally pass a period of 

 their lives in a cyst. The cysts may be coats of jelly or stronger cover- 

 ings, usually organic, but sometimes , as in the Chrysomonadina, chiefly 

 composed of inorganic material. The function of the cyst is nearly 

 always to shield the organism, either from unfavourable circumstances 

 or from stimuli which would interfere with some process, such as re- 



