22 THE INVERTEBRATA 



Another complication sometimes exists in the presence of a "reser- 

 voir" 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 cir- 

 cumstances or from stimuli which would interfere with some process, 

 such as reproduction or digestion, but in a few cases it facilitates 

 syngamy by keeping gametes together. Encystment is less common 

 among species which live in the relatively equable conditions of the 

 sea, than in freshwater and parasitic forms. Cysts which do not sub- 

 serve reproduction may be resistance cysts, against drought, alterations 

 of concentration, or the appearance of poisonous substances in the 

 surrounding medium, either in the habitat in which encystment takes 

 place or in those encountered in the course of distribution. They 

 may on the other hand be resting cysts, which enable the organism 

 to proceed undisturbed with digestion or photosynthesis or by quies- 

 cence to conserve its energy during starvation. Cysts which subserve 

 reproduction may be gamocysts, in which union of gametes takes place 

 (gregarines. Figs. 76-78), oocysts, containing a zygote, or sporocysts 

 containing several small individuals produced by fission. The oocyst 

 frequently becomes a sporocyst by fission of the zygote. Reproductive 

 cysts are often also resistance cysts. 



The nuclei of the Protozoa (Fig. 18) vary greatly in structure. 

 They usually contain masses of some size composed of various 

 materials. Such masses, when they consist only of the substance 

 known as plastin (which takes acid stains), are known as nucleoli', 

 if they also contain chromatin (basic-staining) they are artiphi- 

 nucleoli. A single central mass is an endosome : it may be a temporary 



