20 THE INVERTEBRATA 



not been shown to digest fat, but can usually dissolve starch, and some- 

 times cellulose. The latter faculty becomes of great importance when 

 they are symbionts in the alimentary canal of animals whose food 

 consists of plant tissues (pp. 68, m). In a few cases {Balaiitidium, 

 some Amoebae) contractions of the protoplasm divide large morsels 

 into fragments. Often, but not, for instance, in foraminifera, the 

 food vacuoles circulate in the cytoplasm ; sometimes they do this along 

 a regular track. Their circulation is often due to streaming of the 

 endoplasm, but sometimes (ciliates, etc.) it is brought about by 

 peristalsis of the cytoplasm. Defaecation of the indigestible remains 

 of food takes place at any part of the surface when there is no pellicle, 

 but in pelliculate forms at a fixed spot. Sometimes there is a per- 

 manent rectal passage lined by ectoplasm (Fig. 88 B, an). Saprophytic 

 forms range from some which can subsist on mixtures of substances 

 as simple as aminoacids and acetates (or even, as Polytoma can, upon 

 ammonium acetate alone), to parasites whose food probably differs 

 chemically but little from that of holozoic forms. 



Reserve materials^ for use at times when nutriment is not being 

 taken or when some process, such as rapid multiplication, is making 

 heavy demands upon the resources of the organism, are stored by 

 most protozoa, and are often conspicuous, as granules, vacuoles, 

 crystals, etc., in the cytoplasm. The carbohydrates starch, para- 

 mylum (in the Euglenoidina), and leucosin (in the Chrysomonadina) 

 are formed by holophytic organisms and by some colourless forms 

 related to these (as by Polytoma, Fig. 24, Peranema, etc.). Glycogen 

 is stored by parasitic and other anaerobic forms, in which it is per- 

 haps split with evolution of energy, as in various anaerobic metazoa. 

 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 

 phosphorescent forms (dinoflagellates, radiolarians) the oxidation of 

 fats is the source of the emission of light. 



The nitrogenous excreta of the Protozoa appear to be most often 

 ammonia compounds, less often urea, and occasionally urates. Ex- 

 cretion doubtless frequently takes place from the general surface of 

 the body. Sometimes there are recognizable in the cytoplasm 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, Chlamydomonas and Actino- 

 sphaerium (Figs. 23, 33c.z;«c.), the contractile vacuoles are solitary, 



