124 PHYSIOLOGY OF PROTOZOA 



of oxygen, tliey are able to elaborate relatively simple chemical compounds 

 into the protein materials necessary for their existence. Such forms may 

 be cultivated in solutions of various salts, and, like plants, commonly 

 elaborate starch or other amyloid substances as one of the products of 

 assimilation, and not infrequently build for themselves capsules composed 

 of cellulose. Between these and the completely holozoic forms, which 

 require, in addition to oxygen, ready-formed proteid materials, either solid 

 or in solution, there exists a group of organisms known as saprophytes. 

 These do not possess chromatophores, but are able to live in fluids con- 

 taining oxygen and complex organic compounds, which nevertheless are 

 simpler than the proteid materials required by the truly holozoic types. 



Amongst the holozoic Protozoa two methods of obtaining proteid 

 material occur. In the one the organism ingests solid proteid material, 

 mostly in the form of other living organisms, such as bacteria and other 

 Protozoa, or, as in the case of parasitic forms like Entamoeba histolytica, 

 the cells of the host's body (Fig. 95). This solid matter is ingested either 

 through a definite mouth opening or cytostome, or, when such is not present, 

 through any part of the body surface by means of pseudopodia which 

 surround it, or by a movement of the cytoplasm over the object, which 

 appears to sink into its substance. In the other method, the proteid which 

 is in solution is absorbed in liquid form. There is no mouth opening, the 

 material merely passing into the body by osmosis. The latter method is 

 characteristic of many parasitic Protozoa, such as trypanosomes, malarial 

 parasites, coccidia, and gregarines. Other parasitic forms, such as the 

 amoebse, Trichomonas and Balantidium, ingest solid matter either by means 

 of pseudopodia or definite cytostomes (Figs. 26 and 14). 



In the case of Suctoria, which obtain their food by means of sucking 

 tentacles, these are applied to solid objects, from which the proteid is 

 extracted in a liquid form, probably as a result of ferments acting at the 

 points of contact (Fig. 15). 



As regards the proteid material ingested, two conditions result. When 

 it is absorbed in solid form it is enclosed in food vacuoles, in which the par- 

 ticles are found in various stages of digestion (Fig. 70). When the proteid is 

 absorbed in a state of solution no such food vacuoles are formed. From 

 a study of the changes which occur during digestion in food vacuoles it 

 has been found that when a living organism is ingested it is at first killed 

 and then gradually digested, leaving finally a residuum of fsecal matter 

 which is got rid of by the vacuole approaching the surface of the body 

 and discharging its contents. In the ciliates there frequently exists 

 a definite anal opening or cytopyge, usually at the posterior end of the 

 organism, through which the residue is discharged (Fig. 512). The process 

 of digestion is evidently the result of ferments which are secreted by the 



