Physiology 461 



with the species and with the type of food. Ingestion in Amoebida com- 

 monly involves extension of pseudopodia or formation of "food cups" to 

 engulf the food. A food cup may be quite deep, as in Amoeba vespertilio 

 (233) in which the food is taken in through a temporary "cytostome" 

 (Fig. 1. 15, C) similar to the permanent structures in more specialized 

 Protozoa. In shelled types ingestion is limited to an area of naked proto- 

 plasm. If there is a large enough opening, ingested particles may be 

 passed into the shell, as in Arcella. If the shell contains only small pores, 

 as in many Foraminiferida, the fusion of already extended pseudopodia 

 to enclose trapped food is the essential feature. A comparable process 

 often follows the adherence of food particles to axopodia in Heliozoida 

 and Radiolarida. Ingestion by pseudopodial activity has been reported in 

 various holozoic phytoflagellates and other simple flagellates, while fairly 

 large particles are ingested without marked pseudopodial activity in 

 Lophomonas and Tricbonympha (125). In certain flagellates and in typi- 

 cal ciliates, ingestion is limited to a cytostome. In simple cases, this ap- 

 pears to be merely a thin region of the cortex. More often, the cytostome 

 lies at the base of a groove or pit. The oral groove, or the peristomial 

 area, of ciliates is often equipped with strong cilia, membranelles, or 

 undulating membranes which drive particles into the cytostome (Chap- 

 ter I). 



Food vacuoles 



The wall of the vacuole in Amoeba and similar types is derived 

 from the surface layer of the body. In ciliates feeding on small particles, 

 the vacuole develops at the inner end of the cytopharynx as an enlarging 

 bulb which is eventually pinched off (103, 300). Formation of the vacuole 

 apparently is stimulated by the passage of solid particles through the 

 cytostome into the cytopharynx, since ciliates in a non-particulate me- 

 dium contain few, if any, food vacuoles (340). The ingestion of large 

 masses, as in the engulfment of Paramecium by Didinium nasutum, is a 

 less simple process. In Suctorea (Chapter VII), a food vacuole is formed at 

 the base of a tentacle which is usually attached to the prey and sucks its 

 protoplasm into the captor's body. Food vacuoles apparently may fuse or 

 divide. Fusion of small vacuoles (391) and the division of large vacuoles 

 into several smaller ones (387) have been noted in Amoeba proteus. Also, 

 the collection of small ingested particles into one mass, which becomes 

 surrounded by a common vacuolar membrane, has been described in 

 Ichthyophthirius multifiliis (372). 



A continuous "digestive tract," in which successively formed food vacu- 

 oles remain joined with one another by slender tubes, has been described 

 in Paramecium and VorticeUa (300). In addition, a coiled "canal," extend- 

 ing from cytostome to cytopyge, has been described in Colpidium (74). 

 cyclosis of food vacuoles being merely an optical illusion caused by move- 



