PROTOZOA 41 



Digestion is always intracellular and in the endoplasm. Gastric 

 vacuoles contain fluids taken in with the food. Water is modified 

 by osjnosis to become a digestive fluid weak in HCl. At the begin- 

 ning of the digestive process, when food matters are first ingested, 

 there is a remarkable secretion of acid. This secretion ceases with 

 the beginning of the breakdown of food materials. Usually there 

 is a distinct alkaline reaction in the food vacuoles. Proteins are the 

 main sources of nutrition; starch is affected but slightly, but it is 

 claimed that the Rhizopoda dissolve starch grains and even cellulose. 



Few protozoa are known to digest fats, but oil droplets and fat 

 bodies are found in practically all of the protozoan groups. Dawson 

 and Belkin have shown "^ that Ameba dubia and A. proteus are able 

 to digest several of the oils, including peanut oil and olive oil. 



Some of the injurious bacteria may serve as excellent food for an 

 ameba. Rndameba coli ingests intestinal bacteria and Endameba 

 histolytica engulfs red blood corpuscles. Kofoid finds that a flagel- 

 late {Pentatrichoinonas) ingests red blood corpuscles. The majority 

 of protozoan parasites absorb food by osmosis. Some Ophryo- 

 scolecidae eat chlorophyll grains. 



Excretion is by osmosis and also in the majority of Protozoa by 

 one or more contractile vacuoles. Contractile vacuoles are formed 

 in the endoplasm by accumulation of liquid. The cause of their 

 contraction is unknown. Probably the contractile vacuole is also 

 a respiratory organ. In one-half hour a protozoan throws out a 

 quantity of water equal to the size of its body. 



Irritability. — The Protozoa are sensitive to nearly all stimuli. 

 There is not satisfactory evidence of color vision, although many 

 forms react to changes in light intensity. With regard to reaction 

 to the pull of gravity it is argued that this is not a true positive 

 geotropism (p. 25) in Paramecium. The Protozoa react to tem- 

 perature changes, chemical and electrical stimuli. Ameba is a 

 complete neuromuscular organism capable of responding to stimuli 

 without correlation. Reproduction in Ameba coiiijiAlA of. ( i ) sim p le 

 TJi^ J oi c m o r fiooion^ a). -buddrng^-Xg^-^p ere formatioti ^ 



Reproduction and Regeneration. Reproduction in the Sarco- 

 dina. — (i) Fusion of two amebae is said to occur, resulting in a new 

 organism. The significance and results of such a procedure are 

 unknown. Such a process has been compared to the fusion of the 



7 Proc. Soc. Exp. Biol, and Med., 1928, vol. 25, pp. 790-793; and Biol. Bull., 1929, 

 vol. 56, p. 80. 



