24 



PROTOZOA 



Shell, composed of sond 



Fig. 35. 



Pseud o podia 



Difflugia. (After Leidy.) 



the moveme'tit of ameba is dependent on changes of the protoplasm 

 from sol to gel and back to a sol state again. (See page 4.) It 

 has also been suggested by Mast that form in ameba is dependent on 



water content. 



Digestion. — Food is ingested 

 directly through the ectoplasm^ 

 and having entered the endoplasm, 

 minute quantities of HCl secreted 

 around the food mass form a gas- 

 tric vacuole. Carbohydrates are 

 not acted upon to any extent, 

 digestion being chiefly limited to 

 protein and fat. Solid wastes are 

 extruded at any point, the ameba 

 moving away and allowing the 

 weighty excrement to pass 

 through the ectoplasm. Ameba 

 can nip a paramecium in two, engulfing one-half, and leaving the 

 other half outside. 



Circulation. — There is no definite distribution of food materials 

 but the movements of the animal thoroughly distribute the food 

 granules. 



Respiration. — Oxygen is taken in through the whole surface of 

 the body, and CO2 is extruded. The contractile vacuole is also im- 

 portant in the interchange of gases. 



Excretion. — Besides the ejection of solid feces by merely leaving 

 them behind as the animal moves forward, the contractile vacuole 

 definitely functions in the excretion of liquid and gaseous wastes. 



Reproduction. — The ameba is able to divide its nuclear and cyto- 

 plasmic constituents equally, the process being called binaj-y fission 

 or division. Under adverse conditions or sometimes solely for 

 reproductive purposes the ameba encysts. It then forms daughter 

 cells which mature in about three weeks. The number produced 

 varies in different species. 



Nervous System and Reactions. — Without nerve cells or fibers, 

 the ameba is still a complete neuromuscular organism. It reacts 

 to all sorts of stimuli, including light, heat, touch, gravity, currents 

 of water, chemicals, and electricity. 



Tropisms. — The term "tropism" has long been used to indicate 

 the reaction of an animal to some sort of stimulus. A few of the 



