UNICELLULAR FORMS 67 



vacuoles by the amoeba. This region at the base of the gullet that 

 is thus a special food-receiving organ, is called the cytostome. Here 

 the gastric vacuoles form and their progression by the cyclosis of 

 the cytoplasm and the processes of digestion and assimilation are 

 very similar to those in the amoeba. The undigested materials and 

 foreign particles are finally ejected at a definite spot on the body 

 wall, known as the anal spot. The paramoecium represents a more 

 advanced mechanism than the amoeba in the localization of a defi- 

 nite region for engulfing food and another definite region for the 

 expulsion of indigestible stuflfs, and in other ways. 



Irritability in ParamcEcium. Paramoecium responds much 

 more actively to outside influences, which will henceforth be called 

 stimuli, than does amoeba. Obviously the surface of the animal is 

 the region which receives the stimulus, and recent work has indi- 

 cated that the impulse set up by such stimuli is transmitted by 

 specialized paths through the protoplasm, forming a sort of rudi- 

 mentary nerve net. Thus by the connections between these paths 

 the action of the entire animal is coordinated. This coordination 

 system is not easily visible in paramoecium and is described in 

 more detail in a heterotrychous ciliate, diplodinium (Fig. 18). In 

 diplodinium, just forward of the oral groove in the endoplasm, is 

 a small body, the neuromotorium, which is connected by strands 

 to the base of each cilium. If the strand from a single cilium is 

 cut, that one ceases to beat. This coordination mechanism sharply 

 illustrates two facts: First, the Protozoa are not simple rudimentary 

 systems intermediate between non-living and living, but are in fact 

 complex individuals, although they consist of but one cell; second, 

 that while in many respects the Infusoria contain functions and 

 structures that are similar to those of Amoeba and hence indicate a 

 relationship, the members of the class have developed new and 

 strikingly diflFerent specialized protoplasmic devices. 



Paramoecium responds negatively to gravity; one always finds 

 them in greatest numbers at or about the surface of a culture. By 



