Chap. 21 THE PROTOZOANS 451 



Nutrition. Paramecia live surrounded by swarms of bacteria. These are 

 swept into the oral groove and down the gullet by cilia that move so rapidly 

 that a microscopic stream of water seems to run through the protoplasm. For- 

 tunately for the Paramecium, food is not always pouring into it. Granules 

 containing enzymes form about the food vacuole as soon as the food creates it. 

 In the first stage of digestion, the content of the vacuole is acid and the micro- 

 organisms in it are killed. In the second stage, the granules swell and dissolve; 

 the content of the vacuole becomes alkaline; part of the food is dissolved and 

 absorbed in the protoplasm and the indigestible residue is squeezed along in a 

 regular circuit toward the anal pore. The vacuole disappears when its func- 

 tion ceases, but a successor appears in the same location as soon as more food 

 arrives. 



Respiration, Water Content, Excretion. Oxygen is secured from the sur- 

 rounding water and carbon dioxide is given off into it. The fresh water that 

 surrounds the paramecia has a lower osmotic pressure than protoplasm and 

 therefore is continually diffused into them. This creates an income of oxygen, 

 but necessitates the outlet supplied by the contractile vacuoles or else the ani- 

 mal would burst. The vacuoles eliminate metabolic waste though they also 

 have the very important function of maintaining water balance just as the 

 kidneys of the frog do. 



Mechanisms of Sensory-motor Functions. Complicated neuromotor systems 

 in paramecia and other ciliates have been demonstrated by special preparation 

 and high magnification. Beneath the pellicle each cilium originates in a 

 rounded base and these are connected with one another (Fig. 16.1). These 

 fibrils are associated with a latticelike network of fibrils surrounding the mouth 

 and gullet. It is probable that the fibrils are conductors and serve to coordinate 

 the cilia while ingesting food. Some of the fibrils are joined in a minute body 

 (motorium) located in the lattice. Destruction of this in the ciUate Euplotes 

 upsets the coordination of the animal. 



Behavior. The behavior of a Paramecium consists of only a small number of 

 definite movements. By one, or another, or combinations of these few move- 

 ments, it responds to all the stimuli that act upon it. The basic pattern is that 

 of an avoiding reaction (Fig. 21.18). By means of it, the Paramecium rejects 

 one stimulus and accepts another. An avoiding reaction occurs immediately 

 after a stimulus such as contact with an object. The animal slows up, stops or 

 banks off, then moves in a different direction. In doing so it enters a new 

 place, comes upon different chemical, mechanical, and electrical stimuli, light 

 or temperature. The repetition of the avoiding reaction by trial and error 

 results in the rejection of some stimuli and acceptance of others. However 

 significant the process may be, it ends in a generally consistent choice of 

 favorable food and surroundings. 



Reproduction — Conjugation and Sex. Paramecia reproduce asexually by 



