Chap. 21 THE PROTOZOANS 449 



tions of the stomach, apparently killed by the digestive fluids. It has been 

 estimated that two per cent of a sheep's daily protein requirement may be 

 met by digested ciliates. They are present in the alimentary canals of other 

 animals apparently sharing the food supply, but without damage to their hosts. 

 Most slugs and many snails, planarians and sea urchins contain them. 



Paramecium and other ciliates have systems of contractile fibrils and neuro- 

 fibrils concerned with responses, coordination, and control of the cilia. The 

 trichocysts are minute poisonous rods arranged at right angles to the body 

 surface. They are discharged with great vigor particularly when a Paramecium 

 is attacked by its constant foe, Didinium. Most ciliates are peculiar in having 

 two kinds of nuclei, a large macronucleus important in general metabolism, and 

 one or more smaller nuclei that take part in conjugation. The latter is an 

 approach to the mating relation and the fusion of sex cells in multicellular 

 animals. In certain individuals, there may be a reorganization of nuclei called 

 endomixis that always occurs within single animals. This brings about an 

 upswing of physiological activity similar to that which follows conjugation. 



Paramecium 



Appearance. Paramecia are common animals in both ponds and labora- 

 tories. This "slipper animalcule" was among the "little things" which were first 

 seen in the seventeenth century, when the newly devised microscopes were being 

 tried out with great enthusiasm. A drawing of it was made by Joblot in 1718. 

 Paramecium came on the human stage then and has never left it. No one will 

 go far into the most recent studies of heredity, of variation and sex, of re- 

 sponses and behavior, and of populations, without finding paramecia a focus 

 of attention. 



General Structures. Its form and structure show the definite shape, differ- 

 entiation of front and rear ends, a definite position of mouth and gullet, path- 

 way of food vacuoles, anal opening, and contractile vacuoles (Fig. 21.17). All 

 of these localizations suggest a trend toward permanence in the location of 

 organs familiar to us in multicellular animals. The endoplasm is enclosed by 

 ectoplasm that secretes the flexible non-living pellicle and bears the cilia that 

 extend through the pellicle. 



Support and Movement. A Paramecium swims by the beating of its cilia. 

 Strong oblique backward strokes drive it forward and, in addition to the 

 forward movement, continually rotate the body on its long axis (Fig. 21.18). 

 The forward movement may stop or be reversed, yet the body will continue 

 to turn. The cilia in the oral groove beat more strongly than elsewhere. This 

 turns the anterior end away from the oral side as a boat turns toward the side 

 that is rowed more strongly. The boat eventually swings in circles and the 

 Paramecium would do the same if it were not that it rotates on its long 

 axis. 



