178 REPRESENTATIVE SINGLE-CELLED ANIMALS 



and in the buccal groove, where they are shghtly longer. Within 

 the gullet, the cilia are fused together in a plate to form the undu- 

 lating membrane, which aids in the passage of food to the interior 

 of the cell. Since the egestion of fsecal material occurs at but one 

 point on the surface of the cell, there is a cytopyge, or cell anus, 

 as well as a cell mouth. In this regard the paramoecium is more 

 speciahzed than the amoeba, in which ingestion and egestion occur 

 at any place on the surface. 



The outermost layer of the cell is a thin cuticle, or peUicle, which 

 is like an elastic membrane. This cuticle is striated to correspond 

 to the distribution of the cilia along definite hnes (Fig. 95 F and G). 

 Beneath the cuticle is a thicker, non-granular layer, the ectoplasm, 

 from which the ciHa arise (Fig. 95 E and F), and which contains the 

 numerous trichocysts. The latter are apparently defensive struc- 

 tures since they are discharged as threads upon appropriate stimu- 

 lation (Fig. 95 I). It is the ectoplasm that gives to the paramoe- 

 cium its permanent shape, for the internal portion, or endoplasm, of 

 the cell is semi-fluid. The paramoecium may be pressed out of 

 shape mechanically, but resumes its former outUne because of the 

 elastic ectoplasm. The endoplasm contains a macronucleus, which is 

 related to the vegetative processes of the cell; and a micronucleus, 

 which is the part of the nuclear apparatus that is most intimately 

 related to heredity and reproduction. The endoplasm also con- 

 tains the two contractile vacuoles, with their radiating canals, and 

 the food vacuoles. Larger masses of various sorts may also be 

 found, in addition to the very fine inclusions of the cytoplasm. 



Movements, Locomotion, and Behavior. — The cytosome in 

 paramoecium has a constant shape, unless it is temporarily dis- 

 torted by external pressure. Locomotion and other movements 

 are effected by the ciha, which beat in such a fashion that the 

 animal pursues a spiral course (Fig. 95 A). The same factors of 

 progression, rotation, and swerving, as described for euglena (cf. 

 p. 168), are to be found in the locomotion of paramoecium. In 

 paramoecium, however, these factors are more easily recognizable. 

 The progression is caused by the backward beat of the cilia, and 

 the rotation by the fact that they strike diagonally. It has been 

 supposed that the swerving results from a stronger beat of the cilia 

 in the buccal groove, but there are some ciliates that follow a spiral 

 course without having any such differentiated area, and hence the 

 stroke of the buccal cilia may not be the only factor. These three 



