40 MANUAL OF ANIMAL BIOLOGY 



the cytoplasm. The surface of the animal, including the peristome, 

 is uniformly covered with fine, hair-like filaments, the cilia, which 

 continually beat in the water in a coordinated manner and thus 

 enable the animal to swim in a rapid, vigorous fashion. Some of 

 them also aid in securing food by driving a current of water along 

 the peristome toward the gullet. (W. f. 27.) 



Enclosing the ectoplasm of Paramecium is a thin, secreted cell 

 wall, or pellicle, which shows very fine regular markings, or stria- 

 tions, on the surface. Innumerable fine strands of ectoplasm pro- 

 ject through the pellicle. These constitute the cilia which form a 

 uniform covering over the surface of the animal. The pellicle of 

 Paramecium is much more rigid than that of Euglena, so that 

 normally the animal maintains a definite shape. However, when 

 an individual is pressed against pieces of debris or is under the 

 influence of various other environmental conditions, a considerable 

 variation in shape takes place. 



Embedded in the ectoplasm is a single layer of highly differen- 

 tiated, flask-shaped bodies, known as the trichocysts. They lie 

 with their long axes perpendicular to the surface of the ectoplasm. 

 The base of the structure is toward the interior of the animal, 

 and the other end opens through the ectoplasm to the exterior. 

 These trichocysts are filled with a fluid which hardens and becomes 

 thread-like when it is expelled into the water. The discharge of 

 the trichocysts can be brought about experimentally by adding 

 a little dilute acid to the culture medium and thus irritating the 

 Paramecia. In some species of Protozoa it has been found that 

 the trichocysts have a paralyzing effect upon other living organ- 

 isms, and they are therefore regarded as weapons useful either 

 in the capture of other animals for food or in the repelling of at- 

 tacks by enemies. In Paramecium, however, the function of these 

 bodies seems to be essentially defensive. 



The endoplasm of Paramecium is, as has been found to be the 

 case in other forms, somewhat less viscous than the ectoplasm. 

 It also contains many granules of various sizes, some of which 

 are constituent parts of the cytoplasm, while others are merely 

 temporary metaplasmic particles. The endoplasm also contains 

 numerous gastric vacuoles and a characteristic nuclear appa- 

 ratus composed of a large macronucleus and one or two small 

 micronuclei, 1 the latter lying on or near the macronucleus. 



1 W. p. 54, footnote. 



