PARAMECIUM 193 



relation, however, between cilia and trichocysts, and after the 

 the latter have been exploded, the much shorter and finer cilia 

 can be seen between the inner ends of the trichocyst threads. 

 Nothing is known of the mechanism by which the trichocysts 

 are extended, and very little is known about their function. 

 They are believed to be organs of offence and defence whereby 

 a Paramecium seizes and paralyses its prey or defends itself 

 against enemies. But it must be confessed that hours of patient 

 watching fail to demonstrate that a living and actively feeding 

 Paramecium uses its supposed weapons for capturing prey,, nor 

 does it turn them against the many living organisms which may 

 run up against it. The trichocysts, however, are so like the 

 thread-cells or nematocysts which are the undoubtedly offensive 

 and defensive weapons of ccelenterates and of some flat-worms 

 and mollusca, that it is difficult to avoid the conclusion that 

 they have a similar function. 



Before entering into details about the macronucleus and 

 micronucleus it will be well to consider the other organs. The 

 endoplasm, occupying the centre of the body, forms by far the 

 larger part of its bulk. There is really no sharp distinction 

 between endoplasm and ectoplasm or cortical layer. Both 

 have the alveolar structure characteristic of undifferentiated 

 protoplasm, and the two pass gradually into one another, the 

 passage being sometimes more gradual, sometimes more distinct. 

 As a rule the endoplasm may be distinguished by the rotatory 

 streaming movement or cyclosis of the granules and food 

 particles which it contains. These may be seen to move 

 steadily round a course which begins just above and behind the 

 so-called gullet near the posterior end ; passes forward along the 

 dorsal side, and turns downward at the anterior extremity to 

 return again towards the mouth on the ventral surface. This 

 course is indicated by the arrows in fig. 41, B. 



Whilst the ectoplasm only contains structural constituents 

 of the cell-body,* such as the trichocysts and the macro- and 

 micronuclei, the substance of the endoplasm is loaded with a 

 number of granules and particles, the products of assimilation 



* An exception, however, must be made in the case of Paramecium 

 bursaria, in which the green symbiotic algae, though they are not, strictly 

 speaking, structural constituents of the cell-body, are situated in the 

 ectoplasm, and, when they pass into the endoplasm, are digested and serve 

 as food. 



