THE CILIATA 221 



trichocysts are only discharged at or immediately before the 

 death of the animal, and a specimen killed for preservation is 

 nearly always seen lying in the midst of a cloud of fine 

 filaments formed by the exploded trichocysts. In Paramecium 

 the exploded trichocysts are generally cast right out of 

 the body on explosion, but sometimes they remain 

 sticking in it by one extremity, and the animal then looks as if 

 it were clothed with a coat of very long cilia. There is no 

 relation, however, between cilia and trichocysts, and after 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 un- 

 doubtedly offensive and defensive weapons of ccelenterates, 

 that it is difficult to avoid the conclusion that they have a 

 similar function. 



Before entering into details about the meganucleus 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, sometimesmore 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 

 cytopharynx 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. 47, B. 



Whilst the ectoplasm only contains structural constituents 



