REACTIONS OF DIDINIUM NASUTUM. IO/ 



cysts have already been discharged owing to a previous attack, 

 the connection is never broken; the protuberance is insignificant 

 and the seizing organ is only slightly extended if at all. Thus 

 a second hunter frequently succeeds in remaining attached to 

 its prey after the first has been thrown off. 



In Nassula, a comparatively large ciliate, the trichocysts are of 

 such a nature and so abundant that I have never seen one cap- 

 tured by Didinium, although they are vigorously and freely 

 attacked. 



This is, as far as I know, the first instance in which a defensive 

 function of the trichocysts has been actually demonstrated, 

 although it has generally been assumed to be both offensive and 

 defensive. Calkins (1901, p. 50) after referring to the offensive 

 use of these structures in Didinium, writes as follows : ' : This 

 process is strikingly illustrated by the ciliate Actinobohis radians, 

 which combines the selection of food with the offensive use of 

 trichocysts. This remarkable organism possesses a coating of 

 cilia and protractile tentacles, which may be elongated to a length 

 equal to three times the body-diameter, or withdrawn completely 

 into the body. The ends of the tentacles are loaded with tricho- 

 cysts (Entz, 1883). When at rest, the mouth is directed down- 

 ward, and the tentacles are stretched out in all directions, form- 

 ing a minute forest of plasmic processes, amongst which smaller 

 ciliates, such as Uroccntrnm, Gastrostyla, etc., or flagellates of 

 all kinds may become entangled without injury to themselves and 

 without disturbing the Actinobolns or drawing out the fatal 

 darts. When, however, an Halteria grandinclla, with its quick 

 and jerky movements, approaches the spot, the carnivore is not 

 so peaceful. The trichocysts are discharged with unerring aim, 

 and the Halteria whirls around in a vigorous, but vain, effort to 

 escape, then becomes quiet, with cilia outstretched, perfectly 

 paralyzed." In another section of his book (p. 175), however, 

 referring to the trichocysts in general, Calkins says: "Their 

 function is purely conjectural, although it is generally supposed 

 that they serve as defensive weapons." 



Jennings also thinks that we are uncertain as to their function. 

 He writes (1906, p. 90): "They are usually supposed to be 

 weapons of defence. If a Paramecium is seized by an animal 



