THE INFUSORIA 1 75 



by its fixity in the cell. It is occasionally thickened to form the 

 denser ends of the body, as in the tail of Stentor. In some cases, 

 also, processes from the cortical plasm invade the endoplasm to 

 surround the nucleus and hold it in a fixed position in the body 

 (Dasytricha). The cortical plasm, furthermore, is the seat of the 

 peculiar and characteristic offensive and defensive trichocysts. In 

 some forms, probably representing the primitive condition, these 

 are distributed about the body (Paramcecium), but, even more than 

 the cilia, they have been subject to reduction in most parts of the 

 body until, in the majority of forms, they are restricted to a limited 

 area, while in the Hypotrichida and Peritrichida they occur only 

 sporadically. In Prorodon (Holotrichida), the trichocysts are found 

 in the anterior end only, and in the family Trachelinidce they are 

 found only on the ventral side, while in Trachelitis, Dileptus, etc., they 

 are on the ventral side of the anterior process (Fig. 92). In Lionotus, 

 they are reduced to a single line along the ventral side of the anterior 

 process. 



The trichocysts are so minute that their finer structure has not 

 been definitely made out, although a few different types have been 

 studied (Fig. 12, C, p. 39). Rod-like forms have been seen in Loxopliyl- 

 litm, Lionotus, and Strombidium, and spindle forms in Paramecium, 

 Front onia and Nassula. When protruded from the body they are, 

 for the most part, apparently of the same size and shape as when 

 within the ectoplasm. Occasionally when protruded, however, they 

 have small hooks or swellings on the end (Maupas). They vary in 

 size from three to twelve microns when within the body, but when 

 protruded they measure from thirty to sixty microns. The cause of 

 the protrusion is unknown ; certain reagents act as irritants and 

 cause them to explode and throw out the long threads. Their func- 

 tion, too, is purely conjectural, although it is generally supposed that 

 they serve as defensive weapons. In some cases they appear to serve 

 as weapons of offence as well, especially in those ciliates where they 

 are limited in number to a comparatively few large ones. According 

 to Maupas ('83), these Infusoria chase their prey and launch their 

 trichocyst darts, which penetrate the outer coating of the victim and 

 paralyze it, possibly through the action of some noxious fluid. 

 Forms much larger than the hunters are frequently brought down in 

 this way, to be swallowed either whole or piecemeal. The attack is 

 not necessarily fatal, for the larger forms frequently revive. 1 



The position of the trichocysts is primarily in the cortical plasm, 

 but they are rarely entirely immersed, being much more frequently 

 suspended in the streaming endoplasm. They are occasionally drawn 



1 See ante, p. 50. 



