228 GARY N. CALKINS 



way that spiles in a ferry slip take up the strain. In the center 

 of the conical proboscis is a column of protoplasm somewhat 

 denser than the rfest of the endoplasm. This structure, called 

 by Thon the 'mittlerer Strang' and by Mast the 'seizing organ/ 

 is the apparatus which fastens the prey and precedes it into the 

 body of the captor. The open passage which it leaves by its 

 migration through the protoplasm becomes the cytopharynx, 

 the prey being drawn in largely, if not entirely, by its pull (figs. 



1-6). 



This seizing organ is such a remarkable structure that it well 

 repays careful study. There is no doubt that it contains some 

 toxic substance which partially or wholly paralyses Parame- 

 cium, but no one, as yet, has shown where this substance lies. 

 Balbiani ('73) held that trichocysts are discharged by Didinium 

 and that these penetrate and paralyse the victim. Thon, how- 

 ever, supported by Mast, denies the discharge of trichocysts and 

 holds that the entire process of capture and retention is a func- 

 tion of the seizing organ. Thon figures the seizing organ as stri- 

 ated, and in this he is undoubtedly correct, but he evidently 

 failed to note a zone of thickened granular striae near the apex 

 of the seizing organ and clearly apparent when the organ is ex- 

 tended (fig. 8). This zone is made up of the same sort of thing, 

 apparently, as the granular zone at the apex of a tentacle of Ac- 

 tinobolus. These granules were first described for Actinobolus 

 by von Erlanger ('89) as trichocysts on observations confirmed 

 by Moody ('12) who speaks of them as Hrichocyst material.' 

 They evidently are the poison granules which cause the instan- 

 taneous paralysis of Halteria. In the absence of other physical 

 evidence of poison in the seizing organ of Didinium we are justi- 

 fied in regarding these granules, liberated on penetration of the 

 cortex of Paramecium or other victim, as the cause of paralysis. 

 The entire seizing organ may be regarded as a bundle of structures 

 homologous with the distributed tentacles of Actinobolus. Like 

 the seizing organ, each tentacle of Actinobolus is retracted into 

 the endoplasm, dragging the attached victim to the surface of 

 the body, where it is manipulated by the ciha until swallowed 

 through the mouth. 



