KOPOID AND SWEZY: UNARMORED DINOFLAGELLATA 31 



frequent than positions in which the pole of devagination is turned away from 

 the surface. 



The matured organ lias a length varying from 5m to 22/* in the two genera. 

 In Polykrikos the average length is slightly greater than in the smaller species 

 of Nematodinium. In two species, N. torpedo and N. partitum, of the latter 

 genus the nematocysts range in length from 5i^ to 8/*, while those of N. annatum, 

 wit!) its nnieh greater size of body, range from 14/* to 22/*, a size comparable 

 with that of the organelles in Poltjkrikos. 



The shai)e of the nematocyst is slender oval (fig. F, 2), surmounted by a 

 caplike portion at the broader end. It consists of an external capsule (cap.) 

 of considerable rigidity, which Chatton regards as chitinous in its nature. Be- 

 ginning at the l)ase of the caplike portion in the interior of the capsule is an 

 introverted sacklike extension (inf.), which may reach to near the middle of 

 the capsule. This is continuous with the sides of the capsule as may be seen 

 in the exploded nematocyst (fig. F, 3, inf.), in which this portion is everted 

 and thrown forward. To this structure Chatton has given the name ampulla 

 (ampoide). 



Arising from the Ijottom of the interior of the introvert or ampulla is a 

 small cone-shaped thickening from the apex of which a slender stylet (sty.) 

 extends forward with a length nearly equal to that of the introvert, ending free 

 in the cavity. At the base of the introvert, o})posite the origin of the stylet, 

 the thickening is continued into two lo1:)elike bodies (1.) from the point of inter- 

 section of which springs a slender filament (^7.) of considerable length, prob- 

 ably continuous with the walls of the capsule, the spiral coils of wdiich fill the 

 posterior portion of the cavity. The exact relation of this filament to the stylet 

 is hard to determine. In the figures of lioth Biitschli and Bergh the stylet 

 functions as the base of the filament when the latter has been discharged from 

 the capsule. In Faure-Fremiet's figure the stylet remains within the introvert, 

 or rather is thrown liackward after the discharge of the nematocyst. Chatton, 

 however, finds that the role of the stylet is that of an organ for piercing the 

 opercidum at the time of discharge and is not directly connected with the fila- 

 ment, which passes thi-ough the thickened portion of the introvert at the base 

 of the stylet (fig. G), and is thrown off after the discharge of the nematoe^^st. 



At the anterior end, surmounting the caplike head, is a minute operculum 

 (oper.) which marks the point of emergence of the stylet in tlie discharge of 

 the nematocyst. 



No evidence is forthcoming j-egarding the normal discharge of these 

 organelles. When the l)ody wall is ruptured and the nematocysts come in con- 

 tact with the sea water the discharge is usually instantaneous in the case of the 

 fully niatui'cd organelles. The addition of weak acetic acid, formalin or alcohol 

 will also cause a discharge. This process re(|uires but a small fraction of a 

 second for its accomplishment, hence the details of it are difficult to observe 

 and conflicting reports as to the manner in wliicli it takes place have resvdted. 



