TRICHOCYSTS. 83 



follow it, and being finally extended straight and rigid on the field of the 

 microscope, under the form of a very fine and attenuate acicular crystal. 

 In their most completely extended state these bodies were found to consist 

 of an elongate and rigid spiculum-like moiety, acutely pointed at one 

 extremity and continuous at the opposite end into an excessively fine 

 filiform appendage, less than half the length of the spiculum ; this second 

 portion was usually observed to be bent at an angle upon the first, and 

 to be more or less curved at its free end. The considerable structural 

 resemblance that subsists between the trichocysts of the Infusoria as 

 just described, and the cnidse or thread-cells of the Coelenterata or 

 Zoophyte class, was at the time recognized by Professor Allman, and the 

 circumstance has been cited on various occasions as producing strong 

 evidence against the more recently advocated unicellular nature of the 

 Infusoria. The non-validity of this argument is, however, at once made 

 manifest on regard being given to the fact that the thread-cells, even as 

 they occur among the Coelenterata, do not possess the independent 

 morphologic value of simple cells, many such being frequently enclosed 

 within the bounding membrane of a single cellular element, and of which 

 they are therefore to be regarded as the secreted product. In other words, 

 as maintainedby Professor Allman, " the formative Coelenterate cell may in 

 this respect be compared with the entire body of the Infusorium." Accord- 

 ing to Biitschli, the trichocysts in certain forms, including a species of 

 Nassnla, emit a filament at each extremity of the previously enclosing 

 capsule instead of at one end only, as in the more normal case last 

 described. This investigator has suggested that such a double emission 

 is probably exhibited by the trichocysts of all Infusoria, and which 

 in that case affords a means of distinction between these latter and the 

 genuine thread-cells of the Coelenterata ; this hypothesis has not, however, 

 been confirmed by more recent investigation. The trichocysts of abnormal 

 size, exhibiting an entirely irregular distribution, reported by Biitschli of 

 Potykrikos Swartzii, are apparently, as explained in the account given of 

 the species, accidentally engulphed thread-cells only of some neigh- 

 bouring Coelenterate organism. Although more generally trichocysts 

 occur, as in the cases of Paramecium and Ophryoglena, as an even and 

 crowded series beneath the entire cuticular surface, in others, such as 

 Litoiiotics and Loxophyllum, they present a limited and definite plan 

 of distribution. Thus, in the former genus they form a conspicuous 

 linear series confined entirely to the ventral aspect of the proboscidiform 

 anterior prolongation, while in certain representatives of the latter they 

 exhibit a partly linear and partly fasciculate arrangement. Although with 

 but few exceptions the special bodies now under consideration are entirely 

 limited to the Ciliate section of the Infusoria, Stein has recently shown that 

 the Monas {Raphidomonas) semen of Ehrenberg possesses these structures 

 variously distributed throughout the cortical region, but most abundantly 

 along the anterior border, while a doubtful case of their occurrence in a 



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