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TRICHOCYSTS. 83follow it,and being finally extended straight and rigid on the field of themicroscope, under the form of -.a very fine and attenuate acicular crystal.In their most completely extended state these bodies were found to consistof an elongate .and rigid spiculum-like moiety, acutely pointed at oneextremity and continuous at the opposite end into an excessively finefiliform appendage, less than half the length of the spiculum ;this secondportion was usually observed to be bent at an angle upon the first, andto be more or less curved at its free end. The considerable structuralresemblance that subsists between the trichocysts of the Infusoria asjust described, and the cnidae or thread-cells of the Ccelenterata orZoophyte class, was at the time recognized by Professor Allman, and thecircumstance has been cited on various occasions as producing strongevidence against the more recently advocated unicellular nature of theInfusoria. The non-validity of this argument is, however, at once mademanifest on regard being given to the fact that the thread-cells, even asthey occur among the Ccelenterata, do not possess the independentmorphologic value of simple cells, many such being frequently enclosedwithin the bounding membrane of a single cellular element, and of whichthey are therefore to be regarded as the secreted product. In other words,as maintained by Professor Allman, " the formative Ccelenterate cell may inthis respect be compared with the entire body of the Infusorium." Accordingto Biitschli, the trichocysts in certain forms, including a species ofNassula, emit a filament at each extremity of the previously enclosingcapsule instead of at one end only, as in the more normal case lastdescribed. This investigator has suggested that such a double emissionisprobably exhibited by the trichocysts of all Infusoria, and whichin that case affords a means of distinction between these latter and thegenuine thread-cells of the Ccelenterata this ; hypothesis has not, however,been confirmed by more recent investigation. The trichocysts of abnormalsize, exhibiting an entirely irregular distribution, reported by Biitschli ofPolykrikos Swartzii, are apparently, as explained in the account given ofthe species, accidentally engulphed thread-cells only of some neighbouringCcelenterate organism. Although more generally trichocystsoccur, as in the cases of Paramecium and Ophryoglena, as an even andcrowded series beneath the entire cuticular surface, in others, such asLitonotus and Loxophylhim, they present a limited and definite planof distribution. Thus, in the former genus they form a conspicuouslinear series confined entirely to the ventral aspect of the proboscidiformanterior prolongation, while in certain representatives of the latter theyexhibit a partly linear and partly fasciculate arrangement.Although withbut few exceptions the special bodies now under consideration are entirelylimited to the Ciliate section of the Infusoria, Stein has recently shown thatthe Monas (Raphidomonas) semen of Ehrenberg possessesthese structuresvariously distributed throughout the cortical region, but most abundantlyalong the anterior border, while a doubtful case of their occurrence in aG 2