120 MORPHOLOGY. 



The plienonienon of emission consists here, as in tlic form of thread-cell first described, of 

 two distinct acts (see PL VII, tig. 10). The immediate result of the exciting cause is the projec- 

 tion of a long tulmlar thread from the anterior end of the capsule. This thread attains the length 

 of about eight times that of the longer diameter of the capsule ; and on the completion of its 

 emission one end remains attached to the capsule, while the free end presents a small oval dila- 

 tation, behind which the thread is furnished for a short distance with very minute spines, which 

 seem to be arranged spirally on its walls. 



The projection of this thread thus constitutes the first step in the evolution of the thread- 

 cell ; but it is no sooner completed, than a still iiner thread is shot out with great rapidity 

 from its free extremity to a length about equal to that of the first. The whole act of emission is 

 thus completed, and the capsule seems entirely emptied of its contents. 



It is probable that the smaller kind of thread-cell in Hydra lias a structure similar to that 

 now described, though the minuteness of the parts unfits it for a satisfactory demonstration. It 

 is evident, too, that the long thread with its armature of minute spines, whose emission con- 

 stitutes the first stage in the action of the last-described thread-cell, is the exact homologue of 

 the barbed sac of the first kind, the fine terminal filament in the two being also homologous 

 organs. 



The first stage in the emission of the contents of the capsule in the second kind of thread- 

 cell takes place occasionally so slowly, froui accidental resistance, that the course of the thread 

 may be easily followed by the eye ; and then it will be plainly seen to roll outwards by an 

 act of eversion, reminding us of the mode of extension of the tentacles of a snail. The 

 emission of the finer thread from the extremity of this is, probably, also by an act of eversion ; 

 but I have never succeeded in following it so as to obtain direct evidence that it is so.^ 



A fact which has an important bearing on the structure of the thread-cell is the difference 

 of capacity presented by this body according as it is examined in its unevolved or evolved state. 

 It would seem that, in some cases, the capacity of the capsule after evolution is less than it was 

 previously, and then we can understand that no necessity may exist for its emitted contents being 

 replaced by others. In certain cases, however, the total cavity of the thread-cell after the evolution 

 may occupy a space more than double that which characterised it before the commencement of this 

 act. Now, as this occurs in a sac, one portion, at least, of which must consist of a flexible 

 membrane which would yield to any pressure from without not counterbalanced by an equal 

 pressure from within, and as no tendency to collapsing can be detected in the evolved thread- 

 cell, it is plain that, simultaneously with evolution, a fluid must be admitted into it from without, 



' The observations of Mobius (loc. cit.), made chiefly on the thread-cells of CaryophyUia Smitldi, 

 have led him to describe the axile tube as consisting of three tubes, included by invagination one within 

 the other. I cannot find direct evidence of tliis triple invagination in the thread-cells of any of the 

 Ilydroida I have examined. It must, however, he borne in mind that the much larger size of the 

 thread-cells of Car?/o/i/(?///«a render them more favorable for the determination of structural details ; 

 while this view of the axile tube receives support from the fact that the proximal portion of the exserted 

 filament in the thread-cell of Gemmaria greatly exceeds in length the straight portion of the unexserted 

 filament; for this portion is the representative, in the non-evolved state, of the proximal portion of 

 the evolved filament, and the difference between the two can scarcely be otherwise explained than by 

 supposing the straight unevolved portion to consist of a tubular filament several times invaginated into 

 itself. 



