118 



MORPHOLOGY. 



Fig. 52. 



axilc tube loses itself, is in reality hero, as in other cases, a filament rolled upon itself, but of such 

 extreme tenuity and transparency as to render it impossible to distinguish the individual coils. 



By carefully adjusted illumination, the whole of the capsule appears to l)e lined by a 

 membrane of extreme delicacy. The existence of an external capsule distinct from its lining 

 membrane is particularly apparent in the thread-cell after the emission of its contents, for then 

 the outer capsule is seen to terminate by a distinct slightly everted margin round the orifice 

 through which the contents have been expelled (PI. IV, fig. 5). 



The characteristic action of the thread-cell may be brought into play under the microscope 

 by some force artificially applied, such as the pressure of the compressorium, or the contact of 



alcohol or acetic acid. This action consists 

 in a sudden change of form, the capsule 

 opening at its anterior end, from which a 

 very remarkable body is at the same time 

 projected with a rapidity which renders it 

 impossible for the eye to follow it in its 

 progress (PI. IV, fig. 5, and woodcut, fig. 

 52 B, b). When, however, this act is com- 

 pleted, the projected body may be seen to 

 be an elongated sac whose walls are at one 

 end continuous through the opening in the 

 capsule with the membrane by which the 

 latter is lined, and whose opposite end 

 tapers away to a point. Just where this sac 

 begins to thin away towards its point, three 

 rigid spines are fixed in a verticil to the outer 

 surface of its walls, their points being directed 

 backwards like the barbs of an arrow. 



The projection of the barbed sac does 

 not, however, constitute the whole of the 

 phenomenon presented by the thread-cell 

 under the influence of the force which 

 gave 01 igin to its evolution ; for this first 

 act is followed — in most cases instantane- 

 ously — by a second, which consists in the 

 projection, from the pointed summit of the 

 sac, of a filament (woodcut, fig. 52 c, c) 

 of great tenuity and transparency, which 

 rapidly shoots across the field of the micro- 

 scope until it attains a length of between 

 thirty and forty times that of the longer 

 diameter of the original capsule ; at the termination of the emission, one end of the filament 

 always remains in connection with the free end of the barbed sac. The whole act of evolution is 

 thus completed, and the capsule is now seen to contain nothing but an absolutely colourless homo- 

 geneous fluid. 



the 



Diagrammatic Views of the Thread-cell. 



A. The thread-cell previous to the emission of its contents. _, ... 

 double wall of the capsule; b, reflected portion of the inner membran., 

 of the capsule-wall, forming the axile tube of the unevolved thread- 

 cell ; c, the filament lyiug in complicated coils in the bottom of the 

 capsule. 



B. The thread-cell immediately after the first stage of evolution. 

 a, the double wall of the capsule ; h, the barbed sac formed by the 

 eversion of the axile tube; c, the filament still lying within the cap- 

 sule and barbed sac. 



C. The thread-cell after the complete emission of the contents. 

 '(, the double wall of the capsule; I, the barbed sac; c, the ejected 

 filament. 



