80 THE AMERICAN MONTHLY [May, 



forms have precisely the same shape as the small, but are three or four times 

 as large, and the thread makes numerous turns within the sac (fig. 3) . The 

 discharged thread is extremely long and slender, tapers to a point of extraordi- 

 nary fineness, and is armed near the enlarged base with three hard, sharp 

 spines, or barbs, that are usually directed backwards (fig. 7) . 



The small and the middle-sized forms are discharged simply by the aversion 

 of the thread, as illustrated by the diagrammatic figs, i and 2. The large or 

 barbed forms have a more complicated mode of action, the nature of which was 

 first pointed out to me by Mr. J. H. Emerton while making sketches for the 

 drawings reproduced in the accompanying figures. Before the discharge the 

 barbs lie within the thread-cell, folded exactly together, with their points in 

 contact, like the legs of a closed tripod, or triple compass, thus forming a 

 three-sided pyramid, or dart, with an exceedingly sharp, hard point. The 

 base of the pyramid fits accurately into the bottom of a pouch (the ' barbed sac ' 

 of authors) , that is folded into one end of the thread cell, and is continuous at its 

 mouth with the inner or lining membrane of the double-walled sac ; its point 

 is braced against a membrane that stretches across the opening of the pouch, 

 and is continuous with the outer membrane of the sac (fig. 3). From the 

 base of the pyramid a peculiar folded structure hangs down into the cavity 

 of the sac, but its precise nature has not been made out.* 



Both Mr. Emerton and myself have repeatedly studied these thread-cells at 

 the moment of discharge, taking advantage of the fact that the action occa- 

 sionally takes place slowly or incompletely, so that every step may be accu- 

 rately followed. The discharge may be described as consisting of two dis- 

 tinct acts : — First (as a result no doubt of pressure exerted on the sac by 

 sudden contraction of the surrounding pi'otoplasm) , the point of the pyramid 

 is suddenly forced through the membi'ane, the whole pyramid is violently 

 piojected outward, and the pouch in which it lay is evaginated to form the 

 'neck' of the discharged thread cell (fig. 4 and 5). Second, the pyramid 

 splits asunder into three parts (fig. 6), which instantly turn backwards (be- 

 ing as it were hinged at the base) to form the three barbs, and at the same 

 instant the thread is shot forth, being everted from a point in the middle of 

 the triangle formed by the bases of the barbs (fig. 6 and 7). 



This account helps, I think, to explain certain rather puzzling features of 

 the action of thread-cells of this type. It explains the explosive violence of 

 the discharge, for this is due to the sudden rupture of the membrane by the 

 point of the pyramid at the instant when the pressure upon it reaches a certain 

 maximum limited by the resisting power of the membrane. It explains, 

 also, the peneti^ating power of the thread, for the keen, hard point of the 

 pyramid easily punctures the tissues of the Hydra's victim, held fast by the 

 tentacles, and the subsequent spreading apart of the barbs not onl}' fixes the 

 sac, but also clears a space into which the delicate thread may unroll. 



There can be little doubt that all thread-cells of this type (which are widely 

 distributed among the Hydrozoa) are discharged in a similar manner, and it 

 would be interesting to search for the causes by which so curious and per- 

 fect a microscopic mechanism has been evolved, and the successive stages 

 through which it has passed. f 



Bryn Mavvr, March, jS88. 



* The structure of the undischarged thread-cells may best be studied in specimens treated on the slide with one 

 per cent, acetic acid, and afterwards stained with fuchsin or aqueous magenta. Thehighest available power 

 should be used. 



+ The paper of Miibius already cited deals mainly with the thread-cells of Caryophyliia, which do not possess 

 the three large barbs ; but his figures and brief description of the barbed thr.ead-cells of Hydra show that he 

 clearly understood their action. It may be added that nearly fifty years ago Erdl published recognizable figures 

 of these thread-cells, both before and after discharge, and he seems to have understood the action of the pyramid 

 (' Pfeil' ), though he fell intp tjie error of supposing it to be capable of repeated action by withdrawal into the 

 sac after discharge, / 



