THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL. 



Vol. IX. MAY, 1888. No. 5. 



Note on the mechanism of the thread-cells in Hydra. 



By EDMUND B. WILSON, 



BRYN MAWR, PA. 



I. Since Ehrenberg's discovery of stinging ' thread-cells' (otherwise known 

 as nettle-cells, lasso-cells, nematocysts, etc.) that are so characteristic of the 

 true Coelenterata, many accounts have been given of their structure and mode 

 of action, and they have been, perhaps, nowhere so often examined as in the 

 fresh-water Hydra, a form that has been repeatedly studied by good ob- 

 servers, and is examined by every tyro in practical biology. It is, therefore, 

 rather surprising that even the most recent monographs and text-books, while 

 correctly describing the action of the smaller thread-cells, give but an imper- 

 fect account of the beautiful mechanism by which the larger barbed forms are 

 discharged and are enabled to force their delicate threads into resistant objects, 

 such as the tissues of the Hydra's victim. 



As far as I am aware, the only accurate published account of the matter is 

 that of Mobius,* who gives a brief description, with four rude but recogniza- 

 ble figures, of the barbed thread-cells of Hydra. In all subsequent works 

 that I have seen the precise structure of the thread-cell (nematocyst) , as dis- 

 tinguished from the surrounding protoplasmic structure (cnidoblast), is not 

 very carefully considered, and most of the figures are either incomplete or 

 altogether misleading.! 



II. It is known that Hydra has three kinds of thread-cells, which mav be 

 designated as small, middle-sized, and large (the first of these has been as- 

 serted, though probably on insufficient grounds, to be simply an undeveloped 

 form) . The small forms are ovoid, or slightly pyriform in shape, with a verv 

 short, stout thread which makes but a single turn within the sac and afteV 

 discharge is usually coiled like a snail-shell. The middle-sized forms are 

 narrower and more elongate, regularly ovoid, and the thread is disposed in nu- 

 merous coils. After discharge the thread is long and straight, with a rather 

 blunt tip, and, like the sac, it stains intensely with aniline dyes. The large 



* Ueber den Bau, den Mechanismus und die Entwickelung der Nesselkapsein : Abhandlungen aus dem Gebiete 

 der Naturw., herausg. v. dem Naturwiss, Verein zu Hamburg, V, 1866. 



t It is proper to add that the mechanism of the barbed thread-cells oK Hydra was fully made out by Mr. Emer- 

 ton and myself several years ago without knowledge of the studies of Mobius. 



Explanation op Figures. 



{All the figures were drawn from nature by y. H. Emerton). 



Fig. 1. Diagram of undischarged thread-cell, showing Fig. 5. Completion of the first part of the discharge, 



the thread as an infolding of the wall of the sac. the pouch having evaginated to form the 'neck.' 



Fig. 2, Diagram of discharging thread-cell, showing c- ^ i: .■ r .u u 1 (.1. .1, j u • 1 



the eversion of the thread. Fig. 6. Separation of the barbs (the thread having al- 



Fig. 3. Barbed thread-cell of Hydra surrounded by '^^^^ "-^^^ f°"h ^^""^ h^l^^^^ them). 



the cnidoblast, showing the sac, infolded pouch, f"ig- 7- Completely discharged thread-cell (only a 



pyramid, and coiled thread. Highly magnified. P^" °^ 'he thread is shown), with the sac of the 



Fig. 4. Beginning of the discharge, the pyramid hav- nematocyst still enclosed in the cnidoblast. 



ing ruptured the membrane, and the pouch having 



partly unfolded. 



