HISTOLOGY OF THE ECTODERM. 121 



unless we suppose tliat the cavity of the thread-cell is occupied by a compressible and expansible 

 aeriform fluid. This latter supposition cannot be maintained, and if the former view be 

 accepted, we must either admit the power of rapid imbibition in the walls of the cell, or 

 suppose the presence of some definite opening, not yet detected, through which the surrounding 

 water may gain access to its cavity. 



The two forms of thread-cells now described may be regarded as the principal types 

 under which these bodies occur in the Hydroida. Of the nature of the force by which the 

 emission of their contents is effected we cannot yet speak with certainty. Everything that has 

 been observed, however, is opposed to the supposition that the act of evolution is a vital one, 

 dependent on the irritability of certain tissues which enter into the structure of the thread-cell. 

 It is far more probable that it is simply physical, depending on the mere elasticity of the parts, 

 and brought into play when the internal structures are mechanically released from the tension in 

 which they had been held during the previous state of repose. 



One fact may here be mentioned which is inconsistent with the idea of the evolution of the 

 thread-cell being a vital act ; namely, that prolonged immersion in certain re-agents, such as 

 alcohol, will sometimes have no effect in destroying its characteristic properties ; for I have seen 

 the thread-cells of hydroids which had remained for months immersed in alcohol retain the 

 power of emitting their contents on the application of some other re-agent, such as acetic 

 acid. 



There is reason to believe that, in some cases at least, the thread-cell when brought into 

 use is at the same time forcibly ejected from the ectoderm in which it had been previously 

 imbedded, and not merely drawn out of its berth by its attachment to the prey ; and there is no 

 reason why this act should not be referred to an irritability residing in the ectoderm, and 

 receiving its special stimulus from the conditions which rendered necessary the employment of 

 the thread-cell, such as the contact of living prey. 



The employment of the thread -cell by the hydroid would, under these circumstances, involve 

 both a vital and a physical act — the vital manifested in the ejection of the thread-cell from the 

 body of the hydroid, the physical in the emission of its contents. 



The special purpose fulfilled by the thread-cells in the economy of the animal, and their 

 probable employment as urticating organs, will be considered under the section which treats of 

 the physiology of the Hydroida. 



While the thread-cells in the Hydroida are entirely confined to the ectoderm and its 

 appendages, they are by no means uniformly distributed in it. It is in the tentacles of both 

 hydranth and medusa that they are usually most abundant ; and here we find them either 

 generally scattered through the ectoderm, or distributed through it in knot-like or wart-like or 

 verticellate groups, or collected together in the spherical capitula in which the tentacles of some 

 genera terminate (see Pis. IV, V, &c.). In many medusae the ectoderm forms, at the base of the 

 marginal tentacles, bulb-like thickenings which are loaded with thread-cells (see woodcut, fig. 56e). 

 In many cases, minute thread-cells singly, or in clusters, are scattered superficially in the walls of 

 the umbrella ; in the genus Gennnaria (PI. VII) we also find four piriform chambers extending 

 from the circular canal of the medusa into the umbrella-walls, and filled with small oval thread- 

 cells which have the appearance of lying loose within their cavity (figs. 3 and G) ; while numerous 

 similar but smaller sacs, filled with thread-cells, may be seen in irUlia opening into the circular 

 canal, and thence extending in a meridional direction along the walls of the umbrella. 



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