inSTOLOGY OF THE ECTODERM. 119 



Such are the phenomena presented hy the thread-cell in the act of evolution, but the 

 amazing rapidity with which the whole takes place renders it extremely difficult to determine 

 in what the remarkable series of changes now described essentially consists. There can, however, 

 be no doubt tliat the sudden appearance of the barbed sac is the result of a process of eversion 

 of tiic membranous tube which in the original state of the thread-cell occupies its axis, and 

 which is formed by the lining membrane of the capsule inverted into itself at its anterior end. 

 The eversion of this tube, by which it becomes freed from the restraint of the capsule, is 

 necessarily accompanied by its sudden expansion and development ; and that this is the real 

 nature of the phenomenon in question, receives ample confirmation from what may be (vitnessed 

 in another kind of thread-cell to be presently described. 



It is still more difficult to determine the nature of the act which shows itself in the emission 

 of the filament, than of that which constitutes the emission of the barbed sac. Previously to 

 emission, no trace of a filament can be detected in the thread-cell now under consideration ; 

 but, as already said, there can be little doubt that the transparent homogeneous-looking mass in 

 which the posterior part of the axile tube loses itself before its eversion has commenced is 

 nothing more than the filament closely coiled on itself (as shown diagrammatically in woodcut, 

 fig. 52 A). 



It is probable that the expulsion of the filament, like that of the barbed sac, consists also 

 in an act of eversion ; a view which would of course render it necessary to regard it, notwith- 

 standing its extreme tenuity, as a flexible membranous tube. 



It must be borne in mind, that the barbed sac and the filament are in no way continuous 

 with the outer rigid wall of the capsule, but only with the delicate sac by which this is lined. 

 The outer wall opens at its summit by a definite orifice which appears to have been previously 

 closed by a minute lid which is thrown off" in the act of evolution, and which may be occasionally 

 seen after the completion of this act adhering in the form of a httle disc to the edge of the now 

 expanded orifice, through which the contents of the capsule had been emitted. 



Another form of thread-cell which throws light on the structure and action of that just 

 described, occurs also among the Hydroida. In Hydra, it is in the form of a minute oval 

 capsule, much smaller than the former, and having its cavity occupied by a spirally coiled 

 filament which may be easily seen through its transparent walls. The act of evolution consists 

 in the emission of the filament, but the barbed sac which constitutes so important a feature in 

 the evolved thread-cell of the first kind cannot here be distinguished. 



In some other Hydroida, the second kind of thread-cell acquires a larger size than in 

 Hydra. I have carefully examined it in Gemmaria implexa, where it is met with scattered in the 

 ectoderm of the hydranth, and where, from its comparatively large size, it is well fitted for 

 observation. It here consists of an oval capsule (PI. VII, fig. 9), within which may be seen, pre- 

 viously to emission a long thread, consisting of a straight and a coiled portion. The straight 

 portion crosses the capsule diagonally, and is continuous at one end with the walls of the capsule, 

 while at the opposite end it begins to be rolled up in distinct coils which almost completely till 

 the capsule. The coiled portion of the thread, however, does not surround the straight portion, 

 but is placed entirely on one side of it, as had been already noticed by Clark in the thread-cells 

 of Syncoryne mirahilis} 



' Clark, loc. cit. 



