NEMATOCYSTS. 87 



Indeed in specimens mounted in balsam, it is so transparent as to be 

 most difficult to see. The polypites, while possessed of as great 

 retractile power as those of the Thecate Zoophytes, have not the 

 same rapidity of movement, and answer to a stimulus or irritation 

 much more slowly. 



Among the Calyptoblastic Hydroids, the hydranth or polypite 

 is usually cup-shaped, with the tentacles arranged in one_or more 

 rings around the mouth. In Syncoryne, the body is as a rule 

 spindle-shaped, though by elongation it may at times appear almost 

 cylindrical ; while the tentacles are disposed irregularly over the 

 whole surface, standing out stiffly, so many spikes on a war-club. 

 The form of the tentacles, too, is peculiar, each being swollen at the 

 extremity capitate a characteristic shared by Syncoryne and 

 other members of the family. The reason for this capitate form is 

 not far to seek : it owes origin to the peculiar grouping or massing 

 of the nematocysts at the apex a striking divergence from the 

 prevalent arrangement among other families, where the collections or 

 batteries of stinging cells are situated at intervals on the general 

 surface of the tentacles. It is interesting to note that the tentacles 

 of the medusa-stage of Syncoryne have the ordinary arrangement of 

 stinging cells at intervals along the length, i.e. without any suggestion 

 of the massing seen in the tentacles of the Hydroid stage. 



Viewed with good illumination, the unburst stinging-cells can 

 readily be observed in the terminal knobs as more or less lenticular 

 bodies, and by judicious squeezing of a living polypite, some of these 

 may be pressed out, and a number will be certain to project the long 

 whip-like process which serves as the active agency in conveying the 

 poisonous secretion of the cell into the organism against which it is 

 launched. 



The isolated undischarged nematocyst can be made out to be a 

 cell of unusual size, somewhat ovoid in shape, and in which the most 

 conspicuous content is a great clear cyst, highly refractive, and filled 

 with a clear fluid, lying wherein is a spirally coiled filament. The 

 cyst does not occupy the entire cavity of the parent cell, but leaves 

 a space, most marked towards the base, filled with dense protoplasm, 

 in which lies a well-defined nucleus. From the apex of the cell 

 projects a pointed process of the cell wall, named the ''trigger" 

 and which functions as such on contact with a suitable body 

 (prey). It appears to stimulate the cell to a contraction, resulting in 

 the violent expulsion of the sting thread. The thread thus expelled 

 is not solid but is hollow, and in reality a tenuous prolongation of the 

 upper end of the cyst. It arose as an ingrowth or invagination of 

 the summit of the cyst, and when projected went through an instan- 

 taneous process of evagination, i.e. was turned inside out as the 



