134 JOURNAL OF THE [J^IY- 



pression of the cell, for during and after emission of the thread 

 the capsule retains its oval form, with only a slight narrowing 

 and elongation, as appears in Fig. V. There is therefore ground 

 for the suggestion, which has been made, that the unexcited 

 cell contains a highly expansible liquid or perhaps a mixture of 

 gas and liquid, the expansive force of which is liberated by the 

 protrusion of the neck, above referred to, and the formation of 

 an opening ; and I am inclined to believe that the only effect of 

 the exciting cause, whether simple mechanical pressure upon 

 the separated cell or some kind of nervous irritation in the 

 living tissues, is to initiate the explosion by giving vent to the 

 internal pressure of the confined fluid. 



It is altogether probable, if not entirely certain, that, after the 

 thread has once been emitted, the end of the nematocyst has 

 been fulfilled and that it is then speedily sloughed off. Other 

 cells must therefore come to the surface, ready for a new dis- 

 charge of threads. This accounts for the presence of cells in 

 the lower layers of tissue, where they cannot put forth their 

 lassos, but it leaves still unexplained the existence of what are 

 usually regarded as weapons in the walls of the stomach, in por- 

 tions of the reproductive organs, and in other positions from 

 which it would seem as if they could never reach the ectoderm 

 and come into play against an enemy or an object of prey. 



In the specimen from Isophyllia, which I now have under the 

 microscope, it is possible to find a series of the largest cells ex- 

 hibiting various stages in the process of expulsion, from the 

 first movement, shown in Fig. II., to the full extension of the 

 thread, shown in Fig. V. As the thread is seen coiled in the cells, 

 the setae lie closely appressed, so that the total width is only 

 about iT^T5T)th or la^ffirth of an inch. But, after the thread is 

 completely extended, its diameter, exclusive of the setae, is about 

 ^tnyuth of an inch and the setae themselves are about i^^cirth of 

 an inch in length ; so that the thread and setae together have 

 about four times the diameter they had within the cell. 



When the thread is set free by the violent disruption of the 

 cell, it remains of the same size and as closely coiled as it was 

 before; and,. after it is shot forth from an unbroken cell, in the 

 normal manner, it exhibits no tendency whatever to resume a 

 contracted form. In other words, the thread is in itself a wholly 

 passive instrument, — a mere missile propelled from its miniature 

 gun by a power applied to it at the time of its flight. 



