l888.] NEW-YORK MICROSCOPICAL SOCIETY. 133 



is coiled before ejection, to the dimensions and form of the 

 filament after protrusion, and to the size of the setas and their 

 mode of arrangement on the thread. 



The largest cells are slightly curved, and there is little differ- 

 ence in the size of the two extremities. The point from which 

 the thread is discharged is, however, usually a trifle blunter, or 

 broader, than the other. The thread lies loosely coiled within, 

 filling the whole cavity of the cell, and it is impossible to dis- 

 cover any interior capsule. The anterior end of the thread, 

 which appears to be tipped with a hard point, something like an 

 arrow-head, may be seen distinctly, lying properly directed and 

 ready for emission ; but the posterior end cannot be made out 

 satisfactorily. 



The second form of nematocyst is straighter than the first, of 

 a symmetrical oval outline, and about s^th of an inch in length 

 by about agVoth of an inch in width. At the forward end is a 

 tight roll of thread, occupying less than one-third of the narrow 

 diameter of the cell and extending to not quite its middle, 

 whence it is continued, in a loose, irregular mass, nearly to the 

 posterior wall. 



The third sort of nematocyst is more egg-shaped and much 

 smaller than either of the other two, being from s^i^yth to aijyth 

 of an inch in the longer direction and about Wooth in the 

 shorter. It contains no inner capsule, unless the thread is 

 wound about it and thus conceals it, as may well be the case in 

 any of the three forms under observation. The thread is coiled 

 very tightly at one end and lies in a loose mass at the other, 

 as in the case of the larger cell just described. 



In the largest of these cells, represented in Fig. I., with a 

 magnification of 700 diameters, the setae are easily seen upon 

 the thread before its emission. In the others, however, the 

 parts are too small, and the thread is too closely twisted to per- 

 mit of our distinguishing the setfe with any power I have had 

 at my disposal. 



When these thread-cells are subjected to pressure in the 

 compressorium, a short neck is at first protruded, as shown in 

 F"ig. II., and then the thread is emitted from it and forcibly 

 projected to a length many tirnes greater than that which it 

 possessed while enclosed by the capsule. It is not at all 

 clear that the projectile force is derived entirely from com- 



