120 [Dec. 



with a transparant colorless fluid. When the Hydra brings its arms in contact 

 with its prey, the projecting ends of those nettling cells which touch it appear to 

 adhere to the captured animal, and in the struggles of the latter, a delicate thread 

 from the nettling cell is observed to be attached to the prey which is lengthened 

 in the movements oC the latter, entangling its limbs ; and if the struggling con- 

 tinues or the prisoner escapes, it will be found to have several of the nettling 

 cells, torn from the Hydra, adhering by the long delicate threads. In the detach- 

 ment of the nettling cells, after considerable elongation of the thread, the whole 

 of the interior mechanism is first withdrawn from the cell and adheres to its pro- 

 jecting end, as a cylindrical mass, faintly outlined, with a pyramidal summit 

 from which proceeds the thread, and from the upper third of its sides, rise out- 

 wardly like springs, nearly at right angles to it, the four spinous processes be- 

 fore mentioned. The nettling cell itself, appears darkly outlined from the thick- 

 ness of its wall, and is much narrower than in the ordinary condition from the loss 

 of a considerable part of its contents, and within is nothing else but a clear fluid. 

 The traction continuing, the whole cell and appurtenances become detached from 

 the Hydra. The thread which originated in the interior of the cell appears to be 

 of a viscid character, for if any of these cells of the arms of the Hydra come in 

 contact with its own body, they adhere with such tenacity, that the former can 

 only be detached, at the expense of the loss of several of the nettling cells. From 

 the detached cells often being found attached to the Hydra itself by the long 

 threads, some observers, as Ehrenberg,* have considered that they were organs, 

 which the animal threw out from itself like anchors. 



The second form of nettling organs, are found arranged in more or less regular 

 circles around the first or largest form, usually nine to fourteen in a circle. 

 They are transparent, pyriform, about l-3400th in. in length by 1 -5666th in. in 

 breadth, and have projecting from the prominent extremity a cilium about l-875th 

 in. in length. These cells are described by Corda as containing a thick walled 

 sac, adhering to the outer cell at the base of the cilium. According to my obser- 

 vations, the appearance of an inner sac arises from a contained thread which 

 forms a double spiral, one end of which forms the cilium projecting from the 

 nettling cell. 



The third form of nettling organs, are found in greatest abundance about the 

 head of the animal, but also exist upon the arms, particularly at the lower part, 

 and upon the surface of the body generally. This form appears never to have 

 been before noticed. They are oblonc;, transparent cells, about 1-2 125th in. long 

 by l-5666th in. broad, and contain within them a spiral thread, more delicate than 

 in the second form of cells, and have a greater number of turns which take a 

 direction transversely to the length of the cell. They resemble very much in 

 their appearance one of the forms of nettling cells of Corynactis, figured by All- 

 man in the 17th vol. of the Annals of Natural History, PI. 11, fig. 4. I have never 

 been able to see the threads prolonged externally, on account of their minuteness, 

 in contact with the prey of the Hydra ; but by pressure and the continued en- 

 dosmosis of water I have detected them protruded in this as well as the second 

 form described. 



All the forms of nettling organs are placed within especial organic cells, adher- 



See Arch, fiir Naturgesch. 1842, p. 71. 



