312 ANNUAL OF SCIENTIFIC DISCOVERY. 



The upper cavity is the stomach. In the centre of it is a large open- 

 ing, which communicates with the cavity below ; so that the stomach, 

 by the relaxation of its walls, that is, of its muscular fibres, throws 

 down its own contents into the general cavity of the body. But dur- 

 ing the process of digestion, when food has been introduced into this 

 cavity, the mouth is shut, and the stomach is equally shut below. 

 During the whole time that digestion goes on, the stomach remains 

 as a closed bag; but, as soon as the food has been fully digested, then 

 the lower cavity opens to empty its contents into the general cavity ; 

 but sometimes the upper opening expands first, and the refuse of 

 hard particles is thrown away. The lower opening of the stomach is 

 shut again as soon as the homogeneous mass of the digested food has 

 entered into the wide cavity below, precisely as in Actinia. 



" The hard parts of the Polypi are formed by means of cells, within 

 the thickness of the walls of the animal itself. They are neither an 

 external secretion, nor an internal skeleton, but constitute a calcare- 

 ous deposit within the soft parts of the animal. It is by the accu- 

 mulation of microscopic granules of limestone that a regular wall of 

 stone is produced, within the thickness of the membranes, at their 

 lowest portions. The tentacles of these animals are hollow, having 

 vibrating cilia on their inner surfaces, by which very minute particles 

 of food are brought to their mouth. Besides these cilia on the ex- 

 ternal surface, there are other organs which have been known to 

 exist in other animals, but which have never before been observed in 

 corals, called nettling organs. It is very well known that the jelly- 

 fishes, if handled, leave a painful sensation like that of the burning 

 of nettles. It has been ascertained that this nettling arises from the 

 action of a peculiar apparatus, about the form of slender thread issu- 

 ing from a bulb. Now, in this coral animal, the whole surface of the 

 tentacle is provided with such nettling apparatus, forming heaps, ar- 

 ranged over the surface like warts, nearly in rows. There are hun- 

 dreds of these warts upon one of these tentacles, and if we exam- 

 ine their structure, we shall find that every one of them consists of 

 nettling cells. The whole structure of these cells can scarcely be 

 seen by the best microscopes now at our disposal. Even some of the 

 microscopes, considered among the best, do not reach the limits which 

 are required for such investigations. These heaps of wart-like bodies 

 are accumulations of peculiar cells, and there is in each of them a 

 thread coiled up in a spiral form. In some of these there is a sort 

 of arrow, with the thread coiled up around the arrow. In others we 

 have a conical-shaped cell, and here also a thread coiled up. Upon 

 watching these cells, which, from their contents, I could have no 

 doubt were the nettling cells, I have been fortunate enough to see 

 the manner in which these threads are issued, like a lasso. I have no 

 doubt that it is with this apparatus that they sting, though I cannot 

 say what is the action produced upon the tissues of other animals to 

 cause the painful sensations they produce, as all this apparatus is too 

 minute to be investigated in any other way than through high powers 

 of the microscope, with transmitted light, and the chemical operation 

 of the fluid to produce such a sensation upon the skin cannot be dis- 



