396 E. VON LENDENPELD. 



Physiology. 



F. E. Schulze, who discovered the cnidocil, was of opinion 

 that any foreign body touching the cnidocil would cause an 

 explosion of the Nettlecell; much in the same way as touch 

 invariably causes a sting in the case of the stinging-nettle. To 

 this end it appeared that the cnidocil was so placed as to point 

 towards the ends of tentacles, that is, always in that direction 

 whence an enemy would be most likely to approach. The 

 position of the stinging hairs of Urtica is the same. 



Others who dwelt on the subject endorsed Schulze's purely 

 mechanical explanation, that direct pressure on the cnidocil is 

 transmitted to the cnidoblast and there causes the explosion of 

 the capsule which is already in high tension. 



If this were so there would apparently be no reason for the 

 connection of the cnidoblast with the nervous system of the 

 animal. 



Now, it is a well-known fact that touch by no means invariably 

 causes the explosion of the Nettlecells and the ejection of the 

 tube. If a tentacle of an Actinia is viewed under the micro- 

 scope in seawater under a cover-glass, and if fine grains of sand 

 are placed in the water and a strong current produced by suction 

 on one end, then the sand-grains are canned to and fro with 

 great velocity by the moving water and continually come in 

 contact with the surface of the tentacle. No explosion of a 

 Nettlecell, however, can be observed. But if acetic acid be 

 added to the water then the tubes will be seen shooting forth 

 like rockets all over the surface. 



When the animals, as they often do, contract themselves and 

 draw up their tentacles like the Medusae to one hundredth part 

 of their length, or close them over the mouth like the Actiniae, 

 there must be a very strong pressure, which according to the 

 mechanical theory would immediately explode all the Nettle- 

 cells. 



There are some species of Actinia which live in sand, as, for 

 instance, the Cerianthus. Those which live in shallow water. 



