1903.] On the Nematocysts of Solids. 479 



preparation be subsequently washed out with distilled water, 

 the threads of the nematocysts are at once everted. 

 3. I have obtained similar results with Actinian tentacles. If a 

 tentacle of an expanded Actinian is plucked off and plunged as 

 quickly as possible into a 50-per-cent. concentrated solution of 

 sugar, and then teased up, though many nematocysts are found 

 discharged, it is quite easy to find pieces in which none are 

 discharged. I have isolated such pieces, and after further 



teasing up, have kept them for periods varying from 24 72 



hours. The nematocysts were always still undischarged, but 

 when the sugar solution was washed out with water a certain 

 number (never more than approximately 20 per cent.) discharged 

 themselves. The fact that 80 per cent, did not discharge them- 

 selves is probably due to the presence of the other tissues of 

 the tentacle preventing the access of the water, for in similar 

 experiments with nematocysts from a ceras of an ^Eolid, the 

 whole lot were discharged when washed out with water. 



These facts seem to show that the discharge of a nematocyst is due 

 to osmosis. The capsule apparently contains a solution of such a 

 strength that it takes up water from such a weak solution as sea 

 water, but not from the protoplasm of the nematocytes, or the fluids 

 in the alimentary canal of ^Eolids, or from any of the other solutions 

 mentioned above. 



This hypothesis does away with the necessity of supposing the 

 capsule wall to be impermeable, and so avoids Lendenfeld's first objec- 

 tion. As regards the second objection, it must be remembered that 

 for a stain to reach an undischarged nematocyst it must pass through 

 the protoplasm of the nematocyte ; it diffuses into the layer of liquid 

 immediately surrounding the nematocyst without appreciably altering 

 the degree of concentration, therefore, without upsetting the balance 

 between the liquids within and without the capsule, so that by 

 diffusion in both directions the stain can enter the capsule without 

 causing discharge. This applies to intra-vitam staining in Ccelenterates ; 

 fixing reagents probably so alter both capsule contents and protoplasm, 

 and perhaps also capsule wall, that arguments based on the behaviour of 

 fixed nematocysts are untrustworthy. 



But though this hypothesis explains why nematocysts are not 

 discharged while in the alimentary canal and cnidosacs of Solids, and 

 are discharged when extruded into the water, two other questions 

 suggest themselves 



1. To what is the apparent immunity of ^Eolids towards the 



nematocysts of Cnidaria due 1 



2 How is it that we do not find in the cnidosacs a large number 



2 L 2 



