148 G. T. HARRIS ON THE COLLECTION AND 



the polyps by gently brushing the polypary with a camel's hair 

 brush. They are allowed to recover from the shock, and then 

 a few drops of 1-per-cent. cocain hydrochlorate added to each 

 watch-glass and the glasses set gently aside until all the species 

 have been dealt with. When the polyps are fresh and vigorous 

 narcotisation is not a difficult process, nor one requiring extreme 

 care, but should the hydroids be left twenty-four hours or so 

 before dealing with them the process is likely to be not only 

 tedious but generally unsuccessful. When the polypites are 

 judged to be sufficiently narcotised to permit of killing the 

 tentacles should be pricked with a needle somewhat roughly, to 

 be quite certain that narcotisation is sufficient to prevent retrac- 

 tion of the tentacles. I learnt by experience that even when 

 insensibility was apparently well established, on the application 

 of the killing and fixing agent the polypites would withdraw at 

 least partially, perhaps wholly, into the calycles, so that it is 

 necessary to be quite sure that narcotisation is complete before 

 using the fixing fluid. The killing and fixing agent most con- 

 venient is undoubtedly osmic acid, either a plain 1-per-cent. 

 solution or combined with platinum chloride as in Hermann's 

 solution. I have tried many other solutions for this purpose, 

 but found none more suitable. The osmic-acid solution is sprayed 

 over the colony in the watch-glass with a pipette and allowed to 

 act for several minutes, when it is washed away by repeated 

 changes of clean fresh water, allowing the specimens to soak in 

 each wash water for some time. Finally they are given a 

 weak bath of hydrogen peroxide or potassium ferrocyanide, to 

 thoroughly eliminate the acid, and again well washed. This is 

 the procedure for Calyptoblastic hydroids, the Gymnoblastic 

 may have a little more cavalier treatment. If narcotisation is 

 attempted with them it has the effect of causing them to 

 gradually shorten the tentacles, and once that has taken place 

 they never extend them again while under the influence of the 

 narcotic. This being the case the best method is to kill them 

 suddenly with an energetic killing agent while fully expanded. 

 Some retraction of the tentacles may take place in the killing, 

 but to nothing like the extent that would happen if narcotisa- 

 tion were attempted. It is unfortunate that mono-bromide of 

 camphor is insoluble in sea-water, as I am convinced, from the 

 admirable results it gives with Cordylophora and Hydra, that 



