PRESERVATION OF THE HYDROIDA. 149 



it would form an excellent narcotiser for this division. Lang's 

 fluid is a good killing agent for the Gymnoblastea, and, of course, 

 assists staining if carmine is used. Picric acid also answers well ; 

 and osmic acid or Hermann's solution if the specimens are not 

 too large, otherwise I have found the killing occupy sufficient 

 time to permit of considerable contraction. 



Undoubtedly the great difficulty in preparing hydroids for the 

 microscope lies in getting clean mounts that is, supposing clean 

 mounts are desired and this difficulty becomes augmented with 

 material from between tide-marks. The polyparies are generally 

 encrusted and overgrown with an olla podrida of marine life, so 

 that the mount really becomes a compound object. To me 

 this is anything but a drawback, providing, of course, that no 

 essential part of the hydroid is masked. On some shores, how- 

 ever, the amount of material collected is out of all proportion to 

 its interest, and it becomes necessary to subject it to a cleansing 

 process. This should be done before narcotising and killing the 

 hydroid, otherwise the tentacles are liable to be injured and 

 entangled. The polyps withdraw into their calycles during the 

 application of the brush, but soon recover from their fright when 

 placed in a glass of clean fresh water and allowed to rest quietly 

 for a time. 



If staining and mounting fixed material are deferred until a 

 more convenient time, it has to be stored in some preservative 

 fluid ; formalin at once suggests itself, for which reason I 

 wish to utter a word of warning. Formalin is perfectly satis- 

 factory for objects that are to be mounted unstained, as they 

 will eventually find a permanent home in this medium, but 

 personally I have not been successful in staining material that has 

 been stored for some months in a 5-per-cent. solution of formalin. 

 This, of course, may be due to some error peculiar to myself, but 

 I would offer this warning to inexperienced workers. If time 

 and facilities allow I would strongly advise the beginner to stain 

 straight away, and if unable to mount, then store the stained 

 material in 70-per-cent. alcohol. Tailing this, I think it prefer- 

 able to store those hydroids destined for staining in 70-per-cent. 

 alcohol. In storing avoid the error of putting too many into one 

 tube ; small tubes with a few in each are very much better than 

 a heterogeneous collection of species in a large tube or bottle. 



The microscopist will doubtless have his own pet stain or stains, 



