1884.] 



MICROSCOPICAL JOURNAL. 



49 



to our subject than by mere similarity 

 of form. My present use for such 

 examples, however, is as types of the 

 appearence of circular areolae in sili- 

 cious films. As there is no dispute 

 that the hexagons are true areola, 

 there can be as little that these circles 

 in the same valve are such. It is 

 well, therefore, to obsei"ve them with 

 ditferent powers, and to get with low 

 objectives their appearance under 

 similar apparent magnification to that 

 which the finer dots will have under 

 high powers. With a good glass at 

 best adjustment the margin of the 

 areolcE wmII be sharply defined, the 

 thicker part of tht shell will be of a 

 delicate pink color, and the light in 

 the centre of the ai'eolae a greenish 

 white which corresponds closely with 

 the color of the empty part of the 

 field. It is worth while to spend 

 some time in making these appear- 

 ances familiar ; for by studying them 

 V^oth when beyond and when within 

 the focus, we shall get a good test of 

 the way in which circular areolae 

 behave, by which we may judge of 

 other much smaller dots which we 

 shall have occasion to examine. 

 ( To be lontinucd.) 

 o 



A New Method of Mountiiig Hydra. 



Who that has seen the beautiful 

 Hydra viridis gracefully waving its 

 delicate tentacles, while swaying from 

 a rootlet of Lemna or a filament of 

 Vaucheria^ has not wished to per- 

 manently preserve the pretty zoophyte 

 thus expanded? The ardent micro- 

 scopist will probably have foimd the 

 accomplishment of this to be no easy 

 matter, for Hydra has an aggravating 

 way of dying only vvlien retracted into 

 a dimipy little lump, studded with 

 stubby knobs where a moment before 

 the long arms were waving from a 

 slender body. I have spent many an 

 hour in endeavoring to devise some 

 plan by which this interesting little 

 polype could be killed so quickly as 

 not to give it time to contract. The 

 various fixing fluids which have been 



from time to time recommended, such 

 as osmic, pyroligneous and picro-sul- 

 phuric acids, were duly tried, as was 

 also the method of gradually stupefy- 

 ing the animal with small successive 

 doses of alcohol ; but the results in 

 each case were far from being satis- 

 factory. At last the following simple 

 process was hit upon, which has ac- 

 complished the desired end more 

 perfectly than any I have seen pub- 

 lished : — 



Have in readiness a slide upon 

 which a well-dried cell of sufficient 

 depth has been turned. Then, from 

 a gathering of hydi"a, transfer a suffi- 

 cient number of individuals (the more 

 fidly developed the better) very care- 

 fully by means of a camel's-hair brush 

 or a pipette to a drop of water spread 

 near the end of a plain glass slide, 

 and place the latter upon a table in 

 such a way that the end with the 

 drop projects about two inches over 

 the edge. This is easily done by 

 placing a weight upon the opposite 

 end. After allowing the slide to re- 

 main perfectly undisturbed for three 

 or four minutes, hold a lighted coal- 

 oil lamp so that the top of its chim- 

 ney is very near the slide, but a trifle 

 above it. The hydras will then ap- 

 pear brightly illuminated, and it can 

 be easily determined by the unaided 

 eye whether or not their tentacles are 

 fully extended. If they are, quickly 

 move tlie lamp directly under the 

 drop, with the top of the chimney 

 about an inch beneath the slide, and 

 hold it in that position for about 3—5 

 seconds, the exact time depending 

 principally upon the intensitv of the 

 heat. Then quickly remove the slide, 

 and place it upon a slab of marble or 

 metal. When cool pour the drop 

 containing the zoophytes into the pre- 

 pared cell on the slide which has 

 been held in readiness ; add a drop 

 or two of a suitable preservative fluid, 

 arrange the little animals, if neces- 

 sary, by means of a needle or camel's- 

 hair brush (using very great care, 

 however, as the tentacles will be de- 

 stroyed by the least rough handling) , 



