HYDRAS. 171 



The Hydra also occasionally has another form of In- 

 fusorial parasite running over its skin. This is de- 

 pressed and somewhat kidney-shaped in contour, and 

 has cilia only on one surface, the lower or ventral 

 surface. It is caWtA Kerdna polypdrum. It is not so 

 common as Trichodina. 



If the observer desires to preserve the Hydra as a 

 permanently mounted object for the microscope, he 

 may be easily gratified, thanks to the late Mr. A. H. 

 Breckenfeld, of San Francisco, who has devised an 

 admirable method which the writer has tried and 

 recommends. Transfer the Hydras to a slip in a 

 large drop of water, where they can be seen if the 

 slide is held above white paper. When their tentacles 

 are fully extended, "quickly move the lamp directly 

 under the drop, with the top of the chimney about an 

 inch beneath the slide, and hold it in that position for 

 from three to five seconds, the exact time depending 

 principally upon the intensity of the heat. Then 

 quickly remove the slide and place it upon a slab of 

 marble or of metal. When cool, pour the drop contain- 

 ing the zoophytes into the prepared cell on the slide 

 which has been held in readiness; add a drop or two 

 of a suitable preservative fluid, arrange the little ani- 

 mals, if necessary, by means of a needle or a camel's- 

 hair brush (using very great care, however, as the ten- 

 tacles will be destroyed by the least rough handling), 

 cover with thin glass, and finish as in the case of any 

 fluid mount." I have not found it necessary to use 

 two slips of glass. If a deep shellac-cell that has been 

 made for some time and is perfectly dry and hard is 

 used, the Hydras may be placed in it and there cooked 

 and allowed to remain, as a high degree of heat is not 



