396 The Microscope. 



unfamiliar objects occur. I am aware of two species found in this 

 way, that must certainly have remained undiscovered if the pocket- 

 lens had not been used. Most of the freshwater sponges, however, 

 must be taken home and pei-manently mounted before the finer parts 

 of the spicula or statoblasts, by which species are determined, can be 

 defined. The statoblasts are the small, seed-like bodies distributed 

 through the mass of the sponge. They are considered to be winter- 

 eggs or resting- spores. Specimens may be mounted either before or 

 after drying, the process being the same in both cases. A piece of 

 the sponge, usually the smaller the better, is placed on a slide and 

 subjected to the action of some clearing agent by which it can be 

 made transparent. For this purpose some who are skilled in mount- 

 ing sponges use a drop of strong nitric acid, which is placed on the 

 specimen and afterwards heated over a lamp. The acid is then 

 washed out with pure water and if the object is sufficiently clear it 

 is dried and mounted in balsam. If it is not transparent the process 

 of heating in acid and washing out must be repeated till the desired 

 result is obtained. In my hands, however, the purest crystallized 

 carbolic acid, rendered fluid by a little heat, will produce as good or 

 better results with much less risk to the specimen. It will take but 

 a short time after the application of a drop of the acid to the 

 specimen before some parts of it, including, probably, some of the 

 statoblasts will be clear enough for examination. When sufficiently 

 transparent it may be mounted in balsam without washing, as the 

 carbolic acid and balsam mix freely. Thin sections of dried sponges 

 may be made by free-hand cutting or in the microtome by imbedding 

 in any suitable material. These cuttings if carefiilly placed on the 

 slide will show the statoblasts in section and the spicula in situ, 

 frequently with no other medium that thin balsam. Should this be 

 insufficient, however, a drop of the carbolic acid may be appHed. 

 Sections of the statoblasts are beautiful and instructive objects, 

 affording knowledge of their various parts as well as of the arrange- 

 ment of the spicula better than any other form of mounting. They 

 may be cut through on the slide with a thin knife when that is most 

 convenient, as is often the case. 



The fleshy mass of the sponge may be destroyed by boiling the 

 specimen in nitric acid or by soaking it for a time in Labarraque's 

 solution of chlorinated soda. The spicula thus freed may be mounted 

 in balsam in the same manner as diatoms. The spicula to be noticed 

 first are those of the skeleton, which are the largest. These usually 

 protrude through the dermal covering and give, the slightly bristly 



