226 ANATOMY AND PHYSIOLOGY 



special care that a clue proportion of the dermal membrane 

 is included ; this should be placed in a long narrow test- 

 tube, in about an inch deep of nitric acid, in which it should 

 be gently and cautiously boiled over a very small flame 

 until the sponge is entirely dissolved, and then set by until 

 the acid is quite cold and the spicula have subsided to the 

 bottom of the test-tube, so that the greater portion of the 

 acid may be decanted off and its place be supplied with 

 distilled water ; and this latter operation should be repeated 

 three or four times with much care. The spicula thus pre- 

 pared should be placed in a watch-glass with a little distilled 

 water, and the whole stirred up so that an average sample 

 can be obtained for microscopical examination. By this 

 mode of procedure a general view of the whole of the 

 spicula belonging to the species will be obtained, which will 

 serve as a guide to the subsequent modes of examination. 



The boiling in nitric acid should not be continued beyond 

 the time of the piece of sponge falling completely separated 

 to the bottom. If stopped at this period by the addition 

 of a little distilled water, it frequently occurs that undis- 

 solved gemmules and portions of the membranes are found 

 that are very suggestive for the further examination of the 

 specimen. 



The next step should be to take a thin slice from the 

 surface of the sponge, and place it in a cell in a little 

 distilled water, for the purpose of the examination of the 

 structural peculiarities of the dermal membrane. Then take 

 a thin slice from the body of the sponge at right angles to 

 its surface, and mount it in a similar manner for the purpose 

 of ascertaining the nature and peculiarities of its skeleton 

 and other internal organs. These two sections should be 

 carefully examined with the microscope, and if they be not 

 sufficiently characteristic fresh ones should be mounted. If 

 the specimens thus treated be taken from sponges properly 

 preserved, their tissues will expand and assume very much 

 the appearance of those of the living sponge, and they will 

 as nearly as possible exhibit the natural positions and pro- 

 portions of the internal organs. 



The general characters of these sections should be observed 



