Am) PHYSIOLOGY OF THE SPONGIADAE. 
1131 
with impunity; but the amount of salt thus left within them will in time cause con- 
siderable mischief to the specimen. After such specimens have been once thoroughly 
dried and their examination has been completed, they may be plunged into cold water 
for a few minutes, and the water then ejected by a rapid centrifugal motion of the arm, 
and this operation repeated two or three times ; the specimen should be again rapidly 
dried, and it will then keep well in the cabinet and preserve all its characteristic features. 
It is a bad habit to soak marine specimens for a considerable time in fresh water to 
extract the salt, as by this mode of proceeding the minute and delicate characters of the 
object are to a great extent destroyed. 
The most advisable mode of proceeding, in the examination of an unknown species, is, 
first to note the general peculiarities of form and surface as presented to the unassisted 
eye. After the noting of the external character, the next step should be to cut a slice 
out of the sponge, to about half an inch or more in depth, at right angles to the surface, 
taking special care that a due 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 sub- 
sided to the bottom of the test-tube, so that the greater portion of the acid may be 
decanted oflf and its place be supplied with distilled water ; and this operation should be 
repeated three or four times with much care. The spicula thus prepared 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 pro- 
cedure 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 undissolved gemmules and 
portions of the membranes are found, that are very suggestive for the further examina- 
tions 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 proportions of the internal organs. 
The general characters of these sections should be observed with a half-inch or two- 
