350
PREPARATION OF MARINE ANIMALS.
dissolved out from the shell when kept in alcohol. The dark
eyes are very conspicuous in the partially developed attached
bunch of eggs.
Lastly, I describe cases in which it is desirable to remove
some of the pigments by bleaching. Sepiola, like several other
animals, acquires a spurious brown colour on keeping, which
more or less completely hides the true structure. I hope to find
some method which will prevent this change, but so far my best
results have been obtained by removing much of this colour by
keeping for some time in dilute sulphurous acid the animals
which had been allowed to die in salt-water with their pigment
cells contracted. By this means I have been able to prepare
specimens showing the detached pigment cells, and a good deal
of the internal structure.
Some remarkable results may be obtained with small flat
fish, especially plaice. When mounted in their natural state,
the distribution of the blood vessels is a good deal obscured by the
bones. If the fish are first kept in alcohol, and afterwards in
diluted sulphurous acid, the earthy matter of the bones is
removed, the thickness of the specimen is much reduced, and the
colour made paler, so that the blood-vessels are fairly well seen.
It is, however, far better to put the newly caught animals into
diluted formalin, and afterwards keep them for some weeks in
diluted sulphurous acid exposed to strong light. By this means
the general colour is much reduced, and when dried and
mounted many of the blood vessels and the general structure
are displayed. The action of the formalin differs considerably
from that of alcohol, since the dried muscular tissue shows
beautiful interference colours when illuminated obliquely, which
is not the case if the animals have been kept in alcohol.
In what I have said, I have confined myself mainly to the
more or less chemical part of my subject, but the successful
mounting of the preparations involve many mechanical details.
Moreover, when the animals are properly arranged and so dried
on the glass as not to be distorted by irregular lateral shrinkage,
much management is sometimes necessary to mount them in
balsam entirely free from bubbles of air. To describe all the
details requisite in different cases would, however, make this
communication much too long, and would require another class
of illustrations.