Clear out all the flesh and preserve the skin in a very strong solution of salt and water (what is known to chemists as a "saturated solution"), or in alcohol if you have it to spare.
Preparing Rough Skeletons.—In about seven cases out of ten, it is a far easier and more simple matter to rough out, clean, and mount the complete skeleton of a fish than the uninitiated would naturally suppose. A few fishes, such as the shad, have more bones than the law allows, and the preparation of a complete skeleton thus becomes a practical impossibility. Fortunately, however, most fishes are more reasonable in the matter of bones, and to these we direct our efforts.
First and foremost, study the bony structure of a typical scale fish, learn what its principal parts are, and how they are articulated. Learn how the ribs lie, and how a row of slender, riblike bones called appendices, or epipleural spines, are attached to the true ribs, and at their outer extremities touch the inside of the skin along the lateral line of the fish. If you will take a good-sized perch as your first subject, you will not be troubled with any osteological extras, and the process will be as follows:
Lay the perch upon its side, and with a sharp scalpel cut away the skin from the whole of the exposed side. Remove all the viscera. By careful examination, ascertain the exact location of the ribs, and particularly the row of epipleural spines attached at the upper ends of the former. With a broad, flat bone-scraper, or your knife-blade if you have nothing better, begin at the lateral line of the fish, and work toward the top of the back, taking the flesh away in chunks as you go. In a very short time the vertebræ and the interhæmal spines are exposed, and with a narrower bone-scraper the flesh is easily removed from them.
Now turn the fish around, and with great care cut and scrape the flesh away from the ribs and the epipleural spines. Do not on any account detach the latter from the former, but at this stage leave them attached to each other by a thin strip of flesh for their better protection.
Do not separate the ventral fins by cutting through the pubic arch, but with your small, curve-ended bone-scraper remove the flesh from the angular recesses of these bones, and leave the anterior end of the pubic arch attached to the coracoid. Next, pick out the flesh from around the base of the pectoral fin, remove the eye from its socket, and whatever flesh the skull contains. Thus does the bony structure of one entire side stand revealed. The gills are of course to remain in place, as the skeleton would not be complete without them.
There is but one thing more to add. In treating the other side of the fish in a precisely similar manner, care must be taken to not disturb the attachment of the interneural and interhæmal spines which join the dorsal and anal fin rays to the processes of the vertebral column.
Having thus denuded the fish of its flesh, lay the skeleton in a pan of water, and with a moderately soft tooth-brush, or nailbrush, brush it carefully to wash away all blood and mucus. If the bones are full of blood (which is very rarely the case), the skeleton must be soaked in clear water for an hour or two, or longer if necessary, to soak out the blood, so that it will not dry in the bones and permanently disfigure them.
Rough skeletons of fishes may be preserved in alcohol, but for many reasons it is much the best to dry them. Poison them with dry arsenic; do not put upon them either salt, arsenical soap, or alum, hang each one up by the head, and see that it dries in good shape. The pectoral fins should lie well down upon the ribs for mutual protection.