This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
A. How does a fish breathe? Find the gills, - red fringes back of the head. As the water taken into the fish's mouth passes out through the gills, the air dissolved in it gives oxygen to the blood. B. What covering has the fish? Are the scales attached at their rear or their front ends? Is there a reason for this? Over the scales lies a thin skin, often containing coloring matter. Mackerel, butterfish, and a few others have no scales. C. An air bladder under the spine keeps the fish afloat.
In a fresh fish the gills are a bright red, the eyes bulging and bright, the flesh along the back-bone firm and elastic. If the fish can be dented by a finger, do not buy it.
Scrape off any scales which have not been removed. Work from tail to head, slanting the knife toward you to prevent scales from flying. Wash the fish inside and out with a cloth wet in cold salt water, and dry with a clean cloth. If the fish is to be broiled or fried, cut off the head and the tail and split it down the back; if to be boiled, cut off the head only; if to be baked, leave whole.
Fish suitable for baking whole are: cod, haddock, blue-fish, small salmon, bass, shad, whitefish.