Whatsoever hath no fins nor scales in the waters, that shall be an abomination unto you.
Whatsoever hath no fins nor scales ... Fins are pectoral, ventral, dorsal, anal, and caudal. True fish have all, or at least some, of these. They are instruments of locomotion. But pectoral fins are also in some way connected with breathing, and these are possessed by all the real piscatory tribe. Many aquatic animals, however, have different respiratory organs, and are not furnished with any fins.
With regard to scales, which are a protection to their soft and flexible skin, Kirby ('Bridgewater Treatise,' 2:,
p. 376) remarks, scarcely any species of fish is without them. In some fish, upon which, when living, scales are not discoverable under a microscope, when they are dead, and the skin is dry, scales are readily detected and detached. Others, however, are quite destitute of them. The language of the sacred historian must be considered as used in a popular way, and applied to scales which are easily discernible by the naked eye.
The possession or the want of fins and scales has an essential influence in affecting the flesh of fish as an article of human food. With respect to aquatic animals, some, as the great majority of marine fish, inhabit salt water only, while others live in rivers and fresh-water lakes. Some frequent salt water at one time, and fresh at another, as the salmon, sturgeon, etc., while some have their habitat in brackish waters, as several flat-fish and shellfish: and all these varieties seem to be comprehended by the words, "in the waters, in the seas, and in the rivers." Under the rule prescribed by the sacred historian, the shark, the ray, and the sun-fish, the phoca (seal), and the walrus, frogs, eels, shellfish of all descriptions were included as unclean. Many of the latter (shellfish) enjoy a reputation they do not deserve, and have, when plentifully partaken of, produced effects which have led to a suspicion of their containing something of a poisonous nature.