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Eels have no ventral fins; either they have no scales or these are so small as to be hardly visible; their fins are soft, without spines; the gill openings are very small; the vertebrae extend in a straight line to the tip of the tail; and a single fin runs over the back, around the tail and forward on the belly with no separation into dorsal, caudal, and ventral portions. All the species of eels known from the Gulf of Maine have pectoral fins, but most of the morays of warmer seas are without pectorals. There are several other fishes of eel-like form in the Gulf of Maine, viz., the hag and the lampreys the rock eel (Pholis); the snake blenny (Lumpenus); the wrymouth (Cryptacanthodes); the eel pout (Macrozoarces); and the sand eel (Ammodytes). But the jawless, sucker-like mouth of the first two separates them, at a glance, from the true eels, while there either is a well-marked separation between anal and caudal fins in all the rest; or they have ventral fins (large or small), or the dorsal fin is spiny, not soft.

Only five true eels are known from the Gulf of Maine: the common eel (p. 151), the slime eel (p. 157), the conger (p. 154), the snipe eel (p. 159), and the snake eel (p. 159), which fall into five different families according to American usage. A sixth species, the long-nosed eel (a deep-water form p. 158) is to be expected in the deepest parts of the Gulf though it has not actually been recorded there as yet. The group likewise includes the morays of warm seas and sundry deep-sea forms, some of them exceedingly bizarre in appearance.

Common, conger, slime, and long nosed eels look much alike in general form, but are separated from one another by the size of the mouth and by the relative lengths of the fins. In the snipe eels the two jaws are prolonged into a very long slender beak, recalling that of a silver gar, the tail is whiplike, the neck noticeably slimmer than the head, and the general form extremely slender, while the snake eel is very slender with a hard pointed tail.