=The lampreys and hag-fishes (Cyclostomata).=--The next class of fish-like animals is that of the lampreys (fig. 113) and hag-fishes, the Cyclostomata. The lampreys and hags are easily distinguished from the true fishes by their sucking mouth without jaws, their single median nostril, their eel-like shape and lack of lateral appendages or paired fins. The hag-fishes (_Myxine_), which are marine, attach themselves by means of a sucker-like mouth to living fishes (the cod particularly), gradually scraping and eating their way into the abdominal cavity of the fish. These hags or "borers" "approach most nearly to the condition of an internal parasite of any vertebrate." The lampreys, or lamprey-eels as they are often called because of their superficial resemblance to true eels, are both marine and fresh-water in their habitat, and most of them attach themselves to live fishes and suck their blood. They also feed on crustacea, insects, and worms. The brook-lamprey, _Lampetra wilderi_, is never parasitic. It reaches its full size in larval life and transforms simply for spawning. The sea- and lake-lampreys ascend small fresh-water streams when ready to lay their eggs, few living to return. Sometimes small piles of stones are made for nests. The young undergo a considerable metamorphosis in their development. The largest sea-lampreys reach a length of three feet. The common brook-lampreys are from eight to twelve inches long only. =The true fishes (Pisces).=--All the other fish-like animals are grouped in the class Pisces. They are characterized, when compared with the lower fish-like forms just referred to, by the presence of jaws, shoulder girdle, and pelvic girdle. The class includes both the cartilaginous and bony fishes, and is divided into three sub-classes, namely, the Elasmobranchii, including the sharks, rays, skates, torpedoes, etc., the Holocephali, including the chimæras (a few strange-bodied forms), and the Teleostomi, including all the other fishes, as the trout, catfishes, darters, bass, herring, cod, mackerel, sturgeons, etc., etc.