The Plagiostomi comprise the Sharks (fig. 45) and Dog-fishes, a most voracious tribe; the White Shark often attains the length of twenty feet. They are terribly famous for their dreadful powers of destruction, swallowing all that comes before them; they are able to bite in two, with one snap of their formidable jaws, the body of any one unfortunate enough to fall in their way; their mouths are armed with several rows of short spear-shaped teeth; the mouth is placed a considerable distance under the head, so that when biting they turn on one side or quite over; they have no gills, but the water passes through five oblong openings on each side. Sailors hold this terrible inhabitant of the deep in great detestation, as well they may, for there are few who have spent many years at sea but can tell of a friend or messmate who has been mutilated or devoured by the Sharks. When sailors catch one, it is hauled on deck and killed with great gusto; the stomach is opened and its contents examined, and it generally contains some terrible mementoes in the form of buttons, buckles, &c., the only relics of its victims. 3. CYCLOSTOMI (Round-mouthed). [Illustration: FIG. 46.--THE LAMPREY (Petromyzon fluviatilis) (_b_, the sucker).] The Lamprey or Lampern (fig. 46) inhabits both rivers and lakes, and some species, the sea; it has a round mouth, by which it affixes itself to rocks, &c., and also to other fish, it is furnished with teeth on the outer edge, it has no scales, but is covered with a glutinous skin; in form it much resembles an Eel, and is from twelve to eighteen inches long. The Lamprey was once very plentiful in the Thames, and considered a dainty. [Illustration: FIG. 47.--1, FROG (Rana temporaria). 2, TOAD (Bufo vulgaris). 3, TADPOLES.] REPTILES. These constitute the second class of the Vertebrate animals. They are divided into four orders. 1. AMPHIBIA (double-lived). By some naturalists a separate class is made out of the Amphibia, including the Frog kind, Toads, and Newts (fig. 47), which they call "Batrachia," and which takes a place between Reptiles and Fishes, as the young of this class have a totally different form from the adult animal, and much resemble a fish, breathing by gills and having no legs, but as they become mature the gills are replaced by lungs, and the legs make their appearance.