Cold-blooded Vertebrates (p. 19). The heart has only one ventricle and two auricles (cp. p. 21). The skin is naked, damp, usually slippery and smooth; it helps in respiration. Although several Amphibians resemble various Reptiles in outward appearance (Newt and Lizard, Figs. 50 and 46), the structure of the skeleton is quite different. Upon the whole, Amphibians resemble fishes more closely, and they breathe by gills when they are young, which strengthens the agreement. They undergo a metamorphosis. The just-hatched young at first hold fast by suction to the jelly of the spawn; they have external gills. They quickly develop a membranous margin to the body, especially round the tail, which consequently forms a sort of oar. Meanwhile internal gills develop, and the external ones disappear. The larvæ are now fish-like (“tadpoles”). The limbs quickly grow out, the lungs develop, the tail shrivels up, and the animal leaves the water. Even the adult, however, lives among damp surroundings. In the adult state the Amphibians feed on insects, worms, and snails; many species are exceedingly useful owing to this. Amphibians are divided into two orders, the _Urodela_ (Newts, etc.) and the _Anura_ (Frogs and Toads). To the first belong the =Great Crested Newt= (_Triton cristatus_) and the =Common Newt= (_Lissotriton tæniatus_). Examples of the Anura are the =Edible Frog= (_Rana esculenta_), the brownish =Common Frog= (_R. temporaria_, Fig. 51), the =Common Toad= (_Bufo vulgaris_), and the =Natterjack= (_B. calamita_, Fig. 52). [Illustration: FIG. 51.—Common Frog (_Rana temporaria_). ] Both frogs and toads do good by devouring many noxious insects, and also, in particular, snails. Toads are often kept in greenhouses for this purpose; and in the research garden attached to the Rouen entomological laboratory the snails were entirely exterminated in 1891, as a result of introducing a hundred toads and ninety frogs. [Illustration: FIG. 52.—Natterjack (_Bufo calamita_). ] CLASS V.: =PISCES= (FISHES).