Closely related to the frogs and toads are the efts so common in our ponds. These familiar English forms are represented in other countries of the Northern Hemisphere by creatures, some of which (as we shall hereafter see) are of very great interest indeed. The whole group constitutes the second Batrachian order--the order _Urodela_. One of the most noteworthy forms of the order is the eft _Proteus_, which inhabits the dark, subterranean caverns of Carniola and Istria. Allied to this is the _Menobranchus_ of North America and the Axolotl of Mexico. Other forms of the order are the American eft-genera _Spelerpes_ and _Amblystoma_, the _Menopoma_, and the gigantic Salamander (_Cryptobranchus_) of Japan and China, the eel-like _Amphiuma_--with its very long body and minute legs--and the two-legged _Siren_ of the United States. The third order of Batrachians is one which contains very few species, but these are very strange, for though allied to frogs they have the appearance of snakes, or rather perhaps of worms. With long and slender bodies (marked by many transverse wrinkles), devoid of every rudiment of limb, they remind us of the before-noticed _Anguis_, _Typhlops_, and _Uropeltis_ amongst reptiles. The Batrachians in question (which belong to the genera _Cæcilia_ and _Siphonops_) form the order _Ophiomorpha_. The fourth order of Batrachians is one which has entirely passed away and become extinct. It is the order _Labyrinthodonta_, and the species which composed it were, some of them, of large size, with great heads like those of crocodiles. Others bore more or less resemblance to enlarged _Ophiomorpha_. Every one knows that frogs begin their existence in the water as tadpoles, which have the habits and mode of life of fishes. Thus, the class _Batrachia_ naturally conducts us to the class _Pisces_, the class of true fishes. This class contains a prodigious variety of forms, and is far more rich in species than any other of the classes before enumerated--even that of birds. The fishes most familiar to us--such as the perch, carp, mackerel, cod, herring, sole, turbot, salmon, pike, dory, and eel--all belong to one great order called _Teleostei_, and which is made up of what are called "bony" fishes, though there are some bony fishes which do not belong to it. To the same order also belong the Muroena, the electric eel (_Gymnotus_), the flying fishes (_Exocetus_ and _Dactyloptera_), the sucking fish (_Remora_), the pipe-fish and sea-horse (_Hippocampus_), the diodon, the ostracion, the file-fish (_Balistes_), the largest of all fresh-water fishes (_Sudis gigas_ of South America), with a multitude of other forms. Certain more or less singular Teleosteans are classed together in a subordinate group of "Siluroids" (of which fish the _Silurus_ is a type), and which group includes, amongst others, the singular, cuirassed fish Callichthys.