The commencement of the history of Ichthyology coincides with that of Zoology generally. ARISTOTLE (384–322 B.C.) had a perfect knowledge of the general structure of fishes, which he clearly discriminates from the Aquatic animals with lungs and mammæ, _i.e._ Cetaceans, and from the various groups of Aquatic Invertebrates. He says that “the special characteristics of the true fishes consist in the branchiæ and fins, the majority having four fins, but those of an elongate form, as the eels, having two only. Some, as the _Muræna_, lack the fins altogether. The Rays swim with their whole body, which is spread out. The branchiæ are sometimes furnished with an opercle, sometimes without one, as is the case in the cartilaginous fishes.... No fish has hairs or feathers; most are covered with scales, but some have a rough or smooth skin. The tongue is hard, often toothed; and sometimes so much adherent that it seems to be wanting. The eyes have no lids; nor are any ears or nostrils visible, for what takes the place of nostrils is a blind cavity. Nevertheless they have the senses of tasting, smelling, and hearing. All have blood. All scaly fishes are oviparous, but the cartilaginous fishes (with the exception of the Sea-devil, which Aristotle places along with them) are viviparous. All have a heart, liver, and gall-bladder; but kidneys and urinary bladder are absent. They vary much in the structure of their intestines: for whilst the mullet has a fleshy stomach like a bird, others have no stomachic dilatation. Pyloric coeca are close to the stomach, variable in number; there are even some, like the majority of the cartilaginous fishes, which have none whatever. Two bodies are situated along the spine, which have the function of testicles, and open towards the vent, and which are much enlarged in the spawning season. The scales become harder with age. Not being provided with lungs, they have no voice, but several can emit grunting sounds. They sleep like other animals. In the majority the females exceed the males in size; and in the Rays and Sharks the male is distinguished by an appendage on each side of the vent.” Aristotle’s information on the habits of fishes, their migrations, mode and time of propagation, utility, is, as far as it has been tested, surprisingly correct. Unfortunately, only too often we lack the means of recognising the species of which he gives a description. His ideas of specific distinction were as vague as those of the fishermen whose nomenclature he adopted; it never occurred to him that such popular names are subject to change, or may be entirely lost with time, and the difficulty of deciphering his species is further increased by the circumstance that popular names are often applied by him to the same fish, or that different stages of growth are designated by distinct names. The number of fishes known to Aristotle seems to have been about 115, all of which are inhabitants of the Ægean Sea.