1. Among aquatic animals, there is one class of fish, which embraces many forms, and is separated from other animals, for it has a head, and upper and lower parts, in which last are the stomach and bowels, and a continuous and undivided tail. This is not alike in all. They have neither neck nor limb, nor internal and external testicles, nor mammæ, nor has any other animal mammæ that is not viviparous, nor indeed all viviparous animals, but those only that are internally viviparous, and not first of all oviparous. For the dolphin is a viviparous animal, wherefore it has two mammæ, not indeed above, but near the organs of reproduction. It has not evident nipples, but, as it were, a stream flowing from each side. From these the milk exudes, and the young ones suck as they follow the mother. This has been distinctly observed by some persons. 2. But fish, as we have observed, have neither mammæ nor any external passage for the genital organs. In the branchia they have a distinctive organ, through which they eject the water they have received into their mouths; and they have fins, most fishes have four, but the long fishes, as the eel, have only two placed near the branchia, and in this respect the cestreus,[40] a fish in the lake of Siphæ, is similar to the eel,[41] and so is the fish called tænia.[42] Some of these long fish have no fins, as the muræna, nor have they divided branchia like other fish. [40] Mugil, mullet. [41] Muræna anguilla. [42] Perhaps Cepola tænia. 3. Some fish with branchia have coverings over their branchi; in all the cartilaginous fishes they are uncovered. All fishes that have coverings have the branchia placed on their sides; among the cartilaginous fishes some are broad in the lowest part, as the narce[43] and the batos;[44] some very long in the sides, as all the galeodea.[45] In the batracus,[46] although the branchia are on the sides, they are covered with a coriaceous, not a prickly membrane, like those of fishes which are not cartilaginous. [43] Raia torpedo. [44] Raia batos. [45] The shark tribe. [46] Lophius piscatorius. 4. In some fishes with branchia they are single, in others double, but the last towards the body is always single. Some have but few branchia, others have many; but their number is always equal on both sides, and those with the smallest number have always one on each side; this is double in the capros;[47] others have two on each side, sometimes these are single, sometimes double, as in the conger[48] and the scarus;[49] others have four simple branchia on each side, as the ellops,[50] synagris, muræna, and eel; others have four, all divided except the last, as the cichle,[51] perca,[52] glanis,[53] and cyprinus;[54] all the galeodea have five double branchia on each side, the xiphias[55] has eight, which are double. This is the manner and number of the branchia of fishes. [47] Perhaps Cottus cataphractus. [48] Muræna conger. [49] Scarus cretensis.