1. In some animals the sexes are distinct, in others they are not so, these are said to beget and be with young by a likeness to other creatures. There is neither male nor female in fixed animals, nor in testacea. In the malacia and malacostraca there are male and female individuals, and in all animals with feet, whether they have two or four, which produce either an animal, an egg, or a worm from coition. 2. In other kinds the sexes are either single or not single; as in all quadrupeds there is the male and female, in the testacea it is not so, for as some vegetables are fertile and others barren, so it is in these. Among insects and fishes there are some that have no differences of this kind, as the eel is neither male nor female, nor is anything produced from them. 3. But those persons who say that some eels appear to have creatures like worms, of the size of a hair, attached to them, speak without observation, not having seen how they really are; for none of these creatures are viviparous without being first oviparous, none of them have ever been observed to contain ova; those that are viviparous have the embryo attached to the uterus, and not to the abdomen, for there it would be digested like food. The distinction made between the so-called male and female eel that the male has a larger and longer head, and that the head of the female is smaller, and more rounded, is a generic, and not a sexual distinction. 4. There are some fish called epitragiæ, and among fresh-water fish the cyprinus and balagrus are of the same nature, which never have ova or semen; those which are firm and fat, and have a small intestine, appear to be the best. There are creatures, such as the testacea, and plants, which beget, and produce young, but have no organ of coition; and so also in fishes the psetus,[156] erythrhinus,[157] and the channa. All these appear to have ova. [156] Pleuronectes Lingua and Rhombus. [157] Perca marina, or Sparus erithrinus. 5. In sanguineous animals with feet that are not oviparous, the males are generally larger and longer lived than the females, except the hemionus, but the females of this animal are both larger and longer lived; in oviparous and viviparous animals, as in fish and insects, the females are larger than the males, as the serpent, phalangium,[158] ascalabotes,[159] and frog; in fish likewise, as in most of the small gregarious selache, and all that inhabit rocks. [158] Aranca tarantula. [159] Lacerta Gekko.