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1. All animals possess in common those parts by which they take in food, and into which they receive it. But these parts agree or differ in the same way as all the other parts of bodies, that is, either in shape or size, or proportion or position; and besides these, almost all animals possess many other parts in common, such as those by which they reject their excrements, (and the part by which they take their food,) though this does not exist in all. The part by which the food is taken in is called the mouth, that which receives the food from the mouth is called the stomach. The part by which they reject the excrement has many names.
2. The excrement being of two kinds, the animals which possess receptacles for the fluid excrement have also receptacles for the dry; but those which have the latter are not always furnished with the former. Wherefore all animals which have a bladder have a belly also, but not all that have a belly have a bladder; for the part appropriated to the reception of the liquid excrement is called the bladder, and that for the reception of the dry is called the belly.
3. Many animals possess both these parts, and that also by which the semen is emitted. Among animals that have the power of generation, some emit the semen into themselves, and some inject it into others . The former are called female, the latter male. In some animals there is neither male nor female, and there is a diversity in the form of the parts appropriated to this office. For some animals have a uterus, others have only something analogous to the uterus. These are the most essential organs; some of which exist in all animals, others in the majority only.
Source: Aristotle, History of Animals; translated by Richard Cresswell, M.A. (1862)