It is a widely received notion that the energies of living matter have a natural tendency to decline, and finally disappear; and that the death of the body, as a whole, is the necessary correlate of its life. That all living things sooner or later perish needs no demonstration, but it would be difficult to find satisfactory grounds {128} for the belief that they must needs do so. The analogy of a machine that, sooner or later, must be brought to a standstill by the wear and tear of its parts, does not hold, inasmuch as the animal mechanism is continually renewed and repaired; and, though it is true that individual components of the body are constantly dying, yet their places are taken by vigorous successors. A city remains, notwithstanding the constant death-rate of its inhabitants; and such an organism as a crayfish is only a corporate unity, made up of innumerable partially independent individualities. Whatever might be the longevity of crayfishes under imaginable perfect conditions, the fact that, notwithstanding the great number of eggs they produce, their number remains pretty much the same in a given district, if we take the average of a period of years, shows that about as many die as are born; and that, without the process of reproduction, the species would soon come to an end. There are many examples among members of the group of _Crustacea_ to which the crayfish belongs, of animals which produce young from internally developed germs, as some plants throw off bulbs which are capable of reproducing the parent stock; such is the case, for example, with the common water flea (_Daphnia_). But nothing of this kind has been observed in the crayfish; in which, as in the higher animals, the reproduction of the species is dependent upon the combination of two kinds of living {129} matter, which are developed in different individuals, termed _males_ and _females_. These two kinds of living matter are _ova_ and _spermatozoa_, and they are developed in special organs, the _ovary_ and the _testis_. The ovary is lodged in the female; the testis, in the male. The _ovary_ (fig. 30, _ov_) is a body of a trefoil form, which is situated immediately beneath, or in front of, the heart, between the floor of the pericardial sinus and the alimentary canal. From the ventral face of this organ two short and wide canals, the _oviducts_ (_od_), lead down to the bases of the second pair of walking limbs, and terminate in the apertures (_od′_) already noticed there. [Illustration: FIG. 30.—_Astacus fluviatilis._—The female reproductive organs (× 2); _ov_, ovary; _od_, oviduct; _od′_, aperture of oviduct.] [Illustration: FIG. 31.—_Astacus fluviatilis._—The male reproductive organs (× 2); _t_, testis; _vd_, vas deferens; _vd′_, aperture of vas deferens.]