THE MORTALITY OF CRAYFISHES.

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ning of this chapter, so far as such obscure consciousness accompanies the molecular changes of its nervous substance, it will be right to speak of the mind of a crayfish. But it will be obvious that it is merely putting the cart before the horse, to speak of such a mind as a factor in the work done by the organism, when it is merely a dim symbol of a part of such work in the doing.

Whether the crayfish possesses consciousness or not, however, does not affect the question of its being an engine, the actions of which at any moment depend, on the one hand, upon the series of molecular changes excited, either by internal or by external causes, in its neuromuscular machinery; and, on the other, upon the disposition and the properties of the parts of that machinery. And such a self-adjusting machine, containing the immediate conditions of its action within itself, is what is properly understood by an automaton.

Crayfishes, as we have seen, may attain a considerable age; and there is no means of knowing how long they might live, if protected from the innumerable destructive influences to which they are at all ages liable.

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

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

THE OVARY AND THE TESTIS.

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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

FIG. 30.-Astacus fluviatilis.—The female reproductive organs (× 2); ov, ovary; od, oviduct; oď, aperture of oviduct.

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.

The testis (fig. 31, t) is somewhat similar in form to the ovary, but, the three divisions are much narrower

and more elongated: the hinder median division lies under the heart; the anterior divisions are situated between the heart behind, and the stomach and the liver in front (figs. 5 and 12, t). From the point at which the

vd

vd'

FIG. 31.-Astacus fluviatilis.—The male reproductive organs (× 2); t, testis; rd, vas deferens; vd', aperture of vas deferens.

three divisions join, proceed two ducts, which are termed the vasa deferentia (fig. 31, vd). These are very narrow, long, and make many coils before they reach the apertures upon the bases of the hindermost pair of walking limbs, by which they open externally (fig. 31, vď', and fig. 35, vd). Both the ovary and the testis are very much larger

THE OVARY AND THE EGGS.

131

during the breeding season than at other times; the large brownish-yellow, eggs become conspicuous in the ovary,

FIG. 32.-Astacus fluviatilis.-A, a two-thirds grown egg contained in its ovisac (x 50); B, an egg removed from the ovisac (× 10); C, a portion of the wall of an ovisac with the adjacent portion of the contained egg, highly magnified; ep, epithelium of ovisac gs, germinal spots; gv, germinal vesicle; m, membrana propria; v, vitellus; vm, vitelline membrane; w, stalk of ovisac.

;

and the testis assumes a milk-white colour, at this period.

The walls of the ovary are lined internally by a layer of