FIG. 4.-Astacus fluviatilis. -In A, the gills, exposed by the removal of the branchiostegite, are seen in their natural position; in B, the podobranchiæ (see p. 75) are removed, and the anterior set of arthrobranchiæ turned downwards (x 2): 1, eye-stalk; 2, antennule; 3, antenna; 4, mandible; 6, scaphognath te; 7, first maxillipede, in B the epipodite, to which the line points, is partly removed; 8, second maxillipede : 9, third maxillipede; 10, forceps; 14, fourth ambulatory leg; 15, first abdominal appendage; xv., first, and xvI., second abdominal somite; arb. 8, arb. 9, arb. 13, the posterior arthrobranchiæ of the second and third maxillipedes and of the third ambulatory leg; arb'. 9, arb'. 13, the anterior arthrobranchiæ of the third maxillipede and of the third ambulatory leg; pbd. 8, podobranchiæ of the second maxillipede; pbd. 13, that of the third ambulatory leg: plb. 12, plb. 13, the two rudimentary pleurobranchiæ; plb. 14, the functional pleurobranchia; r, rostrum.

waistcoat, if we suppose the lining of the flaps to be made in one piece with the sides of the waistcoat. Or a closer parallel still would be brought about, if the skin of a man's back were loose enough to be pulled out, on each side, into two broad flaps covering the flanks.

It will be observed that the branchial chamber is open behind, below, and in front; and, therefore, that the water in which the crayfish habitually lives has free ingress and egress. Thus the air dissolved in the water enables breathing to go on, just as it does in fishes. As is the case with many fishes, the crayfish breathes very well out of the water, if kept in a situation sufficiently cool and moist to prevent the gills from drying up; and thus there is no reason why, in cool and damp weather, the crayfish should not be able to live very well on land, at any rate among moist herbage, though whether our common crayfishes do make such terrestrial excursions is perhaps doubtful. We shall see, by-and-by, that there are some exotic crayfish which habitually live on land, and perish if they are long submerged in water.

With respect to the internal structure of the crayfish, there are some points which cannot escape notice, however rough the process of examination may be.

Thus, when the carapace is removed in a crayfish which has been just killed, the heart is seen still pulsating. It is an organ of considerable relative size (fig. 5, h), which is situated immediately beneath the

FIG. 5.-Astacus fluviatilis.—A male specimen, with the roof of the carapace and the terga of the abdominal somites removed to show the viscera (nat. size) :—aa, antennary artery; ag, anterior gastric muscles; amm, adductor muscles of the mandibles; cs, cardiac portion of the stomach; gg, green glands; h, heart; hg, hind gut, or large intestine; Lr, liver; oa, ophthalmic artery; pg posterior gastric muscles; saa, superior abdominal artery; t, testis; vd, vas deferens.

middle region of that part of the carapace which lies behind the cervical groove; or, in other words, in the dorsal region of the thorax. In front of it, and therefore in the head, is a large rounded sac, the stomach (fig. 5, cs; fig. 6, cs, ps), from which a very delicate intestine (figs. 5 and 6, hg) passes straight back through the thorax and abdomen to the vent (fig. 6, a).

FIG. 6.--Astacus fluviatilis. —A longitudinal vertical section of the alimentary canal, with the outline of the body (nat. size) :—a, vent; ag, anterior gastric muscle; bd, entrance of left bile duct; cg, cervical groove; cæ, cæcum; cpv, cardio-pyloric valve; cs, cardiac portion of stomach; the circular area immediately below the end of the line from cs marks the position of the gastrolith of the left side; hg, hind-gut; lb, labrum; lt, lateral tooth of stomach; m, mouth; mg, mid-gut; mt, median tooth; ∞, œsophagus; pc, procephalic process; pg, posterior gastric muscle; ps, pyloric portion of stomach; r, annular ridge, marking the commencement of the hind-gut.

In summer, there are commonly to be found at the sides of the stomach two lenticular calcareous masses, which are known as "crabs'-eyes," or gastroliths, and were, in old times, valued in medicine as sovereign remedies for all sorts of disorders. These bodies (fig. 7) are smooth and flattened, or concave, on the side which is turned towards

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the cavity of the stomach; while the opposite side, being convex and rough with irregular prominences, is something like a "brain-stone" coral.

Moreover, when the stomach is laid open, three large

FIG. 7.-Astacus fluviatilis.-A gastrolith; A, from above; B, from below; C, from one side (all x 5); D, in vertical section (× 20).

reddish teeth are seen to project conspicuously into its interior (fig. 6, lt, mt); so that, in addition to its six pairs of jaws, the crayfish has a supplementary crushing mill in its stomach. On each side of the stomach, there is a soft yellow or brown mass, commonly known as the