FIG. 15--Astacus fluviatilis.--A diagrammatic transverse section of the thorax through the twelfth somite, showing the course of the circulation of the blood ( x 3). arb. 12, the anterior or lower, and arb'. 12, the posterior or upper arthrobranchia of the twelfth somite; av, afferent branchial vessel; bcv, branchio-cardiac vein; bg, branchiostegite; em, extensor muscles of abdomen; ep, epimeral wall of thoracic cavity; ev, efferent branchial vessel; fm, flexor muscles of abdomen; fp, floor of pericardium; gn. 6, fifth thoracic ganglion; h, heart; hg, hind-gut; iaa, inferior abdominal artery, in cross section; la, lateral valvular apertures of heart; lr, liver; mp, indicates the position of the mesophragm by which the sternal canal is bounded laterally; p, pericardial sinus; pdb. 12, podobranchia, and plb. 12, pleurobranchia of the twelfth somite; sa, sternal artery; saa, superior abdominal artery; sc, sternal canal; t, testis; XII., sternum of twelfth somite. The arrows indicate the direction of the blood flow.
[Editor's note: illustration is reproduced here approximately twice the size of the woodcut in the book.]
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