3188. With the protrusion into view of the vascular system, the veins are chiefly developed along with their principal organ, the liver, as being the bond of union between the circulation and the intestine.
3189. The animals, which first bring to bear in addition to the intestine a liver, are the Bivalve Mollusca.
3190. With the veins arteries also originate, but with a preponderance of venosity or the venous character. The blood is lymphatic, colourless.
3191. The cardinal venous organ or the liver evokes also into existence a corresponding organ of respiration, namely, free branchiæ with the tegumental form—branchial laminæ or leaves.
3192. In the middle between the branchial laminæ and the liver, the heart is evolved; this organ comprising always a ventricle with an auricle, but consisting of a venous, membranous tissue almost devoid of fibres.
3193. The first heart is in other respects arteriose; it receives the blood from the branchiæ and transmits it to the liver as well as to the remaining parts of the body, from which it proceeds directly to the branchiæ without entering any venous or right heart.
3194. In these animals, as is well known, four branchial leaves lie externally on the belly, which includes the intestine with a large liver, and hangs together with the branchiæ as a separate purse within the mantle.
3195. In the Mussel a structure originates for the first time, which can be compared with a thoracic or pectoral cavity. What covers the branchiæ, must stand in the signification of the thorax or chest. The pallium or mantle of the Mussels is pleura.
3196. Their shells are branchial opercula (as in the Fishes). They are secretions from the mantle, and everywhere accompany the branchiæ.
3197. The locket or hinge of the shell-valves corresponds to the rachis or spina dorsi, as is especially distinct in the Teredines or "ship-worms."