The shells of Bivalve Mollusca are a calcareo-thoracic box, open in front, inverted behind, and moveable like ribs.
3198. The two sphincter or occlusor muscles signify shoulder and haunch.
3199. These animals begin for the first time to exhibit bilaterality or symmetry, because in them there is stirring the idea of the osseous formation. From the branchiæ being situated symmetrically upon both sides, the cardiac chambers are also symmetrical.
3200. The pectoral tunic (mantle) usually elongates at the anal extremity into two tubes, respiratory tubes, through which the water is drawn in and thrust out or expelled. Such is the case in the highest Worms or Holothuriæ, only there the respiratory tube leads into the body itself. A similar arrangement is found in the Echini. Many aquatic larvæ from all kinds of Insects or out of different classes breathe through anal tubes. All these animals consequently repeat the Mussels, and this formation admits of being followed out up to Man, where it is left as allantois and primordial kidneys. The thorax of the Mussel thus opens by the anus. But, since the thorax is that which is here preponderating and is nearly the whole animal, so does the anus open into the posterior respiratory tube.
3201. The Mussel can be regarded as an animal consisting of three cysts or sacs inclosed within each other, namely, of intestine, around this the ventral pouch, and around this again the thoracic sac or mantle. If, moreover, the shells be regarded as a cyst, the animal then consists of four sacs. The heart and branchiæ lie within the thoracic cavity; intestine, liver, and ovarium within the abdominal or ventral cavity. The Mussel is thus a doubled Acalephan.
What is termed foot in the Bivalve Mollusca is nothing else than the abdominal tegument dilated in front to form a muscular ridge.
3202. The Mussels are embryos, in whom the liver originates, and whose chorion acquires a placenta. Then again, as the embryo is nearly all liver, and hangs surrounded by a watery fluid within the widely-expanded chorion and amnion, so does the abdominal pouch hang within the pallial cavity, or in the wide, water-full chest.
3203. In the abdomen there is still only the ovarium, and that indeed very large. There are properly two ovaria, each of which, according to my observations (vid. Göttinger Gelehrte Anzeigen, 1806. Stück 148), opens laterally under the shoulder-muscle and gives exit to the ova, which then enter within or between the branchial fringes, in order to be there developed.
3204. Here the respiratory organs are still at the same time a kind of uterus. The ova may be oxydized therein, like the embryo is in the uterus. These branchiæ are probably still to be compared with, or designated as, sexual branchiæ.
3205. In the back of the shoulder these animals have a highly vascular organ with two excretory passages, which open near to the orifices of the oviducts. I formerly regarded it as a kidney, but, according to more recent observations, it should be a testis. In the anterior part of the foot there is frequently situated a gland which ejects a gelatinoid moisture, which hardens into glutinous filaments—the byssus or beard. It is probably a memento of the tentacula of the Acalephæ. There is besides also an organ in the feet of many Bivalve Mollusca, which occasionally squirts a watery juice out to a great distance. I have occasionally found this organ in our Teichmuschel. Its signification is probably the same as that of the preceding organ.