are flat, plate-like, unreflected lamellae and are regarded as the most primitive (Nucula, Yoldia, etc.); (2) Filibranch, in which the gills are long curtains folded back against themselves and held close to each other by the
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Figure 15. Diagrammatic cross-sections of clams showing the major types of gill structure. a, protobranch; b, filibranch; c, eulamellibranch; d, septibranch.
interlocking of the tiny cilia on the surface of the gill filaments (arks, mussels, scallops, etc.); (3) Eulamellibranch, similar to the filibranchs except that the gill curtains are united by cross-channels (astartes, cardiums, venus clams, tellins and many others); (4) Septibranch, which have very degenerate gill structures consisting of two pallial chambers with only gill slits or very reduced gill filaments acting as windows to the chambers (Cuspidaria and Poromya).
HOW THEY REPRODUCE
The staid bivalve has made his share of contributions to experiments in sex and reproduction, and throughout the class we find varying degrees of sexual differentiation, as well as all manner of ways of insuring proper fertilization, protection of the young and thus the continuation of the species.
The pelecypods have no copulatory organs or other external sexual characteristics, with the exception that in certain species of fresh-water mussels, the marine astartes and a few other genera, the two sexes can be distinguished by the shape of the adult shell. The majority of the bivalves as a group are predominantly of separate sexes, but at least four percent of those adequately studied are known to deviate from the strictly dioecious, or unisexual, condition.
A few species are true hermaphrodites in which the same individual contains both female and male sex organs which may produce eggs and sperm simultaneously. In this group are found certain species of Pecten, Tridacna (the Giant Pacific Clam), Kellia, Dinocardium, Gemma, Tivela (the Pismo Clam), Thracia, Poromya, the shipworm Teredo diegensis and the fresh-water genera Anodonta, Pisidium and Sphaerium. In some of these
