40 



American Se ash ells 



are flat, plate-like, unreflected lamellae and are regarded as the most primi- 

 tive (Nucula, Yoldia, etc.); (2) Filibranch, in which the gills are long 

 curtains folded back against themselves and held close to each other by the 



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, mus- 

 sels, 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 fertili- 

 zation, 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, Dinocardiwn, Gemma, Tivela 

 (the Pismo Clam), Tlyracia, Porojnya, the shipworm Teredo diegensis and 

 the fresh-water genera Anodonta, Pisidium and Sphaeriwn. In some of these 



