April, 1911.] Anatomy and Physiology of the Unionidae. 333 



opening, or simply through the great slit of the mantle on the 

 ventral side. 



Of the first stages of post-embryonal development, we still 

 know little. It has been observed, in Europe, many years ago, 

 that the glochidia of Anodoiita attached themselves on fins, gills, 

 etc., of fishes, are there inclosed in a cyst-like cavity by local 

 hypertrophy of the host's epidermis or epithelium, and live 

 as parasites for weeks or months. In our country, some observa- 

 tions of this kind have been made, but I have not seen a report on 

 them. There is an excellent opportunity here for observations 

 and experiments. 



At a later stage, small mussels — some less than two milli- 

 meters long — are found with post-embryonal shells, still bearing 

 the glochiclium valves in the centers of the beaks. Young Lamp- 

 silis develop a byssus thread, about the thickness of a horse 

 hair, and several inches long, fastened to a stone, or shell. The 

 young mussel begins to develop its gonad in about the third year, 

 and at that age has comparatively few ova and young in its 

 marsupia. Only from that age on, young Lampsilinae begin 

 to show sexual differences of the shells. 



There is another physiological feature of interest. By examin- 

 ing thousands of speciinens at various seasons of many years, it 

 has been found that the mussels of the several groups are producing 

 their young at different times. The Unioninae, also Margaritana, 

 are found with their branchiae barren through autumn, winter 

 and spring, but ova, and spenus developed in the gonads. In the 

 summer, about June, the ova are transferred to the branchiae 

 develop into glochidia within a week or two, and the young are 

 discharged soon ; the whole process taking about four weeks. In 

 the Lampsilinae, and the Anodontinae, the marsupia become 

 gravid in fall, in some as early as August; the transformation into 

 glochidia here also takes only a week or two, and then the embryos, 

 without any noticeable changes, are retained over winter and 

 early spring, that is for eight to even ten months. The former 

 were called short period or summer breeders, the latter long 

 period or winter breeders. 



To sum up : From these condensed and fragmentary outlines, 

 it becoines evident that our Unionidae are not of the simple and 

 unifomi organization as was supposed, and that their study 

 reveals many interesting features. For these reasons, they well 

 deserve more attention than has been given them, as an object of 

 study in the zoological laboratory, for their morphology, anatomy 

 and physiology. 



In conclusion, it may not be amiss to point out briefly the princi- 

 ple differences between the two groups of our fresh water Pelecypoda : 

 the Unionidae of the Naiadacea, and the Sphaeriidae {Sphaerium, 

 Mncsitlini, Pisidium and En per a) of the Cyrenacea The latter. 



