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 Anodonta 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 glochidium 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 specimens 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 sperms 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 becomes evident that our Unionidae are not of the simple and 
uniform 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 Pelecvpoda : 
the Unionidae of the Naiadacea, and the Sphaeriidae ( Spkaerium , 
Mucsulim, Pisidium and Eupera ) of the Cyrenacea The latter, 
