When, for commercial reasons, it seemed desirable to increase the number 

 of clams in our rivers, the experts of the Bureau of Fisheries discovered that 

 they were attempting a most complicated task. Almost all clams must undergo 

 a parasitic stage on the gills, or sometimes on the fins, of some fish. Com' 

 plicating matters still more, it was found that for each kind of clam there must 

 be a particular kind of fish host. As it was finally worked out, the develop- 

 mental histors^ makes an interesting story. The female clam carries the fer' 

 tilized eggs in the modified portion of the gills called the marsupium. When they 

 have reached a certain stage of development, the clam will extrude them at the 

 slightest stimulus, such as the passing of a fish. The larval clams, called glo' 

 chidia, are shot out in immense numbers and some of them are likely to be taken 

 into the mouth of a fish and to find lodgment in its gills. If the fish is of the 

 right species and has not yet acquired immunity by having carried many young 

 clams previously, the glochidia encyst on the gills and complete their larval 

 development. Later they escape from their cysts and, if they are fortunate 

 enough to emerge into a favorable part of the stream, begin their lives as clams. 

 Much work has been done on glochidia and their hosts, but much more 

 remains to be discovered. Glochidia, as found on the gills of fish, can often 

 be identified to genus and occasionally to species. As the first step, three 

 main types of glochidia, based on their shapes, are recognized: the axe-head 

 type, as in the pink heel-splitter; the hooked, as in the floaters and the white 

 heel-splitter; and the backless, as in the purple pimple-back. By this method 

 of study the natural fish hosts for a number of our common clams have been 

 discovered. In addition to its scientific value, this information is of value 

 to the fisherman. If he finds shells of the yellow or slough sand shell, he may 

 be sure gars are present; pimple-backs point to catfishes; mucket and blue 

 point indicate bass; and so on. The glochidia of some species may be carried 

 by salamanders, as Sim psoni concha avihigua on the gills of J\iectnrus maculosus. 

 The scientific names of clams have been subjected to much revision during 

 the past twenty years. Since shells are attractive and easily preserved, it is 

 not surprising that practically all our early American naturalists, as well as 

 many European scientists who employed collectors here, named our clams, 

 often working in ignorance of what their contemporaries were doing. Under 

 these conditions most of our clams were described at different times under 

 different names, and the application of the law of priority has often been 

 difficult. The soft parts of the animal were long completely ignored, and even 

 localities of collections were overlooked or unrecorded. One Mississippi valley 

 shell was actually described by a European scientist as coming from Peru and 

 was given the scientific name of peruviana. 



Recent studies have shown that clam shells may vary greatly with their 

 environment, and indicate that in several cases clams that have been regarded 



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