BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [14-? ' 



that the male and female differ widely in their shape. The extern 

 sculpture, of which so many species offer examples, is probably initiate* 

 in most instances by growth of the shell over the jjlicated network o 

 the gills, another iield open for much needed research. The very younj: 

 shells often have characteristic sculpture, lost by erosion in the adult 

 and they should be carefully sought for and preserved. The young ( 

 the Gorhimolidce generally nestle in a sort of marsupium within the 

 parent until of considerable size, affording a good opportunity for iden 

 tification and for the study of the larval calyculate beaks found ^i, 

 various species. 



The tropical forms of Sphcerium show black maculatious not fouuy, 

 in northern forms, which on close examination will be found to b 

 external to the shell on its inner surface. Their nature is still to b< 

 explained, and the naturalists of our southern border would do well t. 

 examine into the matter. 



The parasites of fresh-water species are not numerous. They com 

 prise rhizopods, infusoria, and cercarian worms, like the embryoniv, 

 stages of the sheep and other flukes. These occur in Limncea especiallj , 

 Small mites are found on them also, and they may sometimes be seeu, 

 on Unio. The beautiful pearls which occur occasionally in Unio an= ' 

 its allies are said to be frequently initiated by the irritation of a 

 Gregarina, over which the tormented mollusk pours out a film of nacre 

 to protect itself. Other pearls are due to the accidental intrusion of 

 grains of sand or the hooked embryos before referred to. Pearl hunting' 

 is not part of the naturalist's work, though he may sometimes be re- 

 warded by a " find." It has been calculated that one Unio in a hundred 

 contains a pearl, that one j)earl in five hundred has a commercial value^ ' 

 and about one in a thousand is worth more than $5. So it will be seen ^' 

 that pearl-hunting is not a profitable pursuit in the long run, and, by"^' 

 its involving the destruction of myriads of interesting animals in regard ^ 

 to which the naturalist has almost everything yet to learn, it is j)laced 

 in the category of pursuits which those interested in biology may ' 

 reasonably discourage. 



Fresh-water collecting in the tropics does not differ essentially ft-om 

 the same pursuit in more temperate regions, except by the greater preva- 

 lence of paludal fevers in the haunts of the collector's prey. 



In deep lakes interesting collections may sometimes be made by means 

 of dredging, but as this does not differ in any essential way from marine 

 dredging the reader is referred for details to the section of this paper 

 relating to explorations of the sea. 



In closing this part of our remarks we would emphasize the desira- 

 bility of maintaining aquaria for the study of the habits of the hving-^ 

 animals. Fresh- water aquaria may easily be made by anybody and 

 require hardly any attention, while they afford an indispensable inethod 

 of learning the ways, habits, and life history of fresh- water ahimals. 

 and offer an attractive ornament to the study or the i)arlor. 



