532 | Bulletin American Museum of Natural History [Vol. LIIT 
ege-masses of Lymnxa, Planorbis, Bulinus, and Physopsis may 
possibly adhere to the feet or to the bill of aquatic birds. Such acci- 
dental conveyance by birds is, however, hardly possible with species that 
fix their egg capsules firmly to stones, water plants, or other shells, or 
that are viviparous. The distribution of such species over different 
hydrographic basins can in most cases be explained only on the basis of 
present or past connections between certain branches of these systems.* 
Other fluviatile mollusks are carried about by the movements of the 
water, either at times of flood when they are swept over inundated areas 
and thus reach swampy depressions or even the head-waters of other 
rivers®; or else fixed on drift wood or buried in the vegetation of floating 
islands. At Leopoldville fair-sized masses of living vegetation, especially 
papyrus and grass, are commonly seen at the season of high water, 
being swept down the cataracts of the Congo River. 
Fresh-water mollusks are not more free of internal and external 
parasites than other animals. The most important of these are certain 
worms, especially trematodes, which infest certain organs as ‘‘sporocysts’ 
and ‘“‘cerecarie,”’ larval stages which eventually migrate to another host 
in order to become adult. When these worms are present in large 
numbers they may cause the death or sterility of the molluscan host. 
They even occur in fresh-water mussels,* where they frequently become 
encysted in nacreous excrescences or in free pearls if they happen to 
penetrate between the mantle and the shell (Unio margarit?fer, etc.).° In 
other cases they merely produce a discoloration of the nacre.°® 
Among the external parasites the most interesting are certain 
aquatic mites (Hydrachnidee) of the genus Unionicola Haldeman (= Ataa 
Fabricius).’ They are most frequently found in Unionide, often fixed 
on the gills, but some have been described from fresh-water snails. In 
some cases they may also become the center of a pearl growth.’ Leeches 
of the genus Glossiphonia Johnson (=Clepsine Savigny) sometimes feed 
'This is illustrated in Europe and North America by the spreading of certain species along canals. 
connecting different hydrographic basins. 
*Many of the African rivers originate in swamps that flow to different basins. A well-known ex- 
ample is the region of Lake Dilolo which is drained by the Kasai to the Atlantic and by the Zambezi to. 
the Indian Ocean. 
8A special chapter is devoted (pp. 87-96) to those parasites of snails that are of medical or 
bye eee Ae vs 
‘Kelly, H. M. 1899. ‘A statistical study of the parasites of the Unionide.’ Bull. Illinois Stat 
Lab. ae aan pp. ee i — 
’Filippi, F.de. 1852. ‘Sull ’origine delle perle.’ I] Cimento, Torino, I, pp. 429-437 (translated 
_ by F. Kiichenmeister in Miller’s Arch. Anat. Phys., 1856, pp. 251-268). ae ; . 
1856. ‘Encore un mot sur la formation des perles.’ Miller’s Arch. Anat. Phys., pp. 490-493. 
SOsborn, H. L. 1898. ‘Observations on the parasitism of Anodonta plana Lea by a distomid 
trematode at Chatauqua, New York.’ Zo6l. Bull., I, pp. 301-310. 
Wolcott, R. H. 1899. ‘On the North American species of the genus Atax (Fabr. Bruz.’ Trans. 
a sere re he XXVIII-XXx1I; with full bibliography. 
ee Kiichenmeister, F. . ‘Ueber eine der haufigsten Ursachen der Elst len.’ Miiller’s: 
Arch, Anat. Phys., pp. 269-281. . : eit oh 
