1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 531 
appear to be specific distributors of particular Species of unionids, but 
others are mere accidental or occasional hosts. The ecological peculiarities 
of the fish distribution must therefore necessarily be of importance to 
that of the Unionide. On the other hand, certain fresh-water fish deposit 
their eggs in living Unio, where they develop into young fish between the 
gills. Although this curious fact was known since 1787 (Cavolini), it was 
apparently v. Siebold, in 1863, who first showed that the eggs commonly 
found in the European Unio pictorum belong to the fish Rhodeus amarus. 
In the case of Unionide with parasitic embryos there can be little 
doubt that their distribution over the various branches of a river system 
is actively aided by the migrations of fishes that act as their hosts. 
Although most of these fresh-water mussels move about quite freely in 
later life, it would appear from Isely’s experiments with marked speci- 
mens that they migrate but little and probably but seldom far from the 
point where the embryos were dropped from the fish. Their movements 
seem to be prompted mainly by the selection of a sufficient depth of 
water, which is essential to optimum development.? 
It has been noticed many times that certain snails and small mussels 
appear suddenly in newly dug wells, cisterns, or ponds, far away from 
other fresh-water or in localities where these mollusks were entirely 
unknown before.*? It is generally admitted that in such cases they were 
brought in accidentally by birds, to which they had become 
attached either as young or in the egg stage, or more rarely fixed 
to the legs of water beetles. Water fowl are probably more likely 
to carry mollusks and their eggs than other animals and Cawston‘ 
attaches a great importance to wild ducks as conveyors of snails 
that may act as intermediate hosts for flukes. Indeed, the gelatinous 
ena OCRMNOONT EOD COE TEN 1 (PESMARES Ca td ave TENET, Sates Seduce Dh Ce HT RS) PRN On et ey a Me ER Se RUS AT BY te Be NUT A Oia 
‘See Noll, F.C. 1869. ‘Bitterling und Malermuschel.’ Zoolog. Garten, Frankfurt a.M., X, pp. 
257-265, Pl.; 1870, op. cit., XI, pp. 237-238. ; 
2Isely, F. B. 1914. ‘Experimental study of the growth and migration of fresh-water mussels. 
U.S. Bur. Fish. Doc. No. 792, 24 pp., 3 Pls. ; 
8A number of examples are discussed by Kew, H. W. 1893. ‘The dispersal of shells.’ (London), 
mete ino fontee t 1 of mollusks by other animal 
ee also for the dispersal of mollusks by other animals: 
Cockerell, T. D. rt 1921. ‘The dispersal of snails by birds.’ Nature, CVIII, pp. 496-497 (Balea 
perversa adhering to the plumage of birds, probably by means of a sticky slime). ; 
Ramsden, C. T. 1914. ‘The bobolink (Dolichonyx oryzinorus) as a conveyor of Mollusca. The 
Auk, N.S., XX XI, p. 250 |live Succinea riiser (Pfeiffer) hidden among the feathers of migrating birds, 
i uba]. 
me al ee, W.L. 1914. ‘Birds transporting food supplies.’ The Auk, N.8., XXXI, pp. 404-405. 
Presents some evidence tending to show that birds hide mollusks on purpose among their feathers, so as 
to carry with them a supply of food. The upland plover, Bartramia longicauda, was several times found 
by G. E. Beyer in Louisiana, with 20 to 40 snails of the genus Physa concealed among the under wing 
feathers, while the stomachs always contained a number of crushed shells of the snail. 
Tomlin, J. R. le B. 1910. ‘The dispersal of shells by insects.’ Journ, of Conchology, XITI, p. 108. 
Observed a bumble bee in England flying with : Potamias elegans attached to its hind tarsus which was 
ed between the shell and the operculum. 
a J. W. 1884. ‘Ducks transporting fresh-water clams.’ The Auk, I, pp. 195-196. 
Sage, J. H. 1895. ‘A Sora caught by a mussel.’ The Auk, XII, pp. 297-298. 
Ganong, W.F. ‘Do young loons eat fresh-water clams?’ The Auk, XIII. pp. 77-78. 
4Cawston, F.G. 1921. ‘Wild birds a cause of the spread of bilharzia infection.’ Journ. Trop. Med. 
Hyg., XXIV, pp. 109-110. 
