1927] Prlsbry-Bequaert, The Aquatic Moltusks of the Belgian Congo 87 
a ring to which is attached a sack-net of loose burlap or of fine-mesh 
copper screen. The dirt thus gathered is washed in the net t6 remove the 
mud and fine sand, the coarser material thrown out, and the remaining 
finer material which contains the mollusks kept for more thorough ex- 
amination at home. Water plants should be pulled out and the mud 
adhering to their roots washed in the net. A crow-foot dredge trailed on 
the bottom conveniently secures many of the large and medium-sized 
fresh-water mussels. In deeper water dredging is the only means for 
collecting. A solid, rectangular frame, with a very loose burlap sack 
attached, or for the larger mussels a strong net of half-inch mesh, can be 
dragged from a boat or dropped some distance out and then hauled to 
the shore. Certain species live in rather coarse gravel at the bottom of 
creeks and it is necessary to stir up the heavy material before gathering 
it in the net. 7 
Advantage should be taken of the season of the year when the level 
of the stream is at its lowest to visit the sandbanks and rapids and also 
to enter swamps which at other times are hardly accessible. The bottom 
of dried-out ponds should then be excavated to secure certain species 
that are able to estivate in the baked mud. With the beginning of the 
rainy season the first heavy rains often swell the streams rapidly and carry 
débris which is deposited along the banks of more quiet stretches. Such 
~ fine drift almost always contains dead shells, land-snails as well as small 
aquatic species. Many of the Congo species of Cleopatra have been ob- 
tained only in this fashion. 
A modern taxonomy of aquatic mollusks must be primarily based on 
characters furnished by the animal and its anatomy. Since these have 
been examined in but very few of the African species, it cannot be too 
strongly urged that notes be made on the appearance of the living animal 
and that material be suitably preserved in alcohol. Indications for this 
purpose were given in the report on land mollusks. The system of 
African fresh-water mussels, Unionide and Mutelidr, is now merely 
tentative for want of alcoholic material. 
Mepicau IMpoRTANCE OF FRESH-WATER Mo.uusxs! 3 
The study of the ecology and distribution of aquatic mollusks in 
tropical and subtropical regions has acquired in late years consider- 
able importance from a medical and veterinary point of view through 
1The literature bearing on helminth parasites of mollusks and their relatiors tc ya1asitic worms in 
vertebrates has grown to almost fantastic proportions. Only some of the salient points which have a 
direct bearing upon preventive medicine could here be considered. Much of the information has been 
taken from the Tropical Diseases Bulletin. 
