SYSTEMATIC ACCOUNT OF CONGO FRESH-WATER MOLLUSKS 
GASTROPODA 
Aquatic or terrestrial mollusks, with a distinct head, bearing one or two pairs of 
tentacles and, with few exceptions, a pair of eyes. Radula usually present. The body 
is asymmetrical; the ventral side forms a broad and flat foot; the dorsal side is 
covered by the mantle which, as a rule, secretes a univalve shell, usually more or less 
coiled. The mantle contains an open, respiratory cavity serving as a lung or contain- 
ing gills. In some marine forms (Nudibranchiata) the mantle is absent and the gills 
are outgrowths of the integument of the back. 
The fresh-water gastropods are polyphyletic. They have evolved in 
two different ways and in each case from several stocks and at different 
times. : 
In the Streptoneura, the fluviatile types have been derived directly 
from marine forms which gradually became adapted to brackish and 
fresh-water conditions. ‘These forms have retained the ancestral gills, 
but in some cases they have later become more or less terrestrial and have 
acquired an additional lung (Ampullariide). The purely terrestrial, 
lung-breathing Cyclophoride and Pomatiaside have undoubtedly orig- 
inated in some such fashion from a pectinibranchiate stock. The adapta- 
tion from marine to fresh-water or even terrestrial life is still going on at 
present in the Neritide, Cerithiide, Littorinide and others.! 
The Ethiopian fresh-water gastropods belong to the following seven- 
teen families: 
ORDERS SUBORDERS FAMILIES 
’ ( Auriculidee 
| Lymneidee 
Euthyneura, gaan PUPOOWATA oie dos eas Basommatophora..... { Planorbidee 
| Physidee 
| Ancylide 
| Scutibranchiata ....... . Rhipidoglossa ....... _{ Neritidee 
\ Hydrocenidee 
Streptoneura...... Ampullariide 
; Viviparids 
Amnicolide 
Syrnolopside 
Pectinibranchiata...... Teenioglossa...... ....-| Lrunéatellidee 
Synceridez 
Valvatide: 
Littorinidee 
Cerithiide 
Melaniidee 
1See Pelseneer, P. 1895. ‘Prosobranches aériens et Pulmonés branchiféres.’ Arch. de Biologie, 
XIV, 2, pp. 351-398, Pls. x1v—xvii1. 
97 
