49 
P. Manson-Bahr and N. H. Fairley 
IV. OBSERVATIONS ON CERTAIN MOLLUSCAN INTERMEDIARIES, 
ESPECIALLY RELATING TO THOSE OF EGYPTIAN BILHARZIASIS. 
Planorbis Group of Snails. 
(1) Planorbis boissyi, as its name implies, is quite flat when laid on an even 
surface. It acts as the intermediate host for Schistosomum mansoni. We have 
found two varieties of this snail at el Marg (near Cairo). One has a dark gray 
or black thickened shell, often covered with whitish incrustations, the other 
has a light brown thin shell free from any deposits. It is the latter variety 
that is most commonly infested. 
This is the most common snail in the sweet-water canals at el Marg, and 
is to be found in many places in the Delta, at Ismailia, Tel-el-Kebir, Abou 
Soueir, and Tanta. Agglutinated masses may be found floating down stream 
in the canals, and in this way, snails originally infested in the more closely 
populated districts may be transported over wide stretches of country. 
(2) Planorbis mareoticus was also met with at el Marg. Its chief character¬ 
istic was its small size. 
Bullinus Group of Snails. 
The differentiation of the four species of Bullinus and of their many transi¬ 
tional forms is no easy matter, except for the experienced conchologist. The 
four species are Bullinus contortus, Bullinus dybowski, Bullinus innesi, and 
Bullinus forskali. In common, they all possess a non-operculated sinistral 
shell, that is to say, the helix runs from left to right when the apex points 
upwards; and the mouth, facing the observer, is situated on the left. Leiper 
has shown that three of these species, B. contortus, B. dybowski, B. innesi, act 
as intermediary hosts for S. haematobium. The bodies of all these snails con¬ 
tain a red diffusible pigment. 
The only other snail commonly seen in the Canal Zone with the above 
characteristics is Physa subopaca. It is easily distinguished by its rapid move¬ 
ment, by the different shape of the shell which is more pointed and elongated, 
by the different shape of the foot, and by the absence of any red pigment. 
Numerous attempts to infest the Physa artificially failed, and over 100 speci¬ 
mens collected under natural conditions, and dissected, were found to be free 
from cercariae. No attraction for this species of snail was exhibited by mira- 
cidia hatched out of terminal spined ova, though frequent experiments were 
made to investigate this point. 
In colour, the Bullinus vary from greenish black to brown. If infested 
with Schistosomum haematobium, the apical convolutions are yellow when 
viewed by transmitted light. If infested with Gastrodiscus aegyptius 
(Cobbold), the apical whorl is an intense black, but if infested with a yellow 
coloured redia (probably Cercaria lutea (Gilchrist)), it is bright orange. I nfesta- 
tions with the tw o latter parasites are extremely common in these snails. 
Parasitology xn 4 
