P. Manson-Bahr and N. H. Fairley 
65 
agriculture of this rainless country is so dependent upon the life-giving stream 
emanating from them that the water supply cannot be interrupted for any 
length of time. 
Obviously some other method must be devised more in keeping with the 
agricultural needs of the country. 
* 
The Spread of the Disease in Egypt. 
Snails of the two genera Planorbis and Bullinus differ somewhat in their 
habits. 
As a general rule, Planorbis is a surface-feeding snail and favours either 
slowly moving or stagnant and muddy waterways. It feeds on any vegetable 
material, but for the most part on the rushes and grasses which grow at the 
margins of the water. Little difficulty is generally experienced in collecting 
specimens as they occur in masses floating on the surface. 
Bullinus , on the other hand, is generally found attached to stones, posts, 
etc. at some considerable depth below the surface. It is more usually found 
in clear water such as in large tanks and in more swiftly flowing waterways 
than is the former. It feeds upon various kinds of aquatic plants which choke 
these channels in the summer season and may then be found in clusters 
attached to the under surface of the floating leaves. The main breeding season 
of Planorbis in Egypt would appear to be the spring months, March and April, 
while Bullinus deposits its ova at a later date. 
In the case of Planorbis the egg-masses are generally deposited upon the 
shell of another individual of the same species; while Bullinis deposits its ova 
upon stones at the bottom of the canal. The young snails, according to our 
observations, take about three months to mature. 
From our observations the fact emerges that the highest rate of infestation 
of fresh-water snails occurs round certain centres in the Delta, such as Cairo, 
Zag-a-zig and Ismailia. These three towns in particular are situated on the 
main sources of w^ater supply for the Delta and Canal zone, and from them a 
maximal faecal and urinary contamination of w^ater takes place. Now it has 
been showm that the range of action of the miracidium is a comparatively 
limited one. 
In the less sparselv inhabited stretches of the fresh-water canal, between 
these densely populated centres, it is very improbable that the local snails 
would become infested with miracidia; or if they did, that the cercariae thus 
produced would be numerous enough to run anv chances of encountering a 
suitable host during their short lifetime. 
During the spring months of the year the sluice-gates are open and the 
Nile w r ater is coursing through the canals of Lower Egypt bearing with it large 
masses of fresh-water weed and vegetation, and it is to this that most of the 
infested snails can be found to be clinging. Therefore it would seem than an 
annual migration of snails to the most distant parts takes place by these means 
and that this migration corresponds wdth the breeding season. 
Parasitology xn 5 
