March, 1954]                                A. K. VARMA                                      15

of nasal schistosomiasis, actually lie in the centre of this canalised zone and are
watered perennially. The subsidiaries of the canals through which water is taken
to the fields for irrigation and the side-drains provided for draining off the excess
water to low-lying areas are physical features of paramount importance in the
ecology of the snail vectors of the parasite. It is noteworthy that not a single snail
was ever found in the main streams of the canals where the water generally flows
swiftly.

The Eastern Canal, with its few branches, irrigates only the western parts of
the districts of Gaya and Patna, while the rest of the central and eastern region,
particularly of Patna district, is a low tract or depression which is annually flooded
with water from the united streams of the Poonpoon, the Phalgu and the Sakri.
These rivers flowing through low-lying land cannot empty into the Ganges when
the latter itself is in high flood and are consequently held up and spread over the
adjacent country. Throughout its long devious course the Poonpoon is extensively
used for irrigation in the districts of Gaya and Patna, the usual practice being to
raise temporary dams across its channels and to divert the water to subsidiary
channels and thence into the adjacent fields. The river is fed by a number of
tributaries, namely the Batane, the Adri, the Madar, the Morhar and the Dardha,
all joining from the right bank. The predominant feature in the physiography of
this part is the presence of innumerable swampy ditches, ponds, pools and low-lying,
water-logged areas, filled with more or less clean water from the canals and the
over-flooded rivers, providing a suitable abode for the molluscan intermediaries.
These places are readily and regularly accessible to the susceptible animals, mostly
for the purpose of drinking; while some are taken there by their owners for washing
and cleaning. These facts account for the continued infections and re-infections
of both the molluscan and the definitive hosts.

Rainfall

Broadly speaking there are three distinct seasons, each having four months'
duration and gradually merging into the other; (1) the rainy season extending
from June to September, (2) the winter from October to January and (3) the summer
from February to May. The rains are torrential from the south-west monsoon
arising from the Bay of Bengal and deviating westward after striking against the
Assam Hills of Garo, Khasi and Jayanti and also from the same south-west monsoon
coming out from the Arabian sea and deviating towards the east in the Gangetic
plains from the Indus valley. The area also gets some more rains from the north-
east monsoon during the winter months.

Average monthly rainfall, as furnished by the State Agricultural Department
is given in Table I.

                                                     TABLE I

                                        Average rainfall in inches