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alone distinguishes it from all other trematodes. It is a blind sac 
which receives at its anterior end the opening of the genital ducts, and 
it extends backwards to the regions of the testes where it expands to a 
considerable size. Roughly speaking one might liken it to a hollow 
cone lying ventrally in the body of the parasite with its apex placed 
anteriorly. The atrium is lined with a thin cuticle, and directly external 
to the hypodermis many thin muscular fasciculi run longitudinally 
(Fig. 6, M.B.). There are no well defined circular muscles. As stated 
before, the physiology of this organ is not known, but perhaps it may 
act as a kind of brood pouch or as a receptacle for storing the ova during 
Fig. 8. Changes in the atrium of G. bubalis. D. dorsal. V. ventral. 
certain periods. Its exact nature, however, will not be undeistood until 
the development has been investigated. The configuration of the 
atrium on cross section has been used by Fischoeder (1903) as a basis 
for classification. Thus: 
Cross section of atrium triangular with apex directed dorsally— 
crumenifer and covipressus with pointed apex; gregarias and wenyoni 
with bifurcated apex. 
