September 1955]
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the body. It consists of a large, reniform posterior chamber and a small anterior
one. The bladder opens out posteriorly on the dorsal surface of the body at the
point of attachment of the tail, by a small excretory pore. Main excretory trunks
or canals take their origin medially from the anterior chamber of the bladder. At
first they are slightly sinuous and thick-walled, and pursue an outward and forward
course to the sides of the acetabulum where they describe a lateral loop, after which
they continue forward as very wide tubes lateral to the secretory glands. The
dilated portions of the main vessels contain very large and conspicuous excretory
granules, the number of which may vary from four to seven on either side. The
granules are roughly oval in shape and are of different sizes, those occupying the
middle region being larger. They are highly refractile and some of the bigger ones
may exhibit one or two transverse lines denoting the fusion of smaller granules.
Each main vessel on reaching the level of the pharynx becomes suddenly narrower and
forms a loop (characteristic of all echinostome larvae) and passes backwards as a
narrow tube to the acetabular region where it bifurcates into the anterior and posterior
collecting tubules. The anterior collecting tubule divides into two branches, each
one giving off three capillaries capped with flame cells, whereas the posterior collecting
tubule divides into three branches each leading to a set of three flame cells. Thus,
there are 15 flame cells on either side of the body, six connected with the anterior
collecting tubule and nine with the posterior tubule, all of them occurring in groups
of three. The flame cell arrangement of Cercaria nairi can be represented by the
following formula: 2[(3+3)+(3+3+3)]=30. A longitudinal caudal canal which
starts from the posterior aspect of the bladder extends through the greater part of
the length of the tail. About one-sixth the distance from the root of the tail, this
canal gives off a pair of lateral tubes which open out through minute pores.
The genital organs are represented by two masses of round cells, a larger mass
lying posterior to the acetabulum between it and the excretory bladder, and a smaller
mass, probably representing the rudiments of the genital pore, occupying the narrow
triangular space, bounded by the anterior margin of the acetabulum and the proximal
portions of the two diverging caeca.
The cercariae, like those of other echinostomes, develop and attain maturity
in redia, which infest the digestive gland and sometimes the reproductive organs
of the molluscan host. The affected organs of the snail are coloured orange-brown.
The redia resembles in general those of the other allied members of the group. The
mature ones contain golden yellow pigment granules in the body wall and measure
on an average 1.96 X 0.21 mm., while the young or immature forms do not show any
pigmentation and measure 0.380 X 0.073 mm. They are capable of independent
movement. The anterior end is bluntly rounded, and about one-ninth of the total
length from the extreme tip is a circular groove representing birth-pore. At about
one-third the distance from the posterior end of the body are present the characteris-
tic well-developed blunt ventro-lateral protrusions, known as the 'locomotor appen-
dages' or 'ambulatory processes'. The terminal mouth-opening is in direct con-
tinuation of the cavity of the globular pharynx which measures 0.04 mm. in diameter,
This is succeeded by a sac-like gut filled with a dark reddish-brown granular content.
In young specimens, the gut may extend posteriorly to the level of the ambulatory
processes while in grown-up forms it stops short at a point behind the collar region.
The proximal part of the gut is surrounded by a group of small granular salivary
27 AR/55
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