382                            Cotylophoron cotylophorum

a day. They are whitish in colour and ovoid in shape, with a slight attenua-
tion at the opercular end. They measure 0.0126—0.013 × 0. 0068—0.009*
in size. When kept at a temperature of 80° to 90°F., they hatched in eighteen
to twenty-one days. Though hatching of the eggs took place throughout the
twenty-four hours, most of them hatched between 10 A.M. to 4 P.M.

The miracidium is extremely active and is attracted towards the lighted
side of the container, showing marked heliotropism. It is pyriform in shape
and measures 0.014 to 0.02×0.003 to 0.005 in size, with the maximum
breadth occurring across the anterior fifth of body length. It is covered,
except at the apical papilla, with ciliated epidermal plates which are twenty
in number, arranged in four rows round the body. There are a flask-shaped
gland (primitive gut), two pairs of penetration glands, reproductive tissue and
the usual types of excretory and nervous systems. The miracidia were utilised
in infesting parasite-free, laboratory-raised specimens of a common snail
Indoplanorbis exustus. A few snails were placed in a dish of water containing
actively swimming specimens of the miracidia. The larvae attacked the
snails very readily and slowly penetrated into its exposed parts, specially
foot and mantle. The infested snails began to shed cercariae in thirty to
thirty-five days.

                    DEVELOPMENT INSIDE THE SNAIL

The miracidia retain their ciliated plates for a few hours after penetration
into the body of the mollusc. Soon they become ovoid or elongated in shape
with broadly rounded ends—sporocyst—and the miracidial organs begin to
disappear. A very thin cuticle develops round the body. In about twenty-
four to thirty hours, the germ balls break up into a number of germ cells which
fill the central cavity. The germ cells do not appear to possess cell walls.
They develop into rediae which begin to appear in three days' time and attain
their full size inside the sporocyst in about a fortnight. The number of rediae
in a single sporocyst varies from five to eight. There is no birth pore in the
sporocysts and consequently the enclosed rediae come out by rupturing the
wall at the anterior end. The pair of flame cells present in the miracidium
persist in the fully formed sporocysts which measure 0.29—0.35 × 0.12—
0.19 in size.

A fully mature redia is sausage-shaped, measuring 0.58—0.72 × 0.14—
0.16 in size, and possesses a mouth, pharynx, oesophagus and a rhabdocoel
gut, besides excretory and nervous systems. A number of unicellular bodies
are found associated with the gut. There are no lateral appendages. The
birth pore develops only after the rediae have emerged from the sporocyst.
The excretory system consists of three pairs of flame cells. The cells of the
anterior pair lie one on either side of the gut while those of the posterior pair
are situated laterally a little in front of the hind extremity. The cells of

                                          *All measurements are in mm.