Biology Reference
In-Depth Information
Multicotyle purvisi
Multicotyle purvisi naturally infects the stomach and duodenum of several
freshwater turtles in SE Asia. Ectolecithal eggs are laid into freshwater at
the 1-3 cell stage. At 27-29 8C mature larvae develop in about 25 days.
Hatching usually occurs in the morning, released by light stimulus, but
also occurs without such a stimulus when permanently kept in the dark.
Unique in the animal kingdom, the larva (about 200 mm long) is covered
by a tegument drawn out into numerous very thin (120-180 ˚ diameter)
processes, the so-called microfila (Figure 10.2 ) . The microfila are longest
at the anterior end, where they reach a length of about 6 mm. The larva
has a large posterior sucker as well as two dorsal, two ventral, and six
posterior ciliary tufts responsible for the swimming movements of the
larva. After hatching, the larva creeps on the ground or swims freely in the
water. It floats for extended periods in the water, apparently assisted by
the microfila which enlarge the surface area and prevent the larva from
rapidly sinking to the bottom. A larva often remains attached to the water
surface membrane, before detaching and sinking slowly downwards or
remaining in the water column. It survives for maximally about 33 hours
in the water. While floating in the water column, it is carried by inhalation
currents into the breathing chamber of a snail, from where it migrates into
the snail's kidneys, where it grows up. Snails of three families, Viviparidae,
Ampullariidae, and Bithyniidae, could be infected experimentally. Speed
of development within the snail host depends on the snail species and
temperature. Worms from snails fed to turtles led to successful infection.
Larvae possess an extraordinary array of sensory receptors, which have
been examined in detail by electron microscopy. They include a pair
of eyes, a pair of receptor complexes near the anterior end, each consisting
of a cavity containing one normal and several modified dendritic sensory
nerve endings, and 11 unciliated or uniciliated receptor types. The func-
tions of the various types of receptors are unknown, but it is likely that the
eyes contribute to hatching and possibly to swimming upwards in the water
column. The other receptors may help in migration to the snail's kidneys.
Turtles become infected by eating infected snails. Adult worms reach a
length of about 10 mm or more. Like the larvae, they have a great variety
of sensory receptors, 7 or even 9 types, and a remarkable nervous system:
anteriorly there are not - as is usual in flatworms - a number of single
circular commissures, but rather of two, internal and external ones, one of
the former very large and acting as a cerebral commissure. The function of
receptors and the nervous system is not known, but they must contribute
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