somatic or body cells of the miracidium; and the germ cells which give rise to the next
larval generation.
A miracidium hatches out from an egg, a stage common to most digenean life-cycles.
In the majority of cases the miracidium emerges from the egg into water (except
for Dricocoeliumwhose eggs are eaten by a snail) and with the aid of the cilia on its outer
covering is able to actively swim for several hours. If it encounters the appropriate
species of snail it penetrates into the snail’s ‘foot’; if not it dies.
Once inside the tissues of the snail, the miracidium migrates to the snail’s digestive
gland (the hepatopancreas) and there the miracidium’s germ cells develop either into a
sporocyst or directly into a redia. The germ cells within the sporocyst then give rise either
to daughter sporocysts or directly to the next generation of redia.
Germ cells within the daughter sporocysts can develop in one of three ways:
n A free-swimming ceracaria which can penetrate the definitive host, eg Schistosoma
mansoni.
n A metacercaria which encysts on within a second intermediate host, eg Dricocoelium
dendriticum.
n A mesocaria which encysts into a metacercariae, eg Alaria canis.
The first generation of rediae are the mother rediae and these can develop in one of
three ways:
n Develop into daughter rediae and their germ cells give rise to free-swimming cer-
cariae and these then encyst on vegetation, eg Fasciola hepatica.
n Develop directly into free-swimming cercariae which penetrate the second intermedi-
ate host where they encyst, eg Clonorchis sinensis.
n Develop into ceracariae which are eaten by the definitive host, eg Azygia longa.
Rediae which arise directly from the miracidium become the mother redia and these
develop along one of two ways:
n Develop into daughter rediae which produce free-swimming cercariae, which then
become metacercariae, eg Stichorcis subtriquetrus.
n Develop into cercariae which transform into metacercaria, eg Nanophyetus salmonicola.
n 3.10.2 THE HABITAT OF ADULT TREMATODES
The habitat of the adult trematode (applicable to all endoparasites except for the type of
intermediate host) must provide the following:
n Connections with the outside to enable eggs to pass out of the hosts’ body. These can
be any one of faeces, urine, blood or sputum.
n Provide a surface for attachment and also facilities for feeding.
n The environment must provide easily digestible nourishment to satisfy the high level
of egg production.
n An intermediate host — a mollusc — which presumably provides some selective
advantage related to the enormous asexual reproduction that occurs within a the mollusc.Examples are Fasciola hepatica(the liver fluke; see section 7.5.7) and Schistosomaspp
(see section 7.5.4).
PLATYHELMINTHS