Studies on the development of Crepidostomum farionis
Ultrastructural studies on Crepidostomum farionis
have been undertaken in order to elucidate the development
of the redia, cercaria, metacercaria and juvenile stages
in the intermediate and definitive hosts. Particular
attention has been paid to the tegument, gastrodermis,
excretory vesicle and glandular systems. Concurrent
investigations into the bionomics and behaviour of the
larval stages have also been carried out.
Rediae have been shown to have a birth pore
situated at the posterior end of the body and, additionally
paracrystalline structures which may be viruses have been
detected in them. The tegument of the cercaria appears to
arise by a process of delamination from the primitive
epithelium covering the germ ball. Secretory bodies passed
into the cercarial tegument, while the cercaria is still
inside the redia, may play a part in cyst formation and
secretory bodies in the ventral sucker tegument may be
important in adhesion to Gammarus. After excystment these
secretory bodies are no longer evident in the tegument and
new cytons producing different secretory bodies make contact
with. the tegument.
Penetration glands open into the stylet sac of
cercariae; similar glands open at the anterior end of
metacercariae and juveniles but there are indications that
their secretory activity diminishes as growth proceeds.
They may be involved in extracorporeal digestion in juveniles
before completion of development of the gastrodermis.
Two forwardly directed pigment cup ocelli have been
described in the cercaria and presumed sensilla$ of various
kinds have been studied in cercariae, metacercariatand
juveniles. Anteriorly situated lappets of the metacercariae
and juveniles are almost entirely muscular.
Secretory bodies produced by the cercarial bladder
epithelium and absent after encystment may play a part in
cyst formation. Concretions which accumulate in the bladder
of the metacercaria are shed after excystment, lipid
excretion begins and a thick glycocalyx appears on the
tegument lining the excretory canal.
Cercariae, shown to emerge during the hours of
daylight, are strongly photonegative and geopositive.
Metacercariae excyst readily in 1% trypsin solutions at