204
MARINE MOLLTJSCS AS HOSTS FOR SYMBIOSES
lescens ”) or as B. haimeanus in marine pelecypods. For example,
Johnstone (1904) has reported its occurrence in Cardium edule from the
Lancashire coast, Pelseneer (1906) has reported it in Syndosmya alba
from the north of France, and Lebour (1911) has found it occurring
throughout the tissues of Cardium edule collected a t Fenham Flats,
Northumberland, and at Ensworth, Hampshire, both in England. It
is still uncertain whether all these earlier investigators found B.
haimeanus or the sporocysts of some other species of bucephalid
trematode, since from first hand experience it is known that it is
extremely difficult, if not impossible, to distinguish one species of
bucephalid sporocyst from another and inasmuch as a t least 145 species
of adult bucephalids have been described from fishes (Yamaguti, 1958),
and many from marine fishes, it is highly probable that the sporocysts
of various other species of bucephalids have been confused with B.
haimeanus. Recently, James et al. (1966) have reported finding this
parasite in Cardium edule from Llanrhidian Sands, Gower Peninsula,
Wales. They designated it as Cercaria bucephalopsis haimeana.
Description of stages in mollusc. Sporocysts long, tubular, multibranched, and tangled, closely packed in all body tissues of host
except foot; cercariae a t various stages of development in brood
chamber (Fig. 110). Cercaria (Fig. 11 1) with characteristic forked tail,
with each furca attached to prominent triangular stem ; tail extremely
contractile and may be many times longer than body when extended ;
body proper approximately 0.26 mm long, covered with cuticular
spines ; mouth, in center of ventral sucker (pharynx of some authors),
leads into simple sac-like intestine ; anterior holdfast (rhynchus of
some authors) armed with secretory glands ; oval excretory vesicle
posterior to ventral sucker.
James et al. (1966) have studied the fine structure of the sporocyst
wall of Bucephalus haimeanus. Their electron micrographs have
revealed that the sporocyst wall is comprised of an external syncytial
tegument, lying on a basement lamella, and an internal cellular subtegument which surrounds the brood chamber. The syncytial tegument
has areas of dense cytoplasm alternating with sparse reticulate cytoplasm. The dense cytoplasm contains nuclei, a few mitochondria and
secretory products.
Life cycle. The complete life cycle of B. haimeanus is not known.
Although various authors (Williamson, 1911 ; Johnstone, 1904 ; Dawes,
1946 ; and others) have reported what they consider to be the metacercaria of this trematode encysted in the nerves, mainly the cranial
but also in the spinal nerves near the tail of gadoid fishes, commonly
MARINE MOLLTJSCS AS HOSTS FOR SYMBIOSES
lescens ”) or as B. haimeanus in marine pelecypods. For example,
Johnstone (1904) has reported its occurrence in Cardium edule from the
Lancashire coast, Pelseneer (1906) has reported it in Syndosmya alba
from the north of France, and Lebour (1911) has found it occurring
throughout the tissues of Cardium edule collected a t Fenham Flats,
Northumberland, and at Ensworth, Hampshire, both in England. It
is still uncertain whether all these earlier investigators found B.
haimeanus or the sporocysts of some other species of bucephalid
trematode, since from first hand experience it is known that it is
extremely difficult, if not impossible, to distinguish one species of
bucephalid sporocyst from another and inasmuch as a t least 145 species
of adult bucephalids have been described from fishes (Yamaguti, 1958),
and many from marine fishes, it is highly probable that the sporocysts
of various other species of bucephalids have been confused with B.
haimeanus. Recently, James et al. (1966) have reported finding this
parasite in Cardium edule from Llanrhidian Sands, Gower Peninsula,
Wales. They designated it as Cercaria bucephalopsis haimeana.
Description of stages in mollusc. Sporocysts long, tubular, multibranched, and tangled, closely packed in all body tissues of host
except foot; cercariae a t various stages of development in brood
chamber (Fig. 110). Cercaria (Fig. 11 1) with characteristic forked tail,
with each furca attached to prominent triangular stem ; tail extremely
contractile and may be many times longer than body when extended ;
body proper approximately 0.26 mm long, covered with cuticular
spines ; mouth, in center of ventral sucker (pharynx of some authors),
leads into simple sac-like intestine ; anterior holdfast (rhynchus of
some authors) armed with secretory glands ; oval excretory vesicle
posterior to ventral sucker.
James et al. (1966) have studied the fine structure of the sporocyst
wall of Bucephalus haimeanus. Their electron micrographs have
revealed that the sporocyst wall is comprised of an external syncytial
tegument, lying on a basement lamella, and an internal cellular subtegument which surrounds the brood chamber. The syncytial tegument
has areas of dense cytoplasm alternating with sparse reticulate cytoplasm. The dense cytoplasm contains nuclei, a few mitochondria and
secretory products.
Life cycle. The complete life cycle of B. haimeanus is not known.
Although various authors (Williamson, 1911 ; Johnstone, 1904 ; Dawes,
1946 ; and others) have reported what they consider to be the metacercaria of this trematode encysted in the nerves, mainly the cranial
but also in the spinal nerves near the tail of gadoid fishes, commonly