6. THE PHYLA PORIFERA, CNIDARIA AND PLATYHELMINTHES
241
to near posterior level of pharynx, with conspicuous lateral secondary
tubules, stem subovoid.
Nothing else is known about H . ambigua;
however, from what is known about the life cycle pattern in this genus,
Tapes decussatus is undoubtedly the second intermediate host, while
the first is most probably a gastropod and the natural definitive host is
one or more species of shore birds.
Other information.
28. Himasthla compacta Stunkard, 1960
(Superorder Anepitheliocystidia; order Echinostomida; suborder Echinostomata; superfamily Echinostomatoidea; family Echinostomatidae)
Stunkard (1960), in a paper in which is given a concise but excellent
review of the genus Himasthla, also described a new species, H . compacta,
from laboratory-reared Larus argentatus which had been fed tissues of
Mya arenaria, collected near Boothbay Harbor, Maine, which included
an unknown species of metacercaria. He also recovered specimens of H .
cornpacta from Lurus argentatus fed metacercariae from Mya arenaria
which in turn had been exposed t o echinostome cercariae from the
gastropod Hydrobia minutu collected at Woods Hole, Massachusetts.
Since Mya arenaria, and possibly other commercially important marine
pelecypods, are involved in the life cycle of H . compacta, it is being
included therein.
Description of metacercuria (the stage found in economically important molluscs). Unfortunately, although Stunkard described the
adult, redia, and cercaria of H . compacta, he did not give a description
of the metacercaria. However, since both the cercaria and adult
possess twenty-nine collar spines, it may be assumed that the encysted
metacercaria also possesses twenty-nine spines. These are located on
a ventrally interrupted reniform collar. Twenty-seven of the spines
are arranged in a curved linear row while two smaller spines, one at
each end, are located behind the terminal large spine at each end.
Furthermore, since the body proper of the cercaria measures 0.300.60 mm long and 0.08-0.19 mm wide while a very young adult specimen
recovered from Larus argentatus measures 0.46 mm long and 0.12 mm
wide (extrapolated from Stunkard’s drawing), it can be assumed that
the body dimensions of the metacercaria are between these two sets
of limits, most probably closer to that of the very young adult in
which the reproductive organs had not yet developed.
Life cycle. Stunkard (1960) was able to infect juveniles of Hydrobia
minuta by exposing them to eggs from experimentally infected Larw
A.M.B.--G
16
241
to near posterior level of pharynx, with conspicuous lateral secondary
tubules, stem subovoid.
Nothing else is known about H . ambigua;
however, from what is known about the life cycle pattern in this genus,
Tapes decussatus is undoubtedly the second intermediate host, while
the first is most probably a gastropod and the natural definitive host is
one or more species of shore birds.
Other information.
28. Himasthla compacta Stunkard, 1960
(Superorder Anepitheliocystidia; order Echinostomida; suborder Echinostomata; superfamily Echinostomatoidea; family Echinostomatidae)
Stunkard (1960), in a paper in which is given a concise but excellent
review of the genus Himasthla, also described a new species, H . compacta,
from laboratory-reared Larus argentatus which had been fed tissues of
Mya arenaria, collected near Boothbay Harbor, Maine, which included
an unknown species of metacercaria. He also recovered specimens of H .
cornpacta from Lurus argentatus fed metacercariae from Mya arenaria
which in turn had been exposed t o echinostome cercariae from the
gastropod Hydrobia minutu collected at Woods Hole, Massachusetts.
Since Mya arenaria, and possibly other commercially important marine
pelecypods, are involved in the life cycle of H . compacta, it is being
included therein.
Description of metacercuria (the stage found in economically important molluscs). Unfortunately, although Stunkard described the
adult, redia, and cercaria of H . compacta, he did not give a description
of the metacercaria. However, since both the cercaria and adult
possess twenty-nine collar spines, it may be assumed that the encysted
metacercaria also possesses twenty-nine spines. These are located on
a ventrally interrupted reniform collar. Twenty-seven of the spines
are arranged in a curved linear row while two smaller spines, one at
each end, are located behind the terminal large spine at each end.
Furthermore, since the body proper of the cercaria measures 0.300.60 mm long and 0.08-0.19 mm wide while a very young adult specimen
recovered from Larus argentatus measures 0.46 mm long and 0.12 mm
wide (extrapolated from Stunkard’s drawing), it can be assumed that
the body dimensions of the metacercaria are between these two sets
of limits, most probably closer to that of the very young adult in
which the reproductive organs had not yet developed.
Life cycle. Stunkard (1960) was able to infect juveniles of Hydrobia
minuta by exposing them to eggs from experimentally infected Larw
A.M.B.--G
16