6. THE PHYLA PORIFERA, CNIDARIA AND PLATYHELMINTHES
237
surfaces of C. gigas although none were found in the tissues. Among
the eight marine pelecypods experimentally infected, M y a arenaria
has been demonstrated to be the most efficient host, i.e. the greatest
percentage of metacercariae can be recovered from it, thus supporting
Uzmann’s concept that this clam is an important and heavily parasitized one in nature. The natural definitive host of H . quissetensis is
most probably some shore bird that feeds on pelecypods. Stunkard
(1934, 1937, 1938) has been able experimentally to infect the tern,
Sterna hirundo, and the herring gull, Larus argentatus.
Description of metacermria (the stage found in economically
important molluscs). Cysts spherical to oval, 0.14-0-19 mm in diameter,
enclosing metacercaria which is doubled over ventrally (Fig. 145). A
detailed description of the metacercaria is not available ; however,
cercariae and young adults, hence presumably also the metacercariae,
bear thirty-one spines on a ventrally interrupted reniform collar ;
spines arranged in a single row except at the two terminals where
two spines are situated between and behind the others. When cysts
are observed in histological section, the inner parasite-secreted wall
is usually surrounded by an outer wall comprised of the host’s connective tissue or muscle fibers, depending on the site of the cyst, and
leucocytes.
As stated, Stunkard has been able to infect Sterna
hirundo and Larus argentatus by feeding them metacercariae. He also
has compared the adults recovered from his experimentally infected
birds with those of Linton (1928) collected from four species of naturally
infected Larus and from Nycticorax nycticorax which had been identified
as Himasthla elongata.
Although the redial stage in Nassarius obsoletus, the cercaria,
metacercaria, and adult are known, the exact manner in which N .
obsoletus, the first intermediate host, becomes infected remains to be
demonstrated.
Other information. Nothing is known about the ecology of this parasite, except Miller and Northrup’s (1926) report that in Massachusetts
cercariae-bearing rediae predominate during July through October
while immature rediae are more abundant during November and
December.
The effects of the plasma of pelecypods on cercariae have been
reviewed in an earlier section (see Chapter 4, Section 11, A, 1). Furthermore, the reactions of various pelecypods to encysting metacercariae,
as reported by Cheng et al. (1966a), have also been reviewed above
(see Chapter 4, Section 11, A, 2).
Life cycle.
237
surfaces of C. gigas although none were found in the tissues. Among
the eight marine pelecypods experimentally infected, M y a arenaria
has been demonstrated to be the most efficient host, i.e. the greatest
percentage of metacercariae can be recovered from it, thus supporting
Uzmann’s concept that this clam is an important and heavily parasitized one in nature. The natural definitive host of H . quissetensis is
most probably some shore bird that feeds on pelecypods. Stunkard
(1934, 1937, 1938) has been able experimentally to infect the tern,
Sterna hirundo, and the herring gull, Larus argentatus.
Description of metacermria (the stage found in economically
important molluscs). Cysts spherical to oval, 0.14-0-19 mm in diameter,
enclosing metacercaria which is doubled over ventrally (Fig. 145). A
detailed description of the metacercaria is not available ; however,
cercariae and young adults, hence presumably also the metacercariae,
bear thirty-one spines on a ventrally interrupted reniform collar ;
spines arranged in a single row except at the two terminals where
two spines are situated between and behind the others. When cysts
are observed in histological section, the inner parasite-secreted wall
is usually surrounded by an outer wall comprised of the host’s connective tissue or muscle fibers, depending on the site of the cyst, and
leucocytes.
As stated, Stunkard has been able to infect Sterna
hirundo and Larus argentatus by feeding them metacercariae. He also
has compared the adults recovered from his experimentally infected
birds with those of Linton (1928) collected from four species of naturally
infected Larus and from Nycticorax nycticorax which had been identified
as Himasthla elongata.
Although the redial stage in Nassarius obsoletus, the cercaria,
metacercaria, and adult are known, the exact manner in which N .
obsoletus, the first intermediate host, becomes infected remains to be
demonstrated.
Other information. Nothing is known about the ecology of this parasite, except Miller and Northrup’s (1926) report that in Massachusetts
cercariae-bearing rediae predominate during July through October
while immature rediae are more abundant during November and
December.
The effects of the plasma of pelecypods on cercariae have been
reviewed in an earlier section (see Chapter 4, Section 11, A, 1). Furthermore, the reactions of various pelecypods to encysting metacercariae,
as reported by Cheng et al. (1966a), have also been reviewed above
(see Chapter 4, Section 11, A, 2).
Life cycle.