214
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
siphonal muscles of Tapes and in the mantle of Cardium as Jameson
(1902) had indicated. Nicoll (1907a) has expressed the opinion that
these larvae are those of Gymnophallus dapsilis, a parasite of scoters,
Melanitta nigra and M . fusca,, but experimental evidence is wanting.
The identification of this parasite, a t least the larval stages, appears
to have been clarified by Palombi (1924) who has demonstrated that
Jameson’s (1902) “ pearl trematode ”, which he believed to be L.
somateriae from Mytilus edulis, is identical with Dubois’ Gymnophallus
margaritarum, as is Lebour’s Cercaria margaritae’. I n addition, Palombi
has stated that the metacercaria named Metacercaria (Gymnophallus)
duboisi by Dollfus (1923a) from Mytilus galloprovincialis is the same as
G. margaritarum, as is the metacercaria (reported as a cercaria) found
by Sinitsin (1911) in Mytilus edulis and Venus sp.? from Sebastopol on
the Black Sea and named Adolescaria perla. He has also demonstrated
that the sporocysts and cercariae in Tapes and Cardium are the progenitor stages of the metacercaria in Mytilus. 7
Description of stages in mollusc. Sporocyst (Fig. 119) simple, more
or less spherical and colorless, size varies greatly. Cercaria (Figs. 120
and 121) without tail, pear-shaped, more pointed posteriorly, 0.150.3 mm long, covered with cuticular spines ; a pair of brown eye-spots
a t anterior end, one on each side of anterior sucker ; ventral sucker in
posterior half of body ; ratio of anterior to ventral sucker 3 : 2 (Lebour
(1911) reported it to be 2: 3, but, as Cole (1938) has pointed out, this
is probably a misprint); prepharynx absent; pharynx and short
esophagus present ; globose intestinal caeca reaching midlength of
body or slightly beyond; according to Nicoll (1906a), anlagen of two
testes are situated posterior to ventral sucker (Fig. 121) ; excretory
vesicle large, Y-shaped, with arms reaching anteriorly t o as far as
level of pharynx.
Life cycle. The complete life cycle of G. margaritarum is not known
although, as stated, mollusc-eating shore birds have been postulated
to be the definitive hosts. According to Lebour (1911) :
The cercaria presumably leaves its first host by aid of currents or by crawling
on the ground by aid of its suckers, reaches its (second) intermediate host,
Mytilus edulis. Here it settles down between the mantle and shell, feeds and
grows and finally makes its way into the tissues of the mantle, curls itself up
and rests.
The metacercaria does not secrete its own cyst, rather the mantle
grows around it and forms an enclosing epithelial sac. The cells of
this sac eventually secrete a pearly layer around the parasite. If such
a pearly layer is formed, the metacercaria generally dies, forming the
* See note to p. 218 on p. 390. i See note on p. 390.
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
siphonal muscles of Tapes and in the mantle of Cardium as Jameson
(1902) had indicated. Nicoll (1907a) has expressed the opinion that
these larvae are those of Gymnophallus dapsilis, a parasite of scoters,
Melanitta nigra and M . fusca,, but experimental evidence is wanting.
The identification of this parasite, a t least the larval stages, appears
to have been clarified by Palombi (1924) who has demonstrated that
Jameson’s (1902) “ pearl trematode ”, which he believed to be L.
somateriae from Mytilus edulis, is identical with Dubois’ Gymnophallus
margaritarum, as is Lebour’s Cercaria margaritae’. I n addition, Palombi
has stated that the metacercaria named Metacercaria (Gymnophallus)
duboisi by Dollfus (1923a) from Mytilus galloprovincialis is the same as
G. margaritarum, as is the metacercaria (reported as a cercaria) found
by Sinitsin (1911) in Mytilus edulis and Venus sp.? from Sebastopol on
the Black Sea and named Adolescaria perla. He has also demonstrated
that the sporocysts and cercariae in Tapes and Cardium are the progenitor stages of the metacercaria in Mytilus. 7
Description of stages in mollusc. Sporocyst (Fig. 119) simple, more
or less spherical and colorless, size varies greatly. Cercaria (Figs. 120
and 121) without tail, pear-shaped, more pointed posteriorly, 0.150.3 mm long, covered with cuticular spines ; a pair of brown eye-spots
a t anterior end, one on each side of anterior sucker ; ventral sucker in
posterior half of body ; ratio of anterior to ventral sucker 3 : 2 (Lebour
(1911) reported it to be 2: 3, but, as Cole (1938) has pointed out, this
is probably a misprint); prepharynx absent; pharynx and short
esophagus present ; globose intestinal caeca reaching midlength of
body or slightly beyond; according to Nicoll (1906a), anlagen of two
testes are situated posterior to ventral sucker (Fig. 121) ; excretory
vesicle large, Y-shaped, with arms reaching anteriorly t o as far as
level of pharynx.
Life cycle. The complete life cycle of G. margaritarum is not known
although, as stated, mollusc-eating shore birds have been postulated
to be the definitive hosts. According to Lebour (1911) :
The cercaria presumably leaves its first host by aid of currents or by crawling
on the ground by aid of its suckers, reaches its (second) intermediate host,
Mytilus edulis. Here it settles down between the mantle and shell, feeds and
grows and finally makes its way into the tissues of the mantle, curls itself up
and rests.
The metacercaria does not secrete its own cyst, rather the mantle
grows around it and forms an enclosing epithelial sac. The cells of
this sac eventually secrete a pearly layer around the parasite. If such
a pearly layer is formed, the metacercaria generally dies, forming the
* See note to p. 218 on p. 390. i See note on p. 390.