212
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
6. Mefacercaria (Gymnophallus) margaritarum (Dubois, 1901).
(Figs. 119-121)
superfamily Fellodistomatoidea; family Fellodistomatidae)
(Superorder Anepitheliocystidia; order Strigeatoidea; suborder Azygiata;
The majority of gymnophallid trematodes known from commercially
important marine molluscs are only known in their metacercarial
forms. These are usually found between the mantle and shell of pelecypods. From what is known, it may be said that the cercariae of these
trematodes develop in spherical or subspherical sporocysts in pelecypods.
The cercariae possess eye-spots which may be reduced or wanting.
Furthermore, they are usually without a tail by the time they escape
from the sporocyst ; however, a small forked tail does develop in certain
species (Fig. 127), thus suggesting their relationship to the furcocercous
cercariae, but the tail is usually lost prior to emergence. I n at least
three species, Cercaria dichotoma, C. fulbrighti and C. myae, the tail is
retained by the emerging cercaria.
Stunkard and Uzmann (1958) have given an excellent review of the
gymnophallid trematodes. As they have stated, very often in the past
individuals working on these trematodes have mistaken metacercariae
for cercariae. Furthermore, there is little information on the asexual and
adult stages among this group and the descriptions of the metacercariae
in most instances are far from adequate by modern standards. As the
result, to quote from Stunkard and Uzmann : ‘‘ The situation is chaotic
and one of utter confusion.” Nevertheless, so that this section may be
more complete, a brief account of each of the gymnophallids known
from commercially important molluscs is given. I n many instances the
diagnosis is far from satisfactory, even after I have attempted to supplement the original descriptions by describing features shown in the
illustrations of the earlier workers. Perhaps this undesirable situation
will stimulate others to re-examine this group of trematodes.
Relative to the systematic position of the gymnophallids, although
Dawes (1946) and Yamaguti (1958) have continued to consider them
as members of the family Microphallidae, Cable (1953, 1954a,b, 1956)
has shown that they should be included in the family E’ellodistomatidae.
I am following Cable in placing these trematodes in the latter family.
The trematode referred to as Gymnophallus margaritarum is still
not known in its adult form although various individuals (Jameson,
1902 ; Lebour, 1911 ; Palombi, 1924) have postulated, and most probably correctly so, that it is a parasite of certain shore birds which
feed on Mytilus edulis. Our present knowledge concerning this parasite
dates back to a t least 1655 when, according to Giard (1907), Olaus
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
6. Mefacercaria (Gymnophallus) margaritarum (Dubois, 1901).
(Figs. 119-121)
superfamily Fellodistomatoidea; family Fellodistomatidae)
(Superorder Anepitheliocystidia; order Strigeatoidea; suborder Azygiata;
The majority of gymnophallid trematodes known from commercially
important marine molluscs are only known in their metacercarial
forms. These are usually found between the mantle and shell of pelecypods. From what is known, it may be said that the cercariae of these
trematodes develop in spherical or subspherical sporocysts in pelecypods.
The cercariae possess eye-spots which may be reduced or wanting.
Furthermore, they are usually without a tail by the time they escape
from the sporocyst ; however, a small forked tail does develop in certain
species (Fig. 127), thus suggesting their relationship to the furcocercous
cercariae, but the tail is usually lost prior to emergence. I n at least
three species, Cercaria dichotoma, C. fulbrighti and C. myae, the tail is
retained by the emerging cercaria.
Stunkard and Uzmann (1958) have given an excellent review of the
gymnophallid trematodes. As they have stated, very often in the past
individuals working on these trematodes have mistaken metacercariae
for cercariae. Furthermore, there is little information on the asexual and
adult stages among this group and the descriptions of the metacercariae
in most instances are far from adequate by modern standards. As the
result, to quote from Stunkard and Uzmann : ‘‘ The situation is chaotic
and one of utter confusion.” Nevertheless, so that this section may be
more complete, a brief account of each of the gymnophallids known
from commercially important molluscs is given. I n many instances the
diagnosis is far from satisfactory, even after I have attempted to supplement the original descriptions by describing features shown in the
illustrations of the earlier workers. Perhaps this undesirable situation
will stimulate others to re-examine this group of trematodes.
Relative to the systematic position of the gymnophallids, although
Dawes (1946) and Yamaguti (1958) have continued to consider them
as members of the family Microphallidae, Cable (1953, 1954a,b, 1956)
has shown that they should be included in the family E’ellodistomatidae.
I am following Cable in placing these trematodes in the latter family.
The trematode referred to as Gymnophallus margaritarum is still
not known in its adult form although various individuals (Jameson,
1902 ; Lebour, 1911 ; Palombi, 1924) have postulated, and most probably correctly so, that it is a parasite of certain shore birds which
feed on Mytilus edulis. Our present knowledge concerning this parasite
dates back to a t least 1655 when, according to Giard (1907), Olaus