256
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
well (1924) supported this theory to some extent, Jameson (1912), in
whose detailed study a complete account of the controversy is given,
has pointed out the lack of evidence to support Herdman’s theory,
including the demonstration of the absence of any structures remotely
resembling tapeworm larvae in the center of pearls. It is now generally
agreed that Jameson is correct since these globose larvae are found in
the soft tissues on the interior of bivalves and not on the mantle, as are
gymnophallid trematode metacercariae which do stimulate pearl
formation, and hence could not stimulate the nacre-secreting mantle to
form pearls. But, as Southwell (1924) has contended, if moribund
Tylocephalum larvae fail to penetrate the oyster, they may stimulate
pearl formation on the mantle surface. Proof of this, however, is
wanting.
At least four “ species ” of Tylocephalum larvae have been described
from pelecypods. T . Zudificans was described by Jameson (1912)
encysted in the connective tissue of Margaritifera vulgaris in Ceylon ;
T . margaritiferae was reported by Seurat (1904, 1906) encysted in Margaritifera margaritifera cumingi from the Gambier Islands ; T . minus was
reported by Jameson (1912) in M . vulgaris from Ceylon; and T.
unionifactor was reported by Herdman and Hornell (1906) in M .
vulgaris from Ceylon. In addition, Dollfus (1923b) has reported a
Tylocephalum larva in Meleagrina occa and M . irradians from Nossi-BB,
offthe coast of Madagascar. Among these, Southwell (1930) regarded T.
1udiJicans and T . minus as the larvae of and hence synonyms of T.
dieranha, a species described by Shipley and Hornell (1906) from the
elasmobranch Myliobatis maculata in Ceylonese waters. This has
never been demonstrated experimentally. As Yamaguti (1959) has
pointed out, the original descriptions of these larvae are so incomplete
that they are of no value in identification. As the result, I have no
choice but to refer to them collectively as Tylocephalum spp.
It should be pointed out that the genus Tylocephalum was erected
by Linton (1890) for T . pingue, an adult lecanicephalid found in the
elasmobranchs Rhinoptera quadriloba and R. bonasus caught at Woods
Hole, Massachusetts. Although it is generally agreed that the encysted
larvae found in pelecypods are the immature forms of one or more
species of Tylocephalum, attempts to recover adult tapeworms from
sharks which had been fed these larvae have all failed (Jameson, 1912);
hence, despite a morphological similarity between the holdfasts of the
larvae and adults, doubt still exists as to whether the larvae are those
of Tylocephalum.
Relative to the designation applicable to the globose larvae found
in marine pelecypods, Cheng (1966a) has stated :
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
well (1924) supported this theory to some extent, Jameson (1912), in
whose detailed study a complete account of the controversy is given,
has pointed out the lack of evidence to support Herdman’s theory,
including the demonstration of the absence of any structures remotely
resembling tapeworm larvae in the center of pearls. It is now generally
agreed that Jameson is correct since these globose larvae are found in
the soft tissues on the interior of bivalves and not on the mantle, as are
gymnophallid trematode metacercariae which do stimulate pearl
formation, and hence could not stimulate the nacre-secreting mantle to
form pearls. But, as Southwell (1924) has contended, if moribund
Tylocephalum larvae fail to penetrate the oyster, they may stimulate
pearl formation on the mantle surface. Proof of this, however, is
wanting.
At least four “ species ” of Tylocephalum larvae have been described
from pelecypods. T . Zudificans was described by Jameson (1912)
encysted in the connective tissue of Margaritifera vulgaris in Ceylon ;
T . margaritiferae was reported by Seurat (1904, 1906) encysted in Margaritifera margaritifera cumingi from the Gambier Islands ; T . minus was
reported by Jameson (1912) in M . vulgaris from Ceylon; and T.
unionifactor was reported by Herdman and Hornell (1906) in M .
vulgaris from Ceylon. In addition, Dollfus (1923b) has reported a
Tylocephalum larva in Meleagrina occa and M . irradians from Nossi-BB,
offthe coast of Madagascar. Among these, Southwell (1930) regarded T.
1udiJicans and T . minus as the larvae of and hence synonyms of T.
dieranha, a species described by Shipley and Hornell (1906) from the
elasmobranch Myliobatis maculata in Ceylonese waters. This has
never been demonstrated experimentally. As Yamaguti (1959) has
pointed out, the original descriptions of these larvae are so incomplete
that they are of no value in identification. As the result, I have no
choice but to refer to them collectively as Tylocephalum spp.
It should be pointed out that the genus Tylocephalum was erected
by Linton (1890) for T . pingue, an adult lecanicephalid found in the
elasmobranchs Rhinoptera quadriloba and R. bonasus caught at Woods
Hole, Massachusetts. Although it is generally agreed that the encysted
larvae found in pelecypods are the immature forms of one or more
species of Tylocephalum, attempts to recover adult tapeworms from
sharks which had been fed these larvae have all failed (Jameson, 1912);
hence, despite a morphological similarity between the holdfasts of the
larvae and adults, doubt still exists as to whether the larvae are those
of Tylocephalum.
Relative to the designation applicable to the globose larvae found
in marine pelecypods, Cheng (1966a) has stated :