6. THE PHYLA PORIFERA, CNIDARIA AND PLATYRELMIXTHES
255
genus Tylocephalum includes several ‘( species ” the larvae of which
have been reported from pelecypods. According to Fujita (1943), a
cestode tentatively identified as a member of the third order, Trypanorhyncha, has been found in “ oysters ’’ from Belgium. In addition,
Herdman and Hornell (1903) were of the opinion that pearl-formation
in Nargaritifera vulgaris in Ceylonese waters was caused by another
trypanorhynch cestode designated as Tetrarhynchus zcnionifactor.
However, later, Herdman (in Southwell, 1924) has stated :
I am now inclined to think that the globular cysts in the liver of the Ceylon
Pearl Oysters which Hornell and I found in 1902 and regarded as larval
stages of a Tetrarhynchus and which were formally described by Shipley
and Hornell in 1904 under the name Tetrarhynchus unionifactor, are-as
Mr. Southwell says in the present paper-more probably to be referred to
the genus Tylocephalum.
Thus the occurrence of this trypanorhynch in pearl oysters is no longer
accepted.
From the practical viewpoint, interest in cestode parasites of commercially important molluscs has centered primarily around the question: ‘‘ Can these parasites cause sufficient injury so as to result in
death or a severely unhealthy state?” Nosatisfactory answers are yet
available resulting from controlled experiments. From the standpoint
of public health, again no definite answers are available. However,
since the natural definitive hosts of those larval cestodes known from
marine molluscs are all elasmobranch fishes, the possibility of their
becoming established in humans is doubtful.
Reviewed below are the three genera of cestodes which have been
reported from commercially important marine molluscs. The classification employed is that of Yamaguti (1959).
1. Tylocephalum spp. Linton, 1890. (Figs. 168 and 169)
(Order Lecanicephalidea ; family Lecanicephalidae)
The presence of larval lecanicephalid cestodes in oysters and other
pelecypods has been known since Herdman and Hornell (1906),
Shipley and Hornell (1906), Jameson (1912) and Southwell (1924)
reported their occurrence in Ceylonese waters, especially in the pearl
oyster, Margaritifera vulgaris, and Seurat (1904, 1906) reported a species
in M . margaritifera cumingi in the South Pacific. The original interest in
these larval cestodes stems from Herdman’s (1903-06) hypothesie that
they are one of the causes of pearl formation in oysters and that remains
of such larvae could be found in the center of pearls. Although South-
255
genus Tylocephalum includes several ‘( species ” the larvae of which
have been reported from pelecypods. According to Fujita (1943), a
cestode tentatively identified as a member of the third order, Trypanorhyncha, has been found in “ oysters ’’ from Belgium. In addition,
Herdman and Hornell (1903) were of the opinion that pearl-formation
in Nargaritifera vulgaris in Ceylonese waters was caused by another
trypanorhynch cestode designated as Tetrarhynchus zcnionifactor.
However, later, Herdman (in Southwell, 1924) has stated :
I am now inclined to think that the globular cysts in the liver of the Ceylon
Pearl Oysters which Hornell and I found in 1902 and regarded as larval
stages of a Tetrarhynchus and which were formally described by Shipley
and Hornell in 1904 under the name Tetrarhynchus unionifactor, are-as
Mr. Southwell says in the present paper-more probably to be referred to
the genus Tylocephalum.
Thus the occurrence of this trypanorhynch in pearl oysters is no longer
accepted.
From the practical viewpoint, interest in cestode parasites of commercially important molluscs has centered primarily around the question: ‘‘ Can these parasites cause sufficient injury so as to result in
death or a severely unhealthy state?” Nosatisfactory answers are yet
available resulting from controlled experiments. From the standpoint
of public health, again no definite answers are available. However,
since the natural definitive hosts of those larval cestodes known from
marine molluscs are all elasmobranch fishes, the possibility of their
becoming established in humans is doubtful.
Reviewed below are the three genera of cestodes which have been
reported from commercially important marine molluscs. The classification employed is that of Yamaguti (1959).
1. Tylocephalum spp. Linton, 1890. (Figs. 168 and 169)
(Order Lecanicephalidea ; family Lecanicephalidae)
The presence of larval lecanicephalid cestodes in oysters and other
pelecypods has been known since Herdman and Hornell (1906),
Shipley and Hornell (1906), Jameson (1912) and Southwell (1924)
reported their occurrence in Ceylonese waters, especially in the pearl
oyster, Margaritifera vulgaris, and Seurat (1904, 1906) reported a species
in M . margaritifera cumingi in the South Pacific. The original interest in
these larval cestodes stems from Herdman’s (1903-06) hypothesie that
they are one of the causes of pearl formation in oysters and that remains
of such larvae could be found in the center of pearls. Although South-