7. THE PHYLA NEMERTINEA, ASCHELMINTIIES AND ANNELIDA
263
Although earlier workers (van Beneden and Hesse, 1863 ; Semper,
1877 ; Verrill, 1892b ; Burger, 1895) have regarded these as parasites,
subsequent workers (Burger, 1907 ; Guberlet, 1925 ; Coe, 1943, 1945 ;
Hyman, 1951; Barnes, 1963; Cheng, 1964b) have stated that these
nemerteans are definitely commensals which feed on food brought in
by the molluscs’ incurrents. Furthermore, Guberlet (1925) has pointed
out that the lesions sometimes found on pelecypods harboring Malacobdella, as has been reported by McMillin (1924) and others, cannot
be attributed to these nemerteans. There is no reliable evidence to
indicate that Malacobdella spp. are in any way injurious to their hosts.
11. Phylum ASCHELMINTHES
Class Nematoda
Among the aschelminthes, only a few species of nematodes have
been reported from commercially important marine molluscs although
they have been recorded, either as larvae or as adults, from a number
of terrestrial, freshwater, and a few commercially unimportant marine
molluscs (see Pelseneer, 1928 ; Gerichter, 1948). Relative to those
nematodes sometimes encountered during the routine examination of
shellfish, the question invariably arises as to whether these are true
parasites or are free-living species which have become temporarily
attached. That nematodes are fairly commonly found on the surfaces
and in the mantle cavity of marine molluscs is beyond doubt. The
possibility also exists, as with certain protozoa and arthropods which
have been reported from moribund pelecypods, as to whether the
nematodes found in unhealthy molluscan shellfish are obligatory
parasites, facultative parasites, or predators which have invaded
unhealthy, moribund and dead molluscs.
The sparsity of reports on nematodes parasitic in commercially
important marine molluscs would suggest that nematodes rarely occur
in these hosts. As to whether this is true or not cannot be stated since
perhaps the situation merely reflects the lack of competent nematode
taxonomists among marine parasitologists. For example, various
workers (personal communication) have found nematodes in histological sections of certain pelecypods, especially oysters ; however, no
attempts have been made to identify or study the biology of these.
My former associate, R. W. Burton (1961), has found cross-sections of
an unidentified nematode in several specimens of Crassostrea virginica
collected at Marumsco Bar, Pocomoke Sound, Clay Island Bar, and
Malls Bar, Tangier Sound, all in Worcester County, Maryland, during
1960. I n addition, Burton (unpublished) has found unidentified
263
Although earlier workers (van Beneden and Hesse, 1863 ; Semper,
1877 ; Verrill, 1892b ; Burger, 1895) have regarded these as parasites,
subsequent workers (Burger, 1907 ; Guberlet, 1925 ; Coe, 1943, 1945 ;
Hyman, 1951; Barnes, 1963; Cheng, 1964b) have stated that these
nemerteans are definitely commensals which feed on food brought in
by the molluscs’ incurrents. Furthermore, Guberlet (1925) has pointed
out that the lesions sometimes found on pelecypods harboring Malacobdella, as has been reported by McMillin (1924) and others, cannot
be attributed to these nemerteans. There is no reliable evidence to
indicate that Malacobdella spp. are in any way injurious to their hosts.
11. Phylum ASCHELMINTHES
Class Nematoda
Among the aschelminthes, only a few species of nematodes have
been reported from commercially important marine molluscs although
they have been recorded, either as larvae or as adults, from a number
of terrestrial, freshwater, and a few commercially unimportant marine
molluscs (see Pelseneer, 1928 ; Gerichter, 1948). Relative to those
nematodes sometimes encountered during the routine examination of
shellfish, the question invariably arises as to whether these are true
parasites or are free-living species which have become temporarily
attached. That nematodes are fairly commonly found on the surfaces
and in the mantle cavity of marine molluscs is beyond doubt. The
possibility also exists, as with certain protozoa and arthropods which
have been reported from moribund pelecypods, as to whether the
nematodes found in unhealthy molluscan shellfish are obligatory
parasites, facultative parasites, or predators which have invaded
unhealthy, moribund and dead molluscs.
The sparsity of reports on nematodes parasitic in commercially
important marine molluscs would suggest that nematodes rarely occur
in these hosts. As to whether this is true or not cannot be stated since
perhaps the situation merely reflects the lack of competent nematode
taxonomists among marine parasitologists. For example, various
workers (personal communication) have found nematodes in histological sections of certain pelecypods, especially oysters ; however, no
attempts have been made to identify or study the biology of these.
My former associate, R. W. Burton (1961), has found cross-sections of
an unidentified nematode in several specimens of Crassostrea virginica
collected at Marumsco Bar, Pocomoke Sound, Clay Island Bar, and
Malls Bar, Tangier Sound, all in Worcester County, Maryland, during
1960. I n addition, Burton (unpublished) has found unidentified