6. THE PHYLA PORIFERA, CNIDARIA AND PLATYHELMINTHES
259
elasmobranch, hatch in water. The ciliated coracidium is swept up
with other planktonic organisms during the oyster’s feeding process
and is ingested or becomes attached to the gill surfaces. It sheds its
ciliated coat prior to penetrating through either the alimentary wall
or through the gill surface, develops into a metacestode, and eventually
is immobilized when the host forms a capsule around it. The remainder
of its life cycle remains undertermined.
Both Sparks (1963) and Cheng (1966a) have
studied pathological changes in parasitized Crassostrea virginica (see
Chapter 4, Section 11, A, 2). Since it has been found that encysted
metacestodes are eventually resorbed in C. virginica, I have concluded
that this oyster is probably not the natural intermediate host. Further
evidence for this has recently been uncovered while examining parasitized Tapes semidecussatus. I n this clam, the host’s cellular reaction
is even more severe than in Crassostrea virginica, and resorption of the
parasite is commonly observed (Cheng and Rifkin, 1967).
Other information.
2 . Echeneibothrium sp. van Beneden, 1850. (Figs. 170 and 171)
(Order Tetraphyllidea; family Phyllobothriidae)
The genus Echeneibothrium was erected by van Beneden (1850) to
include E. minimum, a parasite found in the spiral valve of elasmobranchs. Since then, a t least thirty-five species of adults have been
described, all from elasmobranchs (Yamaguti, 1959). Hyman (1951)
has depicted the larval stage (plerocercoid) of an undetermined species
of Echeneibothrium encysted in the foot of a “ clam ” collected in
Puget Sound. This larva may be the same as that reported by Sparks
and Chew (1966) encysted throughout the tissues of the littleneck
clam, Venerupis staminea (Figs. 170 and 171), collected in Humboldt
Bay, California, which they have indicated to be similar to E. myxorhynchum although minor differences exist.
Description of plerocercoid (the stage found in molluscs). Body
elongate, tapering towards posterior terminal ; with apical sucker (or
opening) on presumably extensible muscular rostellum (referred to by
some as a myzorhynchus) and four protruding broad and trumpet-like
bothridia ; each bothridium with ten loculi (or areolae) (according to
Hart, 1936; Sparks and Chew, 1966) or eighteen loculi formed by a
single longitudinal and eight transverse septa plus one locule a t each
terminal (according to Riser, 1955) ; pedicels and abothrial surfaces of
bothridia covered with small hooks.
L(fe cycle. The life cycle of Echeneibothrium has not been demonstrated. It is believed that plerocercoids in molluscs are taken in by
259
elasmobranch, hatch in water. The ciliated coracidium is swept up
with other planktonic organisms during the oyster’s feeding process
and is ingested or becomes attached to the gill surfaces. It sheds its
ciliated coat prior to penetrating through either the alimentary wall
or through the gill surface, develops into a metacestode, and eventually
is immobilized when the host forms a capsule around it. The remainder
of its life cycle remains undertermined.
Both Sparks (1963) and Cheng (1966a) have
studied pathological changes in parasitized Crassostrea virginica (see
Chapter 4, Section 11, A, 2). Since it has been found that encysted
metacestodes are eventually resorbed in C. virginica, I have concluded
that this oyster is probably not the natural intermediate host. Further
evidence for this has recently been uncovered while examining parasitized Tapes semidecussatus. I n this clam, the host’s cellular reaction
is even more severe than in Crassostrea virginica, and resorption of the
parasite is commonly observed (Cheng and Rifkin, 1967).
Other information.
2 . Echeneibothrium sp. van Beneden, 1850. (Figs. 170 and 171)
(Order Tetraphyllidea; family Phyllobothriidae)
The genus Echeneibothrium was erected by van Beneden (1850) to
include E. minimum, a parasite found in the spiral valve of elasmobranchs. Since then, a t least thirty-five species of adults have been
described, all from elasmobranchs (Yamaguti, 1959). Hyman (1951)
has depicted the larval stage (plerocercoid) of an undetermined species
of Echeneibothrium encysted in the foot of a “ clam ” collected in
Puget Sound. This larva may be the same as that reported by Sparks
and Chew (1966) encysted throughout the tissues of the littleneck
clam, Venerupis staminea (Figs. 170 and 171), collected in Humboldt
Bay, California, which they have indicated to be similar to E. myxorhynchum although minor differences exist.
Description of plerocercoid (the stage found in molluscs). Body
elongate, tapering towards posterior terminal ; with apical sucker (or
opening) on presumably extensible muscular rostellum (referred to by
some as a myzorhynchus) and four protruding broad and trumpet-like
bothridia ; each bothridium with ten loculi (or areolae) (according to
Hart, 1936; Sparks and Chew, 1966) or eighteen loculi formed by a
single longitudinal and eight transverse septa plus one locule a t each
terminal (according to Riser, 1955) ; pedicels and abothrial surfaces of
bothridia covered with small hooks.
L(fe cycle. The life cycle of Echeneibothrium has not been demonstrated. It is believed that plerocercoids in molluscs are taken in by