380
XOHMAN Y . ARAKAWA
branchia, where the pellets also exhibit considerable variation according
to feeding habit and habitat. The pellets in the deposit feeding d e e p
burrowers such as the Tellinacea, where the labial palps are much enlarged in comparison with the ctenidia which play a much smaller role
in grading of particles, belong to Type 1-1 and l a or t o 2, while the
suspension feeding burrowers with well-developed eulamellibranch
ctenidia such as the Veneracea tend to produce more elongate pellets
(Type 11-4). The Erycinacea have distinctive habits, most of them
being commensal. I n Phlyctaenachlamys lysiosquillina, which is commensal in the burrows of a shrimp, Lysiosquilla maculata, Popham
(1939) in discussing the formation of the pellets in relation to digestive
system and mode of life states, “ The absence of mucous glands in the
rectum is probably also associated with this movement of water through
the mantle cavity. Particles of faecal matter will be shot out through
the exhalent opening. There is thus no necessity for the formation of
firm faecal pellets, because owing to the violent expulsion of water,
there will be less danger of the mantle cavity silting up.” While, on
the other hand, in non-commensal forms of this super-family, such as
Scintilla japonica, the pellets are ellipsoid with many transverse segments and with considerable amount of mucus (Type 1-2). The most
strikingly modified feeding habits are those of the Protobranchia and
Septibranchia. I n the deposit feeding protobranchs, the palp proboscides instead of the ctenidia play the leading role in feeding. The
ctenidia serve primarily as respiratory organs, largely unconcerned with
collecting food. Moore (1931a) shows that the pellets in these protobranchs, such as Nucula, are characteristically resistant sculptured rods
(Type 11-7, 7a and 7 b ) . The carnivorous septibranchs are another
special case. They feed usually on relatively large animals, such as
copepods or the carcases of these, drawn into the infra-branchial
chamber by means of convulsive muscular pumping action of the
ctenidial septum. The pellets in these, the only carnivorous bivalves,
belong to Type 11, which are indeterminate in shape being very soft and
loose in consistency.
I n this connection it is of interest to note that in the relationship
between types of pellet and mode of feeding and habitat, the bivalves
are paralleled by other ciliary feeders such as polyzoans, brachiopods,
tunicates and Amphioxus. To give some remarkable instances, the
faecal pellets of the attached filter-feeding tunicates, Ciona, Ascidia
and Styela are ribbon-shaped, quite resembling in type those of the
filibranch bivalves with similar habits (Fig. 12). According to Edge’s
observations (1934), Styela is less selective in its feeding than Ciona, for
its pellets contain much coarser material of an indigestible nature than
XOHMAN Y . ARAKAWA
branchia, where the pellets also exhibit considerable variation according
to feeding habit and habitat. The pellets in the deposit feeding d e e p
burrowers such as the Tellinacea, where the labial palps are much enlarged in comparison with the ctenidia which play a much smaller role
in grading of particles, belong to Type 1-1 and l a or t o 2, while the
suspension feeding burrowers with well-developed eulamellibranch
ctenidia such as the Veneracea tend to produce more elongate pellets
(Type 11-4). The Erycinacea have distinctive habits, most of them
being commensal. I n Phlyctaenachlamys lysiosquillina, which is commensal in the burrows of a shrimp, Lysiosquilla maculata, Popham
(1939) in discussing the formation of the pellets in relation to digestive
system and mode of life states, “ The absence of mucous glands in the
rectum is probably also associated with this movement of water through
the mantle cavity. Particles of faecal matter will be shot out through
the exhalent opening. There is thus no necessity for the formation of
firm faecal pellets, because owing to the violent expulsion of water,
there will be less danger of the mantle cavity silting up.” While, on
the other hand, in non-commensal forms of this super-family, such as
Scintilla japonica, the pellets are ellipsoid with many transverse segments and with considerable amount of mucus (Type 1-2). The most
strikingly modified feeding habits are those of the Protobranchia and
Septibranchia. I n the deposit feeding protobranchs, the palp proboscides instead of the ctenidia play the leading role in feeding. The
ctenidia serve primarily as respiratory organs, largely unconcerned with
collecting food. Moore (1931a) shows that the pellets in these protobranchs, such as Nucula, are characteristically resistant sculptured rods
(Type 11-7, 7a and 7 b ) . The carnivorous septibranchs are another
special case. They feed usually on relatively large animals, such as
copepods or the carcases of these, drawn into the infra-branchial
chamber by means of convulsive muscular pumping action of the
ctenidial septum. The pellets in these, the only carnivorous bivalves,
belong to Type 11, which are indeterminate in shape being very soft and
loose in consistency.
I n this connection it is of interest to note that in the relationship
between types of pellet and mode of feeding and habitat, the bivalves
are paralleled by other ciliary feeders such as polyzoans, brachiopods,
tunicates and Amphioxus. To give some remarkable instances, the
faecal pellets of the attached filter-feeding tunicates, Ciona, Ascidia
and Styela are ribbon-shaped, quite resembling in type those of the
filibranch bivalves with similar habits (Fig. 12). According to Edge’s
observations (1934), Styela is less selective in its feeding than Ciona, for
its pellets contain much coarser material of an indigestible nature than