412
KOHMAN P. ARAKAWA
might have evolved as an environmental adaptation, this has happened only once (Jegla and Grrenberg, 1968).
I n any case, it may be suggested that within the Gastrotriteia the
ungrooved ribbon (Type 10a) may be more advanced than the grooved
one (Type l o ) , because the latter is generally present in primitive
heteromyarians, such as Mytilus, and the former in higher heteromyarians and monomyarians such as Pin,na and Pinctada.
As previously discussed, the faeces in the Gastrotetartika take on a
great variety of form, probably in correlation with wide diversity in
habit and the structure and function of the digestive organs. To take
the concrete instance of the Pectinacea, the habits of adult members of
this group can be distinguished into three categories : cementation,
byssal attachment, and freedom. According to Yonge (1951), the
phylogenetically primitive habit must have been byssal attachment
which is universal in early life. From this the habit of cementation and
of freedom may have separately diverged. I n the cemented species of
this group, e.g. Plicatula, the pellets are ungrooved ribbons which
resemble those of other cemented members of the Gastrotriteia, such
as Crassostrea, Ostrea and Xaxostrea. I n most, of the byssally attached
members, such as Chlamys, the pellets are grooved rods trefoiled or
triangular in section (Types 9 and 9a) which, though quite different in
appearance, have a basic connection with the grooved ribbon with bicrescentic section (Type 10a) in other byssally attached members of
the Gastrotriteia. Finally, in the unattached members such as Amusium
in the Amusiidae and Pecten in the Pectinidae, the pellets are in the
form of rodlets wound into a ball (Type 8) which is characteristic of
this group. These interesting facts might be interpreted as evidence
that the ancestral pellet form in this group may be a trefoiled section
rod, and then, in correlation with the change of habit, it may have
altered in two directions ; e.g. ungrooved ribbons (cementation) and
rodlets wound into a ball (freedom). It should be mentioned in this
connection that a close parallel to this could be found among the
members of the Anomiacea, e.g. permanently attached Anomia, temporarily attached Enigmonia and unattached Placenta, though the
investigation of this possibility must be the subject of future research.
The evolution of faecal pellets in the Erycinacea may also be worth
special attention, since the members of this group have distinctive
habits most of them being commensal. I n commensal species such as
Phlyctaenachlamys lysiosquillina, the faecal material is discharged as
disjointed particles unlike those of non-commensal ones such as
Scintilla and Lasaea in which they are discarded as firm ovoid pellets.
T t i s also noteworthy that in the byssally attached Cardita (Carditacea)
KOHMAN P. ARAKAWA
might have evolved as an environmental adaptation, this has happened only once (Jegla and Grrenberg, 1968).
I n any case, it may be suggested that within the Gastrotriteia the
ungrooved ribbon (Type 10a) may be more advanced than the grooved
one (Type l o ) , because the latter is generally present in primitive
heteromyarians, such as Mytilus, and the former in higher heteromyarians and monomyarians such as Pin,na and Pinctada.
As previously discussed, the faeces in the Gastrotetartika take on a
great variety of form, probably in correlation with wide diversity in
habit and the structure and function of the digestive organs. To take
the concrete instance of the Pectinacea, the habits of adult members of
this group can be distinguished into three categories : cementation,
byssal attachment, and freedom. According to Yonge (1951), the
phylogenetically primitive habit must have been byssal attachment
which is universal in early life. From this the habit of cementation and
of freedom may have separately diverged. I n the cemented species of
this group, e.g. Plicatula, the pellets are ungrooved ribbons which
resemble those of other cemented members of the Gastrotriteia, such
as Crassostrea, Ostrea and Xaxostrea. I n most, of the byssally attached
members, such as Chlamys, the pellets are grooved rods trefoiled or
triangular in section (Types 9 and 9a) which, though quite different in
appearance, have a basic connection with the grooved ribbon with bicrescentic section (Type 10a) in other byssally attached members of
the Gastrotriteia. Finally, in the unattached members such as Amusium
in the Amusiidae and Pecten in the Pectinidae, the pellets are in the
form of rodlets wound into a ball (Type 8) which is characteristic of
this group. These interesting facts might be interpreted as evidence
that the ancestral pellet form in this group may be a trefoiled section
rod, and then, in correlation with the change of habit, it may have
altered in two directions ; e.g. ungrooved ribbons (cementation) and
rodlets wound into a ball (freedom). It should be mentioned in this
connection that a close parallel to this could be found among the
members of the Anomiacea, e.g. permanently attached Anomia, temporarily attached Enigmonia and unattached Placenta, though the
investigation of this possibility must be the subject of future research.
The evolution of faecal pellets in the Erycinacea may also be worth
special attention, since the members of this group have distinctive
habits most of them being commensal. I n commensal species such as
Phlyctaenachlamys lysiosquillina, the faecal material is discharged as
disjointed particles unlike those of non-commensal ones such as
Scintilla and Lasaea in which they are discarded as firm ovoid pellets.
T t i s also noteworthy that in the byssally attached Cardita (Carditacea)