SCATOLOGICAL STUDIES OF THE BIVALVIA (MOLLWCA)
41 I
lutionary progress of faecal pellets is monophyletic and that the multigrooved rod (such as that of Nucula sulcata (Type 7 b ) ) may have
evolved continually into a simply grooved one (such as that of Nuculana
(Type 6)) through a series of transitional forms (such as those of Nucula
tenuis (Type 7 ) and N . nucleus (Type 7 a ) ) .
It has been traditional to treat the Septibranchia as the climax of
evolution of the Eulamellibranchia on account of the peculiarities in
structure and function of the ctenidia. The available scatological data,
however, are not yet sufficient to warrant final conclusions on this subject*. As described in Section 111, the Septibranchia show no similarity
to the Protobranchia in faecal characteristics, the faeces in the Septibranchia being extruded as shapeless pellets with soft and fragile consistency (Type 11) quite unlike those of the Protobranchia. Kornicker
(1962) in commenting on the evolution of the molluscan faeces suggested
that the shapeless pellets may have arisen directly from sculptured rods
due to the “ scarcity ” of unsculptured rods which might be transitional
between the sculptured rod and shapeless pellets. But there is no
evidence (Table V) in favour of his hypothesis.
From the data given in Table V, it should be noticed that in the
Polysyringia with some exceptions the production of ribbon-like faecal
pellets (Types 10 and 10a) may well be associated with byssal attachment or cementation. All the Gastrotriteia apart from the Limopsidae
in which the habit of byssal attachment seems to be lost early in life
have ribbon-shaped pellets. The present writer cannot satisfactorily
explain the significance of this phenomenon, but it may throw some
light on this problem that byssal attachment or cementation have
some direct or indirect influence on feeding habits, in view of the fact
that the retention of the filibranch ctenidium in the Anisomyaria is
closely associated with byssal attachment (Yonge, 1962). I n giving a
brief explanation of this fact, Yonge states as follows :
“Although the filibranch ctenidium is clearly less efficient than the
much more compact eulamellibranch ctenidium, this will be of no more
than minor disadvantage if the animals are epifaunal and so live in
relatively clear water.”
There are, of course, exceptions to this rule. To mention a single
example, unattached unionids such as Anodonta and Hyriopsis may
produce ribbon-like pellets (Type 10a). The rectal lumen in these
species is flattened to a crescentic shape by an unusually developed
typhlosole which may assist in osmoregulation in view of their freshwater habitat. Speaking phylogenetically, while the unionid rectum
* But justified by the structure of Halicardia nipponensis (Nakazima, 1967).
(C.M .Y.)
41 I
lutionary progress of faecal pellets is monophyletic and that the multigrooved rod (such as that of Nucula sulcata (Type 7 b ) ) may have
evolved continually into a simply grooved one (such as that of Nuculana
(Type 6)) through a series of transitional forms (such as those of Nucula
tenuis (Type 7 ) and N . nucleus (Type 7 a ) ) .
It has been traditional to treat the Septibranchia as the climax of
evolution of the Eulamellibranchia on account of the peculiarities in
structure and function of the ctenidia. The available scatological data,
however, are not yet sufficient to warrant final conclusions on this subject*. As described in Section 111, the Septibranchia show no similarity
to the Protobranchia in faecal characteristics, the faeces in the Septibranchia being extruded as shapeless pellets with soft and fragile consistency (Type 11) quite unlike those of the Protobranchia. Kornicker
(1962) in commenting on the evolution of the molluscan faeces suggested
that the shapeless pellets may have arisen directly from sculptured rods
due to the “ scarcity ” of unsculptured rods which might be transitional
between the sculptured rod and shapeless pellets. But there is no
evidence (Table V) in favour of his hypothesis.
From the data given in Table V, it should be noticed that in the
Polysyringia with some exceptions the production of ribbon-like faecal
pellets (Types 10 and 10a) may well be associated with byssal attachment or cementation. All the Gastrotriteia apart from the Limopsidae
in which the habit of byssal attachment seems to be lost early in life
have ribbon-shaped pellets. The present writer cannot satisfactorily
explain the significance of this phenomenon, but it may throw some
light on this problem that byssal attachment or cementation have
some direct or indirect influence on feeding habits, in view of the fact
that the retention of the filibranch ctenidium in the Anisomyaria is
closely associated with byssal attachment (Yonge, 1962). I n giving a
brief explanation of this fact, Yonge states as follows :
“Although the filibranch ctenidium is clearly less efficient than the
much more compact eulamellibranch ctenidium, this will be of no more
than minor disadvantage if the animals are epifaunal and so live in
relatively clear water.”
There are, of course, exceptions to this rule. To mention a single
example, unattached unionids such as Anodonta and Hyriopsis may
produce ribbon-like pellets (Type 10a). The rectal lumen in these
species is flattened to a crescentic shape by an unusually developed
typhlosole which may assist in osmoregulation in view of their freshwater habitat. Speaking phylogenetically, while the unionid rectum
* But justified by the structure of Halicardia nipponensis (Nakazima, 1967).
(C.M .Y.)