SCATOLOGI('AL YTCTDIES OF THE BIVALVIA (MOLLUSCA)
395
the characteristic sculpturing of the pellets in the Nuculidae is produced
by appropriate structures in the walls of the mid-gut which possess
longitudinal plicate ridges with long cilia which are highly specific in
number and arrangement (Moore, 1931a). I n the carnivorous Septibranchia with stomach ofType 11, the pellets are so soft and loose in
consistency that they are indefinable in shape (Pellet-type 1 1 ) . Though
Purchon (1958, 1959 and 1963) has pointedly emphasized that the
Septibranchia and the Protobranchia are related in structure and function of the stomach more closely to each other than either is to any other
bivalve group, nevertheless there is little resemblance of pellet-characteristics between the two*. This noticeable difference may be due largely
to the kind of food and to the structure of the mid-gut where the removal
of water takes place and the resulting solidification determines the final
form of the faecal pellets. It is noteworthy that the mid-gut in the
septibranchs is much shorter than that in the protobranchs and that
the lengthening of tjhe intestine is associated with the formation of firm
faecal pellets (Yonge, 1960). In the subclass Polysyringia the stomach
is comparatively complex, being characterized by the well-developed
sorting area, the intruded major intestinal typhlosole as well as by the
large number of ducts from the digestive diverticula. Within this subclass, the Gastrotriteia with stomach of Type 111 produce ribbon-like
pellets which are assigned to Pellet-type 111-10 or 10a. But there are
some notable exceptions, namely the pellets of Ohlimopa and Limopsis
in the Limopsidae (Pellet-type 111-4) where the stomach differs considerably from that of other members of this group (Purchon, 1957).
I n the Gastrotetartika with stomach of Type IV, the pellet-t,ypes are
widely distributed among families and genera, ranging from 1-1 to
111-10, or to Ila, exclusive of IA-3, 11-5, 6, 7 and their subtypes. Of
these, 11-8, IIA-9 and 111-10 are of common occurrence in this group.
This uneven distribution of pellet-types seems to have an association
with the anomalies in the distribution of Stomach-Type IV. The
Gastropempta which achieve the highest complexities of gastric organization (Type V) comprise six pellet-types ranging from 1-1 to 11-5,
and the relatively high frequency occurs in Types 1-1 and 11-4. I n view
of the above we find it difficult to avoid the conclusion that the stomach
in the Bivalvia is concerned with the determination of the primary
characteristics of the faecal pellets.
Mid-gut
The mid-gut appears to be still more closely associated with the
After digestion has occurred in the
formation of the faecal pellets.
* See footnote on p. 320.
395
the characteristic sculpturing of the pellets in the Nuculidae is produced
by appropriate structures in the walls of the mid-gut which possess
longitudinal plicate ridges with long cilia which are highly specific in
number and arrangement (Moore, 1931a). I n the carnivorous Septibranchia with stomach ofType 11, the pellets are so soft and loose in
consistency that they are indefinable in shape (Pellet-type 1 1 ) . Though
Purchon (1958, 1959 and 1963) has pointedly emphasized that the
Septibranchia and the Protobranchia are related in structure and function of the stomach more closely to each other than either is to any other
bivalve group, nevertheless there is little resemblance of pellet-characteristics between the two*. This noticeable difference may be due largely
to the kind of food and to the structure of the mid-gut where the removal
of water takes place and the resulting solidification determines the final
form of the faecal pellets. It is noteworthy that the mid-gut in the
septibranchs is much shorter than that in the protobranchs and that
the lengthening of tjhe intestine is associated with the formation of firm
faecal pellets (Yonge, 1960). In the subclass Polysyringia the stomach
is comparatively complex, being characterized by the well-developed
sorting area, the intruded major intestinal typhlosole as well as by the
large number of ducts from the digestive diverticula. Within this subclass, the Gastrotriteia with stomach of Type 111 produce ribbon-like
pellets which are assigned to Pellet-type 111-10 or 10a. But there are
some notable exceptions, namely the pellets of Ohlimopa and Limopsis
in the Limopsidae (Pellet-type 111-4) where the stomach differs considerably from that of other members of this group (Purchon, 1957).
I n the Gastrotetartika with stomach of Type IV, the pellet-t,ypes are
widely distributed among families and genera, ranging from 1-1 to
111-10, or to Ila, exclusive of IA-3, 11-5, 6, 7 and their subtypes. Of
these, 11-8, IIA-9 and 111-10 are of common occurrence in this group.
This uneven distribution of pellet-types seems to have an association
with the anomalies in the distribution of Stomach-Type IV. The
Gastropempta which achieve the highest complexities of gastric organization (Type V) comprise six pellet-types ranging from 1-1 to 11-5,
and the relatively high frequency occurs in Types 1-1 and 11-4. I n view
of the above we find it difficult to avoid the conclusion that the stomach
in the Bivalvia is concerned with the determination of the primary
characteristics of the faecal pellets.
Mid-gut
The mid-gut appears to be still more closely associated with the
After digestion has occurred in the
formation of the faecal pellets.
* See footnote on p. 320.