394
KOHMAN Y. ARAKAWA
The structure and fuiiction of bivalve digestive organs have been
intensively studied by many workers. Owen (1966), in reviewing work
on movement of food through them, mentions that primitively within
the style-sac the muscular action of the walls forms a faecal rod which
is rotated and passed into the mid-gut by the action of the cilia lining
the sac, and that the formation of such a faecal rod or protostyle is
undoubtedly a primitive function of the style-sac. Yonge (1960) suggests, " The stomach itself probably early contained a ciliated sorting
region, an area of cuticle forming a gastric shield, and a ' style-sac ' region initially concerned with the consolidation of faecal material . . . a
matter of prime importance when the gut opened into the respiratory
(i.e. mantle) cavity. Lengthening of the gut posterior to the stomach
was also associated with the formation of firm faecal pellets. . . "
and he (1935) showed that the average p H value of the various parts of
the gut in bivalves is concerned largely with controlling the viscosity of
the mucus in the food string: in the case of Ostrea edulis, the mucus
becomes less viscous in the lower pH of the stomach juice, about 5 . 5 ,
and this assists the shedding of its load; the intestinal pH is higher,
5.7 in the mid-gut and 6.0 in the rectum, where the more viscous mucus
binds the contents into firm pellets.
Let us now speculate about the relationships between the nature
and form of the faecal pellets and the structure and function of the
digestive organs in the bivalves.
Stomach and its associate organs
Purchon ( 1963), who surveyed structures of the stomach throughout the Bivalvia, suggests that the class can be divided according to
function and structural features of the stomach into five orders:
Gastroproteia (Protobranchia), Gastrodeuteia (Septibranchia), Gastrotriteia, Gastrotetartika and Gastropempta.
A comparison of
different types of the faecal pellets with the form of the stomach in
various bivalve orders is shown in Table VI and Fig. 25. This would
lead us to suppose that the nature and form of the pellets is more
closely related to the structure and function of the stomach than to
those of the ctenidia. In the subclass Oligosyringia, the stomach is
very primitive and simple in many ways, being characterized by the
presence of two or three ducts from the digestive diverticula and of
strongly muscular walls which are associated with the need for trituration or squeezing of relatively large and coarse materials. Within
this subclass, the deposit feeding Protobranchia with stomach of
Type I have very resistant faecal rods with longitudinal surface
sculptures (Pellet-type 11-6, 7, 7a and 7b). It is of interest to note that
KOHMAN Y. ARAKAWA
The structure and fuiiction of bivalve digestive organs have been
intensively studied by many workers. Owen (1966), in reviewing work
on movement of food through them, mentions that primitively within
the style-sac the muscular action of the walls forms a faecal rod which
is rotated and passed into the mid-gut by the action of the cilia lining
the sac, and that the formation of such a faecal rod or protostyle is
undoubtedly a primitive function of the style-sac. Yonge (1960) suggests, " The stomach itself probably early contained a ciliated sorting
region, an area of cuticle forming a gastric shield, and a ' style-sac ' region initially concerned with the consolidation of faecal material . . . a
matter of prime importance when the gut opened into the respiratory
(i.e. mantle) cavity. Lengthening of the gut posterior to the stomach
was also associated with the formation of firm faecal pellets. . . "
and he (1935) showed that the average p H value of the various parts of
the gut in bivalves is concerned largely with controlling the viscosity of
the mucus in the food string: in the case of Ostrea edulis, the mucus
becomes less viscous in the lower pH of the stomach juice, about 5 . 5 ,
and this assists the shedding of its load; the intestinal pH is higher,
5.7 in the mid-gut and 6.0 in the rectum, where the more viscous mucus
binds the contents into firm pellets.
Let us now speculate about the relationships between the nature
and form of the faecal pellets and the structure and function of the
digestive organs in the bivalves.
Stomach and its associate organs
Purchon ( 1963), who surveyed structures of the stomach throughout the Bivalvia, suggests that the class can be divided according to
function and structural features of the stomach into five orders:
Gastroproteia (Protobranchia), Gastrodeuteia (Septibranchia), Gastrotriteia, Gastrotetartika and Gastropempta.
A comparison of
different types of the faecal pellets with the form of the stomach in
various bivalve orders is shown in Table VI and Fig. 25. This would
lead us to suppose that the nature and form of the pellets is more
closely related to the structure and function of the stomach than to
those of the ctenidia. In the subclass Oligosyringia, the stomach is
very primitive and simple in many ways, being characterized by the
presence of two or three ducts from the digestive diverticula and of
strongly muscular walls which are associated with the need for trituration or squeezing of relatively large and coarse materials. Within
this subclass, the deposit feeding Protobranchia with stomach of
Type I have very resistant faecal rods with longitudinal surface
sculptures (Pellet-type 11-6, 7, 7a and 7b). It is of interest to note that