SCATOLOCJICAL STUDIES OF THE HIVALVIA (MOLLUSCA)
413
the pellets are emitted in the form of ungrooved ribbon (Type 10) as in
the case of other byssiferous members of the Gastrotriteia and that in
the majority of the Lucinacea the faecal pellets are in the form of a
plain rod (Type 4), a feature they have in common with the primitive
groups (such as the Veneracea) of the Gastropempta. Last but not
least, the facts mentioned above strongly support Purchon’s view that
the Gastrotetartika are an ancestral group which persists to the
present day, and from which the Gastrotriteia and Gastropempta arose
by specialization. The data drawn from Table VI suggest that the
faecal pellets in the Gastropempta exhibit little variation in type, the
majority of them (60% or more) being ovoids (Type 1) and plain rods
(Type 4). However, there is a gradual transition from plain rods such
as those of the Veneracea (Type 41, through rods constricted at regular
intervals to give short cylinders with rounded ends such as those of the
Cardiacea, Solenacea and some spp. of the Mactracea (Type 3) into
ellipsoid (Type 2), ovoid (Type 1) and discoid pellets (Type Ib) such as
those of the Tellinacea.
I n view of the above facts, it may be concluded that, as suggested
by Purchon (1959) on the basis of evidence drawn from anatomical
investigation of the bivalve stomach, the Protobranchia, Septibranchia,
Gastrotriteia and the Gastropempta are monophyletic stocks whereas
the Gastrotetartika must be polyphyletic. I n some families which should
belong to the Gastropempta, there is reversion to the ancestral condition and in others, which should belong to the Gastrotriteia, paedomorphosis has occurred. However, while there is a considerable degree
of correspondence between the views of this writer and those of Purchon
on the phylogenetic trends within the Bivalvia, there remains a serious
difference of opinion about the status of the Septibranchia.
VII. BIODEPOSITION OF SUSPENSION FEEDING BIVALVES
It is a well-known fact that the defecation activities of earthworms
play a major role in the modification of the physical and chemical
characteristics of mould (Darwin, 1881). Recently, Ito and Imai
( 1955), Lund ( 1 957) and Haven and Morales-Alamo (1 966) drew attention to the fact that faecal production of some filter-feeding marine
invertebrates profoundly influences deposition, transport and the
physical and chemical composition of the sediments in estuaries and
bays. As a designation for convenience, faeces and pseudofaeces of
these animals that settle to the bottom are termed “ biodeposits” ;
the process involved in production of these biodeposits-filtration of
seston, compaction within the animal, and subsequent deposition-are
413
the pellets are emitted in the form of ungrooved ribbon (Type 10) as in
the case of other byssiferous members of the Gastrotriteia and that in
the majority of the Lucinacea the faecal pellets are in the form of a
plain rod (Type 4), a feature they have in common with the primitive
groups (such as the Veneracea) of the Gastropempta. Last but not
least, the facts mentioned above strongly support Purchon’s view that
the Gastrotetartika are an ancestral group which persists to the
present day, and from which the Gastrotriteia and Gastropempta arose
by specialization. The data drawn from Table VI suggest that the
faecal pellets in the Gastropempta exhibit little variation in type, the
majority of them (60% or more) being ovoids (Type 1) and plain rods
(Type 4). However, there is a gradual transition from plain rods such
as those of the Veneracea (Type 41, through rods constricted at regular
intervals to give short cylinders with rounded ends such as those of the
Cardiacea, Solenacea and some spp. of the Mactracea (Type 3) into
ellipsoid (Type 2), ovoid (Type 1) and discoid pellets (Type Ib) such as
those of the Tellinacea.
I n view of the above facts, it may be concluded that, as suggested
by Purchon (1959) on the basis of evidence drawn from anatomical
investigation of the bivalve stomach, the Protobranchia, Septibranchia,
Gastrotriteia and the Gastropempta are monophyletic stocks whereas
the Gastrotetartika must be polyphyletic. I n some families which should
belong to the Gastropempta, there is reversion to the ancestral condition and in others, which should belong to the Gastrotriteia, paedomorphosis has occurred. However, while there is a considerable degree
of correspondence between the views of this writer and those of Purchon
on the phylogenetic trends within the Bivalvia, there remains a serious
difference of opinion about the status of the Septibranchia.
VII. BIODEPOSITION OF SUSPENSION FEEDING BIVALVES
It is a well-known fact that the defecation activities of earthworms
play a major role in the modification of the physical and chemical
characteristics of mould (Darwin, 1881). Recently, Ito and Imai
( 1955), Lund ( 1 957) and Haven and Morales-Alamo (1 966) drew attention to the fact that faecal production of some filter-feeding marine
invertebrates profoundly influences deposition, transport and the
physical and chemical composition of the sediments in estuaries and
bays. As a designation for convenience, faeces and pseudofaeces of
these animals that settle to the bottom are termed “ biodeposits” ;
the process involved in production of these biodeposits-filtration of
seston, compaction within the animal, and subsequent deposition-are