350
N. BALAKRISHNAN NAIR AND M. SARASWATHY
the posterior adductor muscle (PA) instead of anterior to it as in most
bivalves. The discoidal foot (F) occupies the anterior-most portion in
relation to its use as an organ holding the shell in position while boring.
The pericardial cavity (PC) with the contained heart has not only considerably elongated but has also undergone rotation around the posterior adductor with the result that it lies above the intestine. Associated with these, the relation of the various parts of the circulatory
system has changed. The posterior aorta (POA) continues in front of
the ventricle while the anterior aorta (AAO) proceeds backwards. The
kidney (K) and ureters come to lie on the upper side of the pericardial
cavity and ventral to the anal canal (AC). The gonads are creamy white
conspicuous bodies situated in the midline dorsally and laterally. The
renal and reproductive openings have shifted back as also has the
visceral ganglion (VG). The digestive organs have not rotated like the
pericardium. The cylindrical stomach (ST) is surrounded ventrally and
posteriorly by the irregular tubules of the digestive diverticula (DD),
the capacity of the stomach being greatly increased by the long cylindrical caecum (CAG), and the intestine (MG) extending posteriorly
around the caecum as a long loop. The mouth (M) and the anus (AN)
retain their usual positions, the former between the anterior adductor
and the foot, the latter on the dorsal side of the posterior adductor.
The mantle cavity extends behind as a long cylindrical canal
accommodating the ctenidia. The distal end of the tube is produced
into the siphons. The pallets (Fig. 4) are situated at the base of the
siphons on either side beneath the muscular collar and are provided
with muscles which control their movements. The two pallets are
pressed against each other so that the straight inner sides become
apposed and the semicircular outer surfaces form a complete conical
plug effectively closing the circular opening when the siphons are retracted within the burrow. If the salinity of the sea water should
change greatly, or if poisons are introduced or the timber is taken out of
water, the pallets are thrust into the opening, closing the burrow and
protecting the soft animal inside. The retraction and protrusion of the
pallets are brought about respectively by sets of retractor and protractor muscles. The adductors of the pallets facilitate the separatioxl
of the right and left blades of the pallet prior to retraction of the
pallets which enables the siphons to extend. In Bankia the pallets
consist of a series of funnel-like cone-in-cone structures which constitute the blade. These have been secreted and cemented in succession
to a calcareous cylindrical handle. The cone-in-cone units of the blade
are not of the same size, the largest one being found towards the base
and the smallest at the tip, the blade tapering from base to tip. They
N. BALAKRISHNAN NAIR AND M. SARASWATHY
the posterior adductor muscle (PA) instead of anterior to it as in most
bivalves. The discoidal foot (F) occupies the anterior-most portion in
relation to its use as an organ holding the shell in position while boring.
The pericardial cavity (PC) with the contained heart has not only considerably elongated but has also undergone rotation around the posterior adductor with the result that it lies above the intestine. Associated with these, the relation of the various parts of the circulatory
system has changed. The posterior aorta (POA) continues in front of
the ventricle while the anterior aorta (AAO) proceeds backwards. The
kidney (K) and ureters come to lie on the upper side of the pericardial
cavity and ventral to the anal canal (AC). The gonads are creamy white
conspicuous bodies situated in the midline dorsally and laterally. The
renal and reproductive openings have shifted back as also has the
visceral ganglion (VG). The digestive organs have not rotated like the
pericardium. The cylindrical stomach (ST) is surrounded ventrally and
posteriorly by the irregular tubules of the digestive diverticula (DD),
the capacity of the stomach being greatly increased by the long cylindrical caecum (CAG), and the intestine (MG) extending posteriorly
around the caecum as a long loop. The mouth (M) and the anus (AN)
retain their usual positions, the former between the anterior adductor
and the foot, the latter on the dorsal side of the posterior adductor.
The mantle cavity extends behind as a long cylindrical canal
accommodating the ctenidia. The distal end of the tube is produced
into the siphons. The pallets (Fig. 4) are situated at the base of the
siphons on either side beneath the muscular collar and are provided
with muscles which control their movements. The two pallets are
pressed against each other so that the straight inner sides become
apposed and the semicircular outer surfaces form a complete conical
plug effectively closing the circular opening when the siphons are retracted within the burrow. If the salinity of the sea water should
change greatly, or if poisons are introduced or the timber is taken out of
water, the pallets are thrust into the opening, closing the burrow and
protecting the soft animal inside. The retraction and protrusion of the
pallets are brought about respectively by sets of retractor and protractor muscles. The adductors of the pallets facilitate the separatioxl
of the right and left blades of the pallet prior to retraction of the
pallets which enables the siphons to extend. In Bankia the pallets
consist of a series of funnel-like cone-in-cone structures which constitute the blade. These have been secreted and cemented in succession
to a calcareous cylindrical handle. The cone-in-cone units of the blade
are not of the same size, the largest one being found towards the base
and the smallest at the tip, the blade tapering from base to tip. They