THE BIOLOGY OF WOOD-BORING TEREDTNID MOLLUSCS
355
regions of the valves, the sculpture, hinge, condyles, apophyses, muscle
scars, etc. Miller's work (1922) brought to light an amazingly wide
range of variations in Teredo navalis involving practically every feature
of the shell especially in the number of ridges, size and shape of the
auricle, nature of denticles, etc. Nair (1959) illustrated a series of valves
of Nototeredo norvagica from western Norway showing such variations
(Fig. 8). Recent studies (Saraswathy and Nair, 1971) at Cochin on
Nausitora hedleyi also showed the occurrence of similar variations
especially in respect of the length/height ratio, number of ridges, length
of the auricle, etc. With our present inadequate knowledge of the
teredine shell it is too early to say how far shell characters may be
employed satisfactorily for the determination of genera and species. In
stray cases well developed shell characters may assist in identification
when used in conjunction with the pallets as in Psiloteredo megotara
(Hanley) and Xototeredo norvagica (Spengler) both living in the same
habitat but having very similar pallets (see Nair, 1959).
Three species of Bankia (Sigerfoos, 1908; Nair, 1955, 1957a; Bade,
Masurekar and Bal, 1961) and one of Nuusitoru (Saraswathy and Nair,
1971) have been examined with regard to the structure of the mantle.
This consists of two lobes which secrete the valves in a typical bivalve.
In shipworms the lobes have fused along their edges resulting in the
formation of a long and delicate tube open in front for the protrusion
of the foot and produced into long siphons posteriorly. The mantle is
generally thin and translucent and not of uniform thickness. On the
hind margins of the valves the mantle is raised up and extends over the
umbonal region as a thickened muscular ridge. This duplication of the
mantle, called the " cephalic hood " by Quatrefages (1849) was considered by him as the organ directly concerned with boring. Sigerfoos
(1908) and Kofoid (1921) thought that it fitted into the burrow like a
washer around the " head " preventing the fine particles of wood from
entering the burrow. In several specimens of Bankia indica examined
the " cephalic hood " appeared very calloused suggesting frequent rubbing against the walls of the burrow. The epithelium of the hood in
genera like Bankia and Nausitora projects on the median dorsal side
behind the shell as a hump with a series of characteristic transverse
foldings. The epithelial cells are tall and columnar containing granular
cytoplasm. The rest of the mantle except the siphons is thin and the
organs of the pallial cavity may be seen through it in fresh specimens. Since the entire surface of the mantle is capable of secreting
calcium carbonate, the initial region of the burrow occupied
longest by the animal normally receives the maximum limy deposition.
Posterior to the shell valves on the dorsal and ventral surfaces the
355
regions of the valves, the sculpture, hinge, condyles, apophyses, muscle
scars, etc. Miller's work (1922) brought to light an amazingly wide
range of variations in Teredo navalis involving practically every feature
of the shell especially in the number of ridges, size and shape of the
auricle, nature of denticles, etc. Nair (1959) illustrated a series of valves
of Nototeredo norvagica from western Norway showing such variations
(Fig. 8). Recent studies (Saraswathy and Nair, 1971) at Cochin on
Nausitora hedleyi also showed the occurrence of similar variations
especially in respect of the length/height ratio, number of ridges, length
of the auricle, etc. With our present inadequate knowledge of the
teredine shell it is too early to say how far shell characters may be
employed satisfactorily for the determination of genera and species. In
stray cases well developed shell characters may assist in identification
when used in conjunction with the pallets as in Psiloteredo megotara
(Hanley) and Xototeredo norvagica (Spengler) both living in the same
habitat but having very similar pallets (see Nair, 1959).
Three species of Bankia (Sigerfoos, 1908; Nair, 1955, 1957a; Bade,
Masurekar and Bal, 1961) and one of Nuusitoru (Saraswathy and Nair,
1971) have been examined with regard to the structure of the mantle.
This consists of two lobes which secrete the valves in a typical bivalve.
In shipworms the lobes have fused along their edges resulting in the
formation of a long and delicate tube open in front for the protrusion
of the foot and produced into long siphons posteriorly. The mantle is
generally thin and translucent and not of uniform thickness. On the
hind margins of the valves the mantle is raised up and extends over the
umbonal region as a thickened muscular ridge. This duplication of the
mantle, called the " cephalic hood " by Quatrefages (1849) was considered by him as the organ directly concerned with boring. Sigerfoos
(1908) and Kofoid (1921) thought that it fitted into the burrow like a
washer around the " head " preventing the fine particles of wood from
entering the burrow. In several specimens of Bankia indica examined
the " cephalic hood " appeared very calloused suggesting frequent rubbing against the walls of the burrow. The epithelium of the hood in
genera like Bankia and Nausitora projects on the median dorsal side
behind the shell as a hump with a series of characteristic transverse
foldings. The epithelial cells are tall and columnar containing granular
cytoplasm. The rest of the mantle except the siphons is thin and the
organs of the pallial cavity may be seen through it in fresh specimens. Since the entire surface of the mantle is capable of secreting
calcium carbonate, the initial region of the burrow occupied
longest by the animal normally receives the maximum limy deposition.
Posterior to the shell valves on the dorsal and ventral surfaces the