THE BIOLOGY OF WOOD-BORING TEREDINID MOLLUSCS
403
extrusion of the polar body ( 2 li), from the extrusion of the polar body
to the appearance of the cilia (9 h), and from the appearance of the
cilia to the formation of the shell (81 h). Nevertheless, undue importance need not be given to the duration of difforent stages since
development depends, among other factors, on the original condition of
the egg, culturing methods, and temperature of the ambient medium
(Loosanoff and Davis, 1963).
In Bankia indica, 13 h after fertilization the larva becomes slightly
broader at the anterior end assuming the shape of a top. I n addition to
the feeble cilia developed all over the surface, conspicuous cilia develop
along two distinct bands. A conspicuous pre-oral band of cilia, the
prototroch is developed in front of the stomodaeal invagination composed of a double row of ciliated cells. A postoral band, parallel with
the pre-oral belt composed of a single row of less conspicuous ciliated
cells, is developed behind the stomodaeum. A third group of cilia is
formednear the proctodaeal invagination. This trochophore larva swims
for nearly 8 h during the course of which a characteristic shell is secreted.
In Bankia indica, when the trochophore is about 8 h old, i.e. within
about 21 h after fertilization, it becomes the early veliger with a shell
which can completely enclose the body. A pronounced development of
the strong biserial band of preoral cilia takes place with the result that
the anterior region of the larva enlarges. This area with its crown of
motile cilia projects beyond the larval shell and is tho rudiment of the
velum. The shell at this stage has a straight side, the " straight-hinge
stage ", measuring 7 5 p in length and 60 p across. A median protuberance developed ventral to the stomodaeum represents the rudiment of the foot (Nair, 1956a).
On the third day of development, i.e. about 7 2 h after fertilization,
the straight-hinge larva has a much better developed velum with
thickened edges bearing stronger and longer cilia. The shell gland
spreads out with a consequent increase in the shell. Loosanoff and
Davis (1963) describe the shells of straight hinge larvae of Teredo
nuvalis as heavy and thick characterized by a dark band around the
edge of the shell, and outside this a conspicuous light band. As the
larvae approach settling size, the bands become less sharply delineated.
The early larval shell is characteristically longer than wide, being
75 x 6 0 p to 90 x 7 0 p in Bankia indica (Nair, 1956); 95 x 8 0 p
(Sullivan, 1948), 80 x 70 p (Loosanoff and Davis, 1963) or 85 x 7 2 p
(Imai et al., 1950) in Teredo nawalis. Isham and Tierney (1953) record
considerably higher values for T . pedicellata (245 & 20 p).
The duration of the early veliger in the tropical oviparous Bankia
indica is about 6 days during which it grows from 75 x 6 0 p t o 90
A.P.B.-9
14
403
extrusion of the polar body ( 2 li), from the extrusion of the polar body
to the appearance of the cilia (9 h), and from the appearance of the
cilia to the formation of the shell (81 h). Nevertheless, undue importance need not be given to the duration of difforent stages since
development depends, among other factors, on the original condition of
the egg, culturing methods, and temperature of the ambient medium
(Loosanoff and Davis, 1963).
In Bankia indica, 13 h after fertilization the larva becomes slightly
broader at the anterior end assuming the shape of a top. I n addition to
the feeble cilia developed all over the surface, conspicuous cilia develop
along two distinct bands. A conspicuous pre-oral band of cilia, the
prototroch is developed in front of the stomodaeal invagination composed of a double row of ciliated cells. A postoral band, parallel with
the pre-oral belt composed of a single row of less conspicuous ciliated
cells, is developed behind the stomodaeum. A third group of cilia is
formednear the proctodaeal invagination. This trochophore larva swims
for nearly 8 h during the course of which a characteristic shell is secreted.
In Bankia indica, when the trochophore is about 8 h old, i.e. within
about 21 h after fertilization, it becomes the early veliger with a shell
which can completely enclose the body. A pronounced development of
the strong biserial band of preoral cilia takes place with the result that
the anterior region of the larva enlarges. This area with its crown of
motile cilia projects beyond the larval shell and is tho rudiment of the
velum. The shell at this stage has a straight side, the " straight-hinge
stage ", measuring 7 5 p in length and 60 p across. A median protuberance developed ventral to the stomodaeum represents the rudiment of the foot (Nair, 1956a).
On the third day of development, i.e. about 7 2 h after fertilization,
the straight-hinge larva has a much better developed velum with
thickened edges bearing stronger and longer cilia. The shell gland
spreads out with a consequent increase in the shell. Loosanoff and
Davis (1963) describe the shells of straight hinge larvae of Teredo
nuvalis as heavy and thick characterized by a dark band around the
edge of the shell, and outside this a conspicuous light band. As the
larvae approach settling size, the bands become less sharply delineated.
The early larval shell is characteristically longer than wide, being
75 x 6 0 p to 90 x 7 0 p in Bankia indica (Nair, 1956); 95 x 8 0 p
(Sullivan, 1948), 80 x 70 p (Loosanoff and Davis, 1963) or 85 x 7 2 p
(Imai et al., 1950) in Teredo nawalis. Isham and Tierney (1953) record
considerably higher values for T . pedicellata (245 & 20 p).
The duration of the early veliger in the tropical oviparous Bankia
indica is about 6 days during which it grows from 75 x 6 0 p t o 90
A.P.B.-9
14