402
N. BALAKRISHNAN NAIR AND M. SARASWATHY
incubatory habit and hermaphroditism are, as suggested by Fretter and
Graham (1964), related to the stress of reproduction in a habitat in
some way unfavourable. The number of eggs produced is related to
their size and yolkiness which in turn'depends on whether they are to
be spawned in the ambient water for external fertilization or retained
within the body of the female during early embryonic development. In
the former case, they are usually numerous and small, in the latter
fewer but larger. Thus the diameter of the mature egg of the oviparous
species Bankia indica (=B. carinata) averages 45 p (Nair, 1956a),
whereas that of the viviparous Teredo navalis has been given as ranging
between 50 and 60 p (Loosanoff and Davis, 1963) which agrees with the
measurements given by Jmgensen (1946) and Costello et al. (1957).
Ripe eggs and sperms obtained from healthy individuals may be
used for artificial fertilization t o obtain developmental stages. Early
development of the egg of Bankia and other shipworms seems to be
basically the same as that of other marine bivalves. Brief descriptions
of this phase of development have been given by Quatrefages (1849),
Hatschek (1880), Yonge (1924), Nair (1956a) and Costello et al. (1957).
B. Development
I n Bankia indica early developmental stages were obtained through
artificial fertilizations (Nair, 1956a) and the formation of the different
stages (Fig. 32) is timed from the point of fertilization unless otherwise
mentioned. The two celled stage is reached in about 50 min and within
about 30 min after the completion of the first, the second cleavage, a
meridional one, divides the micromere into two ; within 5 h, the embryo
assumes a mosaic shape measuring 65 p. Gastrulation is by epiboly and
takes place within 6 h after fertilization. When the gastrula is about
2 h old, the rudiments of the enteron and a few mesoderm cells are
discernible in stained preparations. The stomodaeum is formed as a
result of ectodermal invagination in the region where the blastopore
has closed. Three hours later the proctodaeum is also distinguishable
as another ectodermal invagination. The shell gland appears about this
time on the dorsal side of the embryo and soon spreads out and the
shell material deposited assumes the form of a cap covering the whole
gland area. At this stage the embryo is elliptical in shape, 75 p in
length and 52 p broad. The minute cilia covering its surface begin to
beat and rotate the embryo. The free-swimming stage is reached about
11 h after fertilization. Sigerfoos (1908) found that Bankia god&
hatches within 3 h. Quatrefages (1849) who studied the gametes and
different phases of development of Teredo fatalis ( = Notoferedo norvagica
(Spengler 2)) collected at San Sebasti&n has recorded the time required for
the different phases of development such as from fecundation to the
N. BALAKRISHNAN NAIR AND M. SARASWATHY
incubatory habit and hermaphroditism are, as suggested by Fretter and
Graham (1964), related to the stress of reproduction in a habitat in
some way unfavourable. The number of eggs produced is related to
their size and yolkiness which in turn'depends on whether they are to
be spawned in the ambient water for external fertilization or retained
within the body of the female during early embryonic development. In
the former case, they are usually numerous and small, in the latter
fewer but larger. Thus the diameter of the mature egg of the oviparous
species Bankia indica (=B. carinata) averages 45 p (Nair, 1956a),
whereas that of the viviparous Teredo navalis has been given as ranging
between 50 and 60 p (Loosanoff and Davis, 1963) which agrees with the
measurements given by Jmgensen (1946) and Costello et al. (1957).
Ripe eggs and sperms obtained from healthy individuals may be
used for artificial fertilization t o obtain developmental stages. Early
development of the egg of Bankia and other shipworms seems to be
basically the same as that of other marine bivalves. Brief descriptions
of this phase of development have been given by Quatrefages (1849),
Hatschek (1880), Yonge (1924), Nair (1956a) and Costello et al. (1957).
B. Development
I n Bankia indica early developmental stages were obtained through
artificial fertilizations (Nair, 1956a) and the formation of the different
stages (Fig. 32) is timed from the point of fertilization unless otherwise
mentioned. The two celled stage is reached in about 50 min and within
about 30 min after the completion of the first, the second cleavage, a
meridional one, divides the micromere into two ; within 5 h, the embryo
assumes a mosaic shape measuring 65 p. Gastrulation is by epiboly and
takes place within 6 h after fertilization. When the gastrula is about
2 h old, the rudiments of the enteron and a few mesoderm cells are
discernible in stained preparations. The stomodaeum is formed as a
result of ectodermal invagination in the region where the blastopore
has closed. Three hours later the proctodaeum is also distinguishable
as another ectodermal invagination. The shell gland appears about this
time on the dorsal side of the embryo and soon spreads out and the
shell material deposited assumes the form of a cap covering the whole
gland area. At this stage the embryo is elliptical in shape, 75 p in
length and 52 p broad. The minute cilia covering its surface begin to
beat and rotate the embryo. The free-swimming stage is reached about
11 h after fertilization. Sigerfoos (1908) found that Bankia god&
hatches within 3 h. Quatrefages (1849) who studied the gametes and
different phases of development of Teredo fatalis ( = Notoferedo norvagica
(Spengler 2)) collected at San Sebasti&n has recorded the time required for
the different phases of development such as from fecundation to the