THE BIOLOQY OF ASCIDIANS
16
larva was retained in the oviduct until in the final stages of development
it had become narrow enough to pass the oviducal sphincter. Thoracic
contractions, initiated by the presence of the tadpole in the base of the
atrial cavity, then moved it forward and out through the siphon. A
similar process may take place in Pycnoclavella stanleyi (Berrill and
Abbott) (Trason, 1963). Levine (1962) observed some larvae of
Eudistoma ritteri Van Name to leave the parent by active swimming and
others to be carried out passively by the exhalent current, and in
Metandrocarpa taylori the larvae may be expelled by vigorous contraction of the zooid (Abbott, 1955).
In some genera, notably Distaplia and Sycozoa, the embryos and
larvae are accommodated in an outgrowth of the thorax containing the
terminal part of the oviduct. In Distaplia the brood pouch with its
larvae becomes separated from the zooid which eventually dies and the
colony then contains numerous isolated pouches. These are exposed
and release their larvae when the common test of the colony disintegrates, following the disappearance of the zooids (Berrill, 1948a). The
same process apparently occurs in Sywzoa (Millar, 1960), and in
Synoicum adureanum (Herdman) (Kott, 1060) which is one of the few
species of the family Polyclinidae in which larvae are not released
through the atrial cavity.
A somewhat different mechanism exists in those ascidians with a
small zooid and a large egg which is unable to pass forward through the
thorax. Here a single embryo generally develops at a time and as it
grows, bulges from the zooid perhaps to be released by rupture of the
body wall. Examples in the family Clavelinidae are Eudistoma digitatum
Millar, E. vastum (Millar), and Distaplia durbanensis Millar (Millar,
1963, 1964a). In Botrylloides the tadpole breaks through the body wall
to reach the common cloaca1 space (Berrill, 1947b).
The family Didemnidae shows the greatest specialization in this
direction, for the eggs pass downwards from the abdomen directly into
the test of the colony, there to be fertilized and undergo their dcvelopment. An exception in the Didemnidae is Diplosomu cupuliferum
(Kott), in which fertilization and development take place in the abdomen of the zooid (Lafargue, 1968). Release of didemnid larvae must
involve partial or complete dissolution of the test matrix, and it is perhaps not surprising that they sometimes metamorphose while still
within the colony (Millar, 1952). Kott (1969) has suggested that zooids
are also produced from larvae retained in the colonies of the unrelated
Synoicum adareanum and Distaplia cylindricu.
There is some evidence that larvae, like gametes, may be released
principally at certain times of day, since Grave and McCosh (1924)
16
larva was retained in the oviduct until in the final stages of development
it had become narrow enough to pass the oviducal sphincter. Thoracic
contractions, initiated by the presence of the tadpole in the base of the
atrial cavity, then moved it forward and out through the siphon. A
similar process may take place in Pycnoclavella stanleyi (Berrill and
Abbott) (Trason, 1963). Levine (1962) observed some larvae of
Eudistoma ritteri Van Name to leave the parent by active swimming and
others to be carried out passively by the exhalent current, and in
Metandrocarpa taylori the larvae may be expelled by vigorous contraction of the zooid (Abbott, 1955).
In some genera, notably Distaplia and Sycozoa, the embryos and
larvae are accommodated in an outgrowth of the thorax containing the
terminal part of the oviduct. In Distaplia the brood pouch with its
larvae becomes separated from the zooid which eventually dies and the
colony then contains numerous isolated pouches. These are exposed
and release their larvae when the common test of the colony disintegrates, following the disappearance of the zooids (Berrill, 1948a). The
same process apparently occurs in Sywzoa (Millar, 1960), and in
Synoicum adureanum (Herdman) (Kott, 1060) which is one of the few
species of the family Polyclinidae in which larvae are not released
through the atrial cavity.
A somewhat different mechanism exists in those ascidians with a
small zooid and a large egg which is unable to pass forward through the
thorax. Here a single embryo generally develops at a time and as it
grows, bulges from the zooid perhaps to be released by rupture of the
body wall. Examples in the family Clavelinidae are Eudistoma digitatum
Millar, E. vastum (Millar), and Distaplia durbanensis Millar (Millar,
1963, 1964a). In Botrylloides the tadpole breaks through the body wall
to reach the common cloaca1 space (Berrill, 1947b).
The family Didemnidae shows the greatest specialization in this
direction, for the eggs pass downwards from the abdomen directly into
the test of the colony, there to be fertilized and undergo their dcvelopment. An exception in the Didemnidae is Diplosomu cupuliferum
(Kott), in which fertilization and development take place in the abdomen of the zooid (Lafargue, 1968). Release of didemnid larvae must
involve partial or complete dissolution of the test matrix, and it is perhaps not surprising that they sometimes metamorphose while still
within the colony (Millar, 1952). Kott (1969) has suggested that zooids
are also produced from larvae retained in the colonies of the unrelated
Synoicum adareanum and Distaplia cylindricu.
There is some evidence that larvae, like gametes, may be released
principally at certain times of day, since Grave and McCosh (1924)