TEE BIOLOGY OF ASCIDIAXS
21
natural conditions and do little more than suggest some possibilities.
Grave (1935) believed, however, that a metabolic product of swimming
activity is essential for metamorphosis. Nevertheless, larvae irnmobilized by narcotization will metamorphose at about the same time as free
swimming controls (Bell, 1955), and although metabolic products may
be concerned, their effect does not appear to be related in a simple way
to the muscular activity of the larva.
Fixation to the substratum is not always essential for metamorphosis; Cloney (1961) for instance found that, although most larvae
of Boltenia villosa (Stimpson) in a culture attach before metamorphosing, others do not. The larvae of Eudistomu ritteri Van Name also vary
in this respect (Lcvine, 1962), and Carlisle (1961) reported postmetamorphic stages of Diplosomu listerianum (Milne Edwards) and
Cionu intestinalis in the plankton of the Plymouth aquarium. It may
be usual for a proportion of the larvae, failing to contact a solid object,
to metamorphose while still planktonic and subsequently to become
attached. Ciona retains the ability to fix itself even as an adult (Berrill,
1929; Millar, 1953a). Before fixation takes place the larval papillae
become sticky and in some species this precedes contact with the substratum. Thus in Perophora viridis a drop of viscid material is secreted
by each papilla towards the end of the free-swimming period and attachment follows contact with a solid surface (Grave and McCosh, 1924).
In Eudistomu ritteri the papillae are already in an everted condition
while the larva is swimming (Levine, 1962). Rapid eversion of the
papillae, with exposure of the adhesive surface, precedes attachment in
Euherdmunia claviformis (Ritter) (Trason, 1957) and in Pycnoclavella
stanleyi (Berrill and Abbott) (Trason, 1963). Trason (1963) believed
that a band of circular muscles in the larval trunk of E . claviformis may
cause eversion of the papillae. Although the family Polyclinidae have a
different kind of papilla, consisting of a goblet containing central
secretory cells, extrusion is also effected by deformation of the papilla,
which forces out the secretory cells and the secretion (Grave, 1920;
Sebastian, 1954). In this case the mechanism is unknown, for there do
not appear to be muscles in the papillae.
Very little is known regarding the choice of substratum by the larva,
or indeed if much choice is exercised. The presence of nerve fibres in
the adhesive papillae of botryllid larvae (Grave, 1934) suggests that
some response is made, presumably on contact with solid surfaces, but
there is nothing to indicate whether surfaces are tested until a suitable
one is found for attachment. Goodbody (1963a) believed that larvae of
Ascidia nigra may be attracted to iron, since unpainted iron develops a
dense growth of the species. It is possible, however, that observed
21
natural conditions and do little more than suggest some possibilities.
Grave (1935) believed, however, that a metabolic product of swimming
activity is essential for metamorphosis. Nevertheless, larvae irnmobilized by narcotization will metamorphose at about the same time as free
swimming controls (Bell, 1955), and although metabolic products may
be concerned, their effect does not appear to be related in a simple way
to the muscular activity of the larva.
Fixation to the substratum is not always essential for metamorphosis; Cloney (1961) for instance found that, although most larvae
of Boltenia villosa (Stimpson) in a culture attach before metamorphosing, others do not. The larvae of Eudistomu ritteri Van Name also vary
in this respect (Lcvine, 1962), and Carlisle (1961) reported postmetamorphic stages of Diplosomu listerianum (Milne Edwards) and
Cionu intestinalis in the plankton of the Plymouth aquarium. It may
be usual for a proportion of the larvae, failing to contact a solid object,
to metamorphose while still planktonic and subsequently to become
attached. Ciona retains the ability to fix itself even as an adult (Berrill,
1929; Millar, 1953a). Before fixation takes place the larval papillae
become sticky and in some species this precedes contact with the substratum. Thus in Perophora viridis a drop of viscid material is secreted
by each papilla towards the end of the free-swimming period and attachment follows contact with a solid surface (Grave and McCosh, 1924).
In Eudistomu ritteri the papillae are already in an everted condition
while the larva is swimming (Levine, 1962). Rapid eversion of the
papillae, with exposure of the adhesive surface, precedes attachment in
Euherdmunia claviformis (Ritter) (Trason, 1957) and in Pycnoclavella
stanleyi (Berrill and Abbott) (Trason, 1963). Trason (1963) believed
that a band of circular muscles in the larval trunk of E . claviformis may
cause eversion of the papillae. Although the family Polyclinidae have a
different kind of papilla, consisting of a goblet containing central
secretory cells, extrusion is also effected by deformation of the papilla,
which forces out the secretory cells and the secretion (Grave, 1920;
Sebastian, 1954). In this case the mechanism is unknown, for there do
not appear to be muscles in the papillae.
Very little is known regarding the choice of substratum by the larva,
or indeed if much choice is exercised. The presence of nerve fibres in
the adhesive papillae of botryllid larvae (Grave, 1934) suggests that
some response is made, presumably on contact with solid surfaces, but
there is nothing to indicate whether surfaces are tested until a suitable
one is found for attachment. Goodbody (1963a) believed that larvae of
Ascidia nigra may be attracted to iron, since unpainted iron develops a
dense growth of the species. It is possible, however, that observed