22
R. H. MlLLAR
differences in population densities may have resulted from variations in
thc surface texture rather than from differential settlement, since
texture may affect the security of attached larvae and young stages.
Nevertheless, he also observed that, while larvae of A . nigra will settle
on almost any clean surface, those of A. interrupta Heller apparently
require a degree of fouling on the substratum before they will settle.
There is scope for experiment and observation in this field to show
whether the physical and chemical nature of the surface is important or
whether, as Dybern (1963) found in Cionu, the requirement is simply for
a site with generally adequate conditions for adult life.
5. Evolution
Many authors have discussed the origin of the tadpole larva in
relation to the ancestry of the vertebrates, but little fresh evidence has
appeared recently. Both Berrill(l955) and Millar (1966b) take the view
that this particular larval form arose within the tunicates. An elongated
muscular body may have been a consequence of the loss of external
ciliation due to the development of test. Jefferies (1968), however,
believes that the tadpole may indicate the line of descent from Cambrian members of the subphylum Calcichordata (Jefferies, 1967), a
view implying that the ancestral tunicates were free-swimming animal8
which later adopted a sessile habit for the adult while retaining a
pelagic larva. Whatever may be the significance of the ascidian tadpole
in this context, the striking similarity in the fine structure of the larval
ocellus and the vertebrate eye (Dilly, 1961) can scarcely be a coincidence
or the result of convergent evolution.
IV. LIFE CYCLE: GROWTH, SUCCESSION OF
GENERATIONS AND MORTALITY
The life cycle of solitary ascidians is relatively simple, and can be
analysed as the establishment of the new individual, its growth, breeding and death; and appropriate measurements can be made at each
stage. In compound forms asexual reproduction introduces a complication, and the resulting colony is to be regarded as the biological
unit. It undergoes a complex series of changes involving various
degrees of decay, renewal and, occasionally, division.
Increase in body length has been used almost universally as a
measure of growth in simple ascidians, and although filr from ideal,
since the rclationship between length and weight is rarely known, it
does givc a useful index. Practical difficulties arise because the body in
most species is attached to a solid object which may interfere with easy
and accurate measurement, and the habit of contracting when disturbed
R. H. MlLLAR
differences in population densities may have resulted from variations in
thc surface texture rather than from differential settlement, since
texture may affect the security of attached larvae and young stages.
Nevertheless, he also observed that, while larvae of A . nigra will settle
on almost any clean surface, those of A. interrupta Heller apparently
require a degree of fouling on the substratum before they will settle.
There is scope for experiment and observation in this field to show
whether the physical and chemical nature of the surface is important or
whether, as Dybern (1963) found in Cionu, the requirement is simply for
a site with generally adequate conditions for adult life.
5. Evolution
Many authors have discussed the origin of the tadpole larva in
relation to the ancestry of the vertebrates, but little fresh evidence has
appeared recently. Both Berrill(l955) and Millar (1966b) take the view
that this particular larval form arose within the tunicates. An elongated
muscular body may have been a consequence of the loss of external
ciliation due to the development of test. Jefferies (1968), however,
believes that the tadpole may indicate the line of descent from Cambrian members of the subphylum Calcichordata (Jefferies, 1967), a
view implying that the ancestral tunicates were free-swimming animal8
which later adopted a sessile habit for the adult while retaining a
pelagic larva. Whatever may be the significance of the ascidian tadpole
in this context, the striking similarity in the fine structure of the larval
ocellus and the vertebrate eye (Dilly, 1961) can scarcely be a coincidence
or the result of convergent evolution.
IV. LIFE CYCLE: GROWTH, SUCCESSION OF
GENERATIONS AND MORTALITY
The life cycle of solitary ascidians is relatively simple, and can be
analysed as the establishment of the new individual, its growth, breeding and death; and appropriate measurements can be made at each
stage. In compound forms asexual reproduction introduces a complication, and the resulting colony is to be regarded as the biological
unit. It undergoes a complex series of changes involving various
degrees of decay, renewal and, occasionally, division.
Increase in body length has been used almost universally as a
measure of growth in simple ascidians, and although filr from ideal,
since the rclationship between length and weight is rarely known, it
does givc a useful index. Practical difficulties arise because the body in
most species is attached to a solid object which may interfere with easy
and accurate measurement, and the habit of contracting when disturbed