54
R. H. MILLAR
Bourdillon (1950) found that the mussel does not penetrate the test
of its host, but merely makes a depression by pulling the shells down
after attaching byssus threads. The mussel is apparently not attracted
by the ascidian, but preferentially settles down on the test following an
accidental encounter. The bedding reaction, however, is probably a
response to a chemical factor of the test, since it occurs even when an
eviscerated test is offered. There can be little question of mutual benefit
from the association, and the ascidian tolerates the mussel without
suffering by its presence.
A few gastropod molluscs have adopted a similar home in the
ascidian test (Harant, 1931).
C. Monniot (1965a) found polychaetes in the cloacal cavity of
Microcosmus multitentaculatus Tokioka and the nemertine Tetrastemma
vittatrtm in 90% of Microcosmus sabatieri Roule collected at Banyulssur-Mer.
As already mentioned in relation to copepods, the distinction between commensalism and parasitism is difficult to establish, but undoubted parasites of ascidians are numerous amongst other systematic
groups (Harant, 1931 ; F. Monniot, 1965) although we still know little
of the effect on the host or of its response.
One hydroid species, Endocrypta huntsmani (Fraser) has been reported from the branchial wall of ascidians (Fraser, 1937).
What is often termed symbiosis, and more strictly mutualism, implies some degree of metabolic dependence between the partners.
Droop (1 963) has reviewed the significance of relationships between
certain animals and symbiotic algae, and both he and Yonge (1937)
pointed out that these animals usually have intracellular digestion. The
fact that in ascidians digestion is extracellular may explain why they
so seldom have symbiotic algae. Smith (1935) found that a number of
tropical species of Didemnidae have algae in the cloacal cavities or the
test, but doubted whether the ascidians could benefit from the association. PBrits (1960) also described algal cells in a species of Trididemnum
from the Red Sea and Tokioka (1967) found zoochlorellae in lacunae of
the common test of Trididemnum cyclops Michaelsen from the Gilbert
Islands, and embedded in the test substance of T . v i d e (Herdman)
from the Philippine Islands. In the last named species the zoochlorellae
are apparently surrounded by the densely packed calcareous spicules of
the test and it is uncertain if efficient photosynthesis would take place
in such a situation. The following Indonesian didemnid species are also
reported to have zoochlorellae in spaces of the test (Tokioka, 1967):
Lissoclinum patella (Gottschaldt), Lissoclinum pulvinum (Tokioka),
Diplosoma virens (Hartmeyer). A remarkable case was found by
R. H. MILLAR
Bourdillon (1950) found that the mussel does not penetrate the test
of its host, but merely makes a depression by pulling the shells down
after attaching byssus threads. The mussel is apparently not attracted
by the ascidian, but preferentially settles down on the test following an
accidental encounter. The bedding reaction, however, is probably a
response to a chemical factor of the test, since it occurs even when an
eviscerated test is offered. There can be little question of mutual benefit
from the association, and the ascidian tolerates the mussel without
suffering by its presence.
A few gastropod molluscs have adopted a similar home in the
ascidian test (Harant, 1931).
C. Monniot (1965a) found polychaetes in the cloacal cavity of
Microcosmus multitentaculatus Tokioka and the nemertine Tetrastemma
vittatrtm in 90% of Microcosmus sabatieri Roule collected at Banyulssur-Mer.
As already mentioned in relation to copepods, the distinction between commensalism and parasitism is difficult to establish, but undoubted parasites of ascidians are numerous amongst other systematic
groups (Harant, 1931 ; F. Monniot, 1965) although we still know little
of the effect on the host or of its response.
One hydroid species, Endocrypta huntsmani (Fraser) has been reported from the branchial wall of ascidians (Fraser, 1937).
What is often termed symbiosis, and more strictly mutualism, implies some degree of metabolic dependence between the partners.
Droop (1 963) has reviewed the significance of relationships between
certain animals and symbiotic algae, and both he and Yonge (1937)
pointed out that these animals usually have intracellular digestion. The
fact that in ascidians digestion is extracellular may explain why they
so seldom have symbiotic algae. Smith (1935) found that a number of
tropical species of Didemnidae have algae in the cloacal cavities or the
test, but doubted whether the ascidians could benefit from the association. PBrits (1960) also described algal cells in a species of Trididemnum
from the Red Sea and Tokioka (1967) found zoochlorellae in lacunae of
the common test of Trididemnum cyclops Michaelsen from the Gilbert
Islands, and embedded in the test substance of T . v i d e (Herdman)
from the Philippine Islands. In the last named species the zoochlorellae
are apparently surrounded by the densely packed calcareous spicules of
the test and it is uncertain if efficient photosynthesis would take place
in such a situation. The following Indonesian didemnid species are also
reported to have zoochlorellae in spaces of the test (Tokioka, 1967):
Lissoclinum patella (Gottschaldt), Lissoclinum pulvinum (Tokioka),
Diplosoma virens (Hartmeyer). A remarkable case was found by