TEE BIOLOOY OF ASCLDIANS
3
gathered by the dorsal lamina or languets and rolled into the form of a
cord, which is then pulled into the oesophagus (Millar, 1953a).
Jergensen (1949) investigated the efficiency of particle retention in
Molgula sp. and Ciona intestinalis, and found that particles of colloidal
graphite of 2-46 p were retained by the mucous sheet. In later experiments J~rgensen and Goldberg (1953) showed that Ciona can completely remove graphite particles of 1-2 p, but that protein molecules
(haemocyanin and haemoglobin) mainly escape. The fact that some
protein molecules are captured, however, suggests that processes other
than purely mechanical ones may be involved. Korringa (1952) believed that the electrical charges on the mucus and the food particles of
filter-feeding animals may determine whether or not small particles,
and molecules, are trapped. I n this connection it is worth noting that
vanadium, which is present in high concentrations in certain ascidians,
appears to be taken up from the sea water initially by adsorption on the
mucus of the branchial sac (Goldberg et al., 1951 ; Bielig et al., 1961).
Stephens and Schinske (1961) found that the three species of ascidians
which they investigated all removed considerable quantities of amino
acids from solution, but there was no direct evidence that mucus was
responsible.
Not all organisms in the feeding current reach the mucous sheets,
since the oral tentacles retain many of the larger particles (Werner and
Werner, 1954) and those which reach the branchial sac may fail, in
some unknown way, to be incorporated in the mucous sheets, and ase
subsequently expelled through the oral siphon (MacGinitie, 1939).
Moreover, MacGinitie briefly mentioned the rejection of some particles
already caught by mucus and suggested that " cilia bordering the dorsal
groove " may be responsible. This interesting possibility deserves
further study, since rejection mechanisms play an important part in
filter-feeding molluscs, and might be expected to occur also in ascidians.
Although we have little indication of how rejection might take place,
there is some indirect evidence that it does, for in Dislaplia cylindricu
(Lesson) and Eugyra aernbaeckae Millar the branchial sac was found to
contain a mixture of sand and cells of ph-ytoplankton, but in the
stomach only the cells were present (Millar, 1960). The basis of selection
is apparently not merely the size of particle, since the stomach contained
cells as large as the sand grains which had been rejected.
Ascidians can also control their feeding by cutting off the secretion
of mucus from the endostyle, with or without maintenance of the water
current (MacGinitie, 1939 ; Werner and Werner, 1954).
The efficiency of feeding depends not only on the ability to filter a
wide range of particles but also on the rate of water transport. This has
3
gathered by the dorsal lamina or languets and rolled into the form of a
cord, which is then pulled into the oesophagus (Millar, 1953a).
Jergensen (1949) investigated the efficiency of particle retention in
Molgula sp. and Ciona intestinalis, and found that particles of colloidal
graphite of 2-46 p were retained by the mucous sheet. In later experiments J~rgensen and Goldberg (1953) showed that Ciona can completely remove graphite particles of 1-2 p, but that protein molecules
(haemocyanin and haemoglobin) mainly escape. The fact that some
protein molecules are captured, however, suggests that processes other
than purely mechanical ones may be involved. Korringa (1952) believed that the electrical charges on the mucus and the food particles of
filter-feeding animals may determine whether or not small particles,
and molecules, are trapped. I n this connection it is worth noting that
vanadium, which is present in high concentrations in certain ascidians,
appears to be taken up from the sea water initially by adsorption on the
mucus of the branchial sac (Goldberg et al., 1951 ; Bielig et al., 1961).
Stephens and Schinske (1961) found that the three species of ascidians
which they investigated all removed considerable quantities of amino
acids from solution, but there was no direct evidence that mucus was
responsible.
Not all organisms in the feeding current reach the mucous sheets,
since the oral tentacles retain many of the larger particles (Werner and
Werner, 1954) and those which reach the branchial sac may fail, in
some unknown way, to be incorporated in the mucous sheets, and ase
subsequently expelled through the oral siphon (MacGinitie, 1939).
Moreover, MacGinitie briefly mentioned the rejection of some particles
already caught by mucus and suggested that " cilia bordering the dorsal
groove " may be responsible. This interesting possibility deserves
further study, since rejection mechanisms play an important part in
filter-feeding molluscs, and might be expected to occur also in ascidians.
Although we have little indication of how rejection might take place,
there is some indirect evidence that it does, for in Dislaplia cylindricu
(Lesson) and Eugyra aernbaeckae Millar the branchial sac was found to
contain a mixture of sand and cells of ph-ytoplankton, but in the
stomach only the cells were present (Millar, 1960). The basis of selection
is apparently not merely the size of particle, since the stomach contained
cells as large as the sand grains which had been rejected.
Ascidians can also control their feeding by cutting off the secretion
of mucus from the endostyle, with or without maintenance of the water
current (MacGinitie, 1939 ; Werner and Werner, 1954).
The efficiency of feeding depends not only on the ability to filter a
wide range of particles but also on the rate of water transport. This has