THE PHYSIOLO(1Y OF ASCIDIANS
65
Webb (1939) and Robertson ( I!)li4) for P. mammillata, and Endean
( 1 9 8 h ) for I’!/ura tdolonifmz. Robertsoli has also made determinations
on a pelagic t uiiicate, Salpa rrLa.&rm Forslrill. Other and conflicting
determinations have been made by Bialaszewicz (1 933) for Phallmia
mammillata a d Cionn intestinalis tinct by Kobayashi (1936) for Chelyosoma sibqja Oltti. All qf thew wsiilts we summarized in Table 1X.
Although there are considerable differences in the ratios found by
various authors, especially thaw of’ Bialaszewicz, there is a clear
indication thiit the plrismi is normally near to neutral in pH, is isoosmotic with sea water and contains a very low concentration of
sulphate ions. Ionic bttlance is tiohieved by clcvation of chloride concentration. The blood also has a low carbon dioxide capacity; Robertson (1 984) providcd data to ~Jiow t h t in /’hallusin marnmilhta a
normal gritdielit, of carbon dioxide oxiRts betwcen the blood and sea
water but that the alkdi restvve of the blood is less than in the surrounding water.
The significance of the low sulphatc concrwtration remains uncertain. Robertson (1964) has pointrcl out that such concentrations could
be maintained against inward diffusion from the sea if the sulphate was
removed from the plasma by blood corpuscles which are then eliminated
from the body. The maintenance of high coiicentrations of sulphuric
acid by certain blood cells suggests a mechanism for withdrawal from
the plasma. The ultimate fate of the cells is concerned with the deposition of test substance and during this process rytolysis occurs and the
sulphmic acid must be lost to the surrounding medium. It ix tempting
therefore to suppose that a dynamic situation might exist in which
sulphate ions are removed from the plasma by blood cells and released
into the test. If this was correvt, we should expect to find large yuantities of sulphur or sulphates in the test but, in Halocynthia aurantium,
Hmith and Dehnc-1 (1970) find negligible qriantities of sulphur. In many
ascidians, but not in Halocynthin, the trst contains large veHicular cells,
or bladder cells, with strong acid propertiw (Webb, 1939 ; Endean, 1961)
and it is conceivable th%t these accumulatc the excess sulphuric acid
eventually discharging it to the surface of the test.
There is thus a potential piithway for the elimination of sulphate
from the body through the test but according to Bielig et al. (196lb) the
uptake of ionic sulphate from the plasma is very slow. If sulphate ions
are not being actively removed by morula cells and discharged through
the test there must be some other mechanism for maintaining a low
plasma concentration. One such mechanism might be through a
Donnan equilibrium, but Robertson (1954) has shown that the concentration of plasma proteins is low (about 0.3 rtig/ml) and Bielig and
65
Webb (1939) and Robertson ( I!)li4) for P. mammillata, and Endean
( 1 9 8 h ) for I’!/ura tdolonifmz. Robertsoli has also made determinations
on a pelagic t uiiicate, Salpa rrLa.&rm Forslrill. Other and conflicting
determinations have been made by Bialaszewicz (1 933) for Phallmia
mammillata a d Cionn intestinalis tinct by Kobayashi (1936) for Chelyosoma sibqja Oltti. All qf thew wsiilts we summarized in Table 1X.
Although there are considerable differences in the ratios found by
various authors, especially thaw of’ Bialaszewicz, there is a clear
indication thiit the plrismi is normally near to neutral in pH, is isoosmotic with sea water and contains a very low concentration of
sulphate ions. Ionic bttlance is tiohieved by clcvation of chloride concentration. The blood also has a low carbon dioxide capacity; Robertson (1 984) providcd data to ~Jiow t h t in /’hallusin marnmilhta a
normal gritdielit, of carbon dioxide oxiRts betwcen the blood and sea
water but that the alkdi restvve of the blood is less than in the surrounding water.
The significance of the low sulphatc concrwtration remains uncertain. Robertson (1964) has pointrcl out that such concentrations could
be maintained against inward diffusion from the sea if the sulphate was
removed from the plasma by blood corpuscles which are then eliminated
from the body. The maintenance of high coiicentrations of sulphuric
acid by certain blood cells suggests a mechanism for withdrawal from
the plasma. The ultimate fate of the cells is concerned with the deposition of test substance and during this process rytolysis occurs and the
sulphmic acid must be lost to the surrounding medium. It ix tempting
therefore to suppose that a dynamic situation might exist in which
sulphate ions are removed from the plasma by blood cells and released
into the test. If this was correvt, we should expect to find large yuantities of sulphur or sulphates in the test but, in Halocynthia aurantium,
Hmith and Dehnc-1 (1970) find negligible qriantities of sulphur. In many
ascidians, but not in Halocynthin, the trst contains large veHicular cells,
or bladder cells, with strong acid propertiw (Webb, 1939 ; Endean, 1961)
and it is conceivable th%t these accumulatc the excess sulphuric acid
eventually discharging it to the surface of the test.
There is thus a potential piithway for the elimination of sulphate
from the body through the test but according to Bielig et al. (196lb) the
uptake of ionic sulphate from the plasma is very slow. If sulphate ions
are not being actively removed by morula cells and discharged through
the test there must be some other mechanism for maintaining a low
plasma concentration. One such mechanism might be through a
Donnan equilibrium, but Robertson (1954) has shown that the concentration of plasma proteins is low (about 0.3 rtig/ml) and Bielig and