TllE PlIYSIOl.O(tY OF ARCIDIANS
43
transport appears to be froin hatarnolymph to gland and not vice versa.
This still leaves the role of the parapyloric cells unaccounted for but
they may be cwwerned with dirert liptake frorn the intestine rather
than from the tLmpullu.
There are two other h.ypothcscs which should be explored and which
do not appear to have becw conc;idcrocl t)y previous authors. The gland
may be concerncd either with ionic regulution of the blood or with the
control of pH in the alimentary aanal. Ascidianu are known to regulate
sulphate but the mechnnistns for doing so are unknown. Thomas (1970b)
has drawn attention to ccrt,ctin cells in thc: stomach which he believed
might perform this function, hut neither these nor the cells of the
pyloric gland ttppcw to be a suitable Rite for such regulation ; sulphate
'ions withdrawn frorn the blood l),y these edls would be released in such
a position that they would be carried back over the intestinal absorptive
surface.
Attentioii h w iilready beeii tlrawn to the fact that t,he enzyme
secreting channels of the stornsch converge towards the gastrointestinal junction where the pyloricb duct enters the alimentary canal
and where there is ~t ring of cilia. The food cord is still wrapped in
mucus or muco-protein and it would be reasonable to believe that the
pyloric secretions are involved iii some wity in softening the food cord
to permit mixing of the enzymes. This might, be achieved by control of
pH a t this particwlur point.
VIl. RIWII~ATION
In view of their ~rwsilr mode of liffi aiitl low level of activity it is to
be expected that respiratory exchange in itscitlians will hc* small and
cvm be accommodated without the intervention of Hpwialized reflpiratory carriers in the blood. ARcidiari hlootl cwitain~ a complrx orgmometal compoiintl usually including (Bither vanadiitm or iron hut thin in
concerned with test formution and not with oxygon transport (p. 81).
The only nviiilable data on oxygen uptake by ascidians are those of
Jorgensen ( 1 952) and Goodbody aiitl Young (unpublished). Jergensen
gives values of 850 p1 of oxygen per hour taken up by five specimens of
('iona intPstinnlis- and comparable values for Molgula manhattensia but
these are not related to any spt~*ific body weight. Goodbody and
Young found that Ascidia n i g m at 88°C consumed 806 p1 O,/h/g total
dry weight, or I630 pl 02/h/g body weight exrluding the test. Because
the test is largely in& it is difficult to know how metabolic activities
in iwiditins should be related to body weight itnd it is advisable to
express them in these two ways (Goodbody, 1957). The level of oxygen
43
transport appears to be froin hatarnolymph to gland and not vice versa.
This still leaves the role of the parapyloric cells unaccounted for but
they may be cwwerned with dirert liptake frorn the intestine rather
than from the tLmpullu.
There are two other h.ypothcscs which should be explored and which
do not appear to have becw conc;idcrocl t)y previous authors. The gland
may be concerncd either with ionic regulution of the blood or with the
control of pH in the alimentary aanal. Ascidianu are known to regulate
sulphate but the mechnnistns for doing so are unknown. Thomas (1970b)
has drawn attention to ccrt,ctin cells in thc: stomach which he believed
might perform this function, hut neither these nor the cells of the
pyloric gland ttppcw to be a suitable Rite for such regulation ; sulphate
'ions withdrawn frorn the blood l),y these edls would be released in such
a position that they would be carried back over the intestinal absorptive
surface.
Attentioii h w iilready beeii tlrawn to the fact that t,he enzyme
secreting channels of the stornsch converge towards the gastrointestinal junction where the pyloricb duct enters the alimentary canal
and where there is ~t ring of cilia. The food cord is still wrapped in
mucus or muco-protein and it would be reasonable to believe that the
pyloric secretions are involved iii some wity in softening the food cord
to permit mixing of the enzymes. This might, be achieved by control of
pH a t this particwlur point.
VIl. RIWII~ATION
In view of their ~rwsilr mode of liffi aiitl low level of activity it is to
be expected that respiratory exchange in itscitlians will hc* small and
cvm be accommodated without the intervention of Hpwialized reflpiratory carriers in the blood. ARcidiari hlootl cwitain~ a complrx orgmometal compoiintl usually including (Bither vanadiitm or iron hut thin in
concerned with test formution and not with oxygon transport (p. 81).
The only nviiilable data on oxygen uptake by ascidians are those of
Jorgensen ( 1 952) and Goodbody aiitl Young (unpublished). Jergensen
gives values of 850 p1 of oxygen per hour taken up by five specimens of
('iona intPstinnlis- and comparable values for Molgula manhattensia but
these are not related to any spt~*ific body weight. Goodbody and
Young found that Ascidia n i g m at 88°C consumed 806 p1 O,/h/g total
dry weight, or I630 pl 02/h/g body weight exrluding the test. Because
the test is largely in& it is difficult to know how metabolic activities
in iwiditins should be related to body weight itnd it is advisable to
express them in these two ways (Goodbody, 1957). The level of oxygen