siphon dianic~ttv eiinurcs thtit t tic c.xhalant discharge of water is driven
II long way from the 1)ivinc~hial siphon c ~ i i d in most ascidians the siphons
iirc so oririitcited ILS tro c~nsrrr~ t h t mixing will not) occur. E’urthermore
ibscitiians iiorrnally livv in pluws whcw nt~tirral renewal of thc water will
occur. It swms more likely t hat nquirting is functioiially rclated to the
food collecting mechanism rathcr than to the water supply. One
posgibility is that it is used to “ clean ” the branchial wall of accumulated mucus rind particdate material not otherwise removed by ciliary
wtivity. However, this will only take place if the atrial siphon is
closed arid pressure differcntid allowed to develop between the two
sides of thca branchis1 wall ; this appears not to be the case in normal
squirting during which water is not exchanged to any degree between
atrial and branc*hiaI cavities and the mucus sheets are not dislodged
(Jnrgenwn, 1966). Squirting must, however, be important for the
maintenance of the citrial cavity tint1 far too little attention has been
paid to this aspect of the problem. Faeces and other particulate matter
are bound to accumulate in the atrial cavity and while normal reflexes
may ensure their discharge, regular spontaneous contractions may
materially mlrunce the sanitary processes of the cavity. Little attention
has also bren givcn to the qucstioii of pseudo-faeces formation and
ejection from thc bninchial sac anti as to whether spontaneous
squirting ma-v be iLssocieited with such 21 process as in bivalve molluscs.
It seems to I)e gencrally assri mcd that ascidians do not sort their food
tmd thcit d l partioulatc mattvr which enters the branchial sac is ultimately transferred to the oesoi)hagus. The orrd tentacles act as a coarse
filter to exclude psrticles beyond IL certain size, but Croxall(1971) points
out that they niay also serve to direct food into the centrc of the inhalant
stream. Onw II particle ticks pnssed this screen there seems to be no way
in which it niay Hubsequcntly be rejectd unless mucus secretion in
interrupted i i t i d particles are permitted to pass through the stigmata.
However, MacGinitie ( 1939) suggests that ascidians are capable of
rtwioving lrirgr partides from t,he food supply and states: “ Such large
particles as (10 find thcxir way into the branchial basket are not incorporated in the mucus, but are in some wtiy dropped from it into
thc branchid basket, arid at intervals are forcibly ejected from the oral
funnel by a sudden contraction of the body wail of the tunicate. ”
Later in the stme paper he suggests that specially large cilia in the
dorsal groove iire rrsed to force out unwanted particles from the mucus.
No other workrra appear to have examined the question of partide
sorting utrd pseudo-fcieces prodrrction in a.rcidian3 and the mattt:r
tlescrvcs ren(ww1 at tent ion r.spwially in relation to ~lwItu,ncoirs
syu irt i ng.
II long way from the 1)ivinc~hial siphon c ~ i i d in most ascidians the siphons
iirc so oririitcited ILS tro c~nsrrr~ t h t mixing will not) occur. E’urthermore
ibscitiians iiorrnally livv in pluws whcw nt~tirral renewal of thc water will
occur. It swms more likely t hat nquirting is functioiially rclated to the
food collecting mechanism rathcr than to the water supply. One
posgibility is that it is used to “ clean ” the branchial wall of accumulated mucus rind particdate material not otherwise removed by ciliary
wtivity. However, this will only take place if the atrial siphon is
closed arid pressure differcntid allowed to develop between the two
sides of thca branchis1 wall ; this appears not to be the case in normal
squirting during which water is not exchanged to any degree between
atrial and branc*hiaI cavities and the mucus sheets are not dislodged
(Jnrgenwn, 1966). Squirting must, however, be important for the
maintenance of the citrial cavity tint1 far too little attention has been
paid to this aspect of the problem. Faeces and other particulate matter
are bound to accumulate in the atrial cavity and while normal reflexes
may ensure their discharge, regular spontaneous contractions may
materially mlrunce the sanitary processes of the cavity. Little attention
has also bren givcn to the qucstioii of pseudo-faeces formation and
ejection from thc bninchial sac anti as to whether spontaneous
squirting ma-v be iLssocieited with such 21 process as in bivalve molluscs.
It seems to I)e gencrally assri mcd that ascidians do not sort their food
tmd thcit d l partioulatc mattvr which enters the branchial sac is ultimately transferred to the oesoi)hagus. The orrd tentacles act as a coarse
filter to exclude psrticles beyond IL certain size, but Croxall(1971) points
out that they niay also serve to direct food into the centrc of the inhalant
stream. Onw II particle ticks pnssed this screen there seems to be no way
in which it niay Hubsequcntly be rejectd unless mucus secretion in
interrupted i i t i d particles are permitted to pass through the stigmata.
However, MacGinitie ( 1939) suggests that ascidians are capable of
rtwioving lrirgr partides from t,he food supply and states: “ Such large
particles as (10 find thcxir way into the branchial basket are not incorporated in the mucus, but are in some wtiy dropped from it into
thc branchid basket, arid at intervals are forcibly ejected from the oral
funnel by a sudden contraction of the body wail of the tunicate. ”
Later in the stme paper he suggests that specially large cilia in the
dorsal groove iire rrsed to force out unwanted particles from the mucus.
No other workrra appear to have examined the question of partide
sorting utrd pseudo-fcieces prodrrction in a.rcidian3 and the mattt:r
tlescrvcs ren(ww1 at tent ion r.spwially in relation to ~lwItu,ncoirs
syu irt i ng.