Orton ( 1 !)I :j), MacGinitic~ ( I !):j!j). W c r w r iuid Werner ( 1 954), .Jergenseri
( 1 960), Millsr ( I 97 1) and Croxdl ( 1 97 I ). It, is generally agreed that i1
continuoiis mucus sheet is produwd hy the endostyle and that cilia
on the branchiid wdl pans this dorsally to the dorsal lamina. It is the
mucus shect, atlid not, the branchid stigmata which acts to strain off
food particles from thr water. Awording to J~lrgensen (1949) and
,Jorgensen ttnd Goldhorg (1953) t h c n i u ~ i i s sheet of Cionu intestinali8 is
capable of rctuining partides u s small a6 I p in diameter. In aplousobranchs thr. miirus sheet must) be moved by the cilia of the stigmata1
margin, but, in phlebobranchs ;Lnd stolidobranchs the mucus sheet is
held uwu-v from thc stigmata1 siirfi~rr by the longitudinal bars and
hcnw trunsport8 of water uiid transport of the mucus sheet become
functionidly separiLted. The murus sheet is now moved by the cilia of
the branvhial papillae and longitudinul bars and Hecht (191th) has
suggested that in Ascidia nigrn wuvt's of muscular contraction in the
branchid papillw nssist in passing the mucus sheet across the branchial
wall. h ' l ~ own observations on A.widia nigra suggest that waves of
artivit!y puss wross the longitiidintd bars and that while the mucus
shcet iri prirnaril,y movcd by cilia its passage is aided by undulating
movetnrntjs of the whole system of intcrnsl longitudinal bars. A similar
observution was madc by Orton (191 3) on Ascidia mentula.
Tlic wholc question of food roll(lc.tioii and transport of the mucus
shret, hus recently been studicd o i i a wmpariLtive basis by Croxall
( I97 1 ). In thc phlebobraiichiatr uscidian Ciona the longitudinal bars
have front,iil cilia only (i.e. on thr surface facing into the branchial sac)
und undcr conditions of low food drnsity the muciis sheet is transported
solely by the ciliary activity of the papillae and bars. In conditions of
high food density small but signifimnt papillar movements occur and
result in a fast moving shect of niii(~its across the branchial wall. In
('orella (also ti phlebobranch) thrre a~re no papillae and the longitudinal
btws huve lateral as well tts frontal (*ilia. In addition the stigmata are
spiral, thiis iiicrcasing the filtration rate, and are three-dimensionally
manged so that they crcate vorti(*es of watcr. Unlike C i o m the mucus
sheet moves ut i i uniform rate irrespective of particle density ; this
probably permits maximum efficient use of the vortices and also prevents aog tendencay toward fragmentation of the mucus sheet in the
vortex. In the stolidobranch ascidians Asteromrpn and Cmrnidmmyu
there arc four branchid folds on eithrr side. The longitudinal bars are
not borric on pedicels but are only just raked off the surface of the
brrbnchittl sac : they are also coiwcntratcd towards the tips of the foldn
itnd are widely spaced in the troughs betwren folds. The bars are ciliated
frontally anti laterally, froritd cilia rtid in the passage of mucus, lateral
( 1 960), Millsr ( I 97 1) and Croxdl ( 1 97 I ). It, is generally agreed that i1
continuoiis mucus sheet is produwd hy the endostyle and that cilia
on the branchiid wdl pans this dorsally to the dorsal lamina. It is the
mucus shect, atlid not, the branchid stigmata which acts to strain off
food particles from thr water. Awording to J~lrgensen (1949) and
,Jorgensen ttnd Goldhorg (1953) t h c n i u ~ i i s sheet of Cionu intestinali8 is
capable of rctuining partides u s small a6 I p in diameter. In aplousobranchs thr. miirus sheet must) be moved by the cilia of the stigmata1
margin, but, in phlebobranchs ;Lnd stolidobranchs the mucus sheet is
held uwu-v from thc stigmata1 siirfi~rr by the longitudinal bars and
hcnw trunsport8 of water uiid transport of the mucus sheet become
functionidly separiLted. The murus sheet is now moved by the cilia of
the branvhial papillae and longitudinul bars and Hecht (191th) has
suggested that in Ascidia nigrn wuvt's of muscular contraction in the
branchid papillw nssist in passing the mucus sheet across the branchial
wall. h ' l ~ own observations on A.widia nigra suggest that waves of
artivit!y puss wross the longitiidintd bars and that while the mucus
shcet iri prirnaril,y movcd by cilia its passage is aided by undulating
movetnrntjs of the whole system of intcrnsl longitudinal bars. A similar
observution was madc by Orton (191 3) on Ascidia mentula.
Tlic wholc question of food roll(lc.tioii and transport of the mucus
shret, hus recently been studicd o i i a wmpariLtive basis by Croxall
( I97 1 ). In thc phlebobraiichiatr uscidian Ciona the longitudinal bars
have front,iil cilia only (i.e. on thr surface facing into the branchial sac)
und undcr conditions of low food drnsity the muciis sheet is transported
solely by the ciliary activity of the papillae and bars. In conditions of
high food density small but signifimnt papillar movements occur and
result in a fast moving shect of niii(~its across the branchial wall. In
('orella (also ti phlebobranch) thrre a~re no papillae and the longitudinal
btws huve lateral as well tts frontal (*ilia. In addition the stigmata are
spiral, thiis iiicrcasing the filtration rate, and are three-dimensionally
manged so that they crcate vorti(*es of watcr. Unlike C i o m the mucus
sheet moves ut i i uniform rate irrespective of particle density ; this
probably permits maximum efficient use of the vortices and also prevents aog tendencay toward fragmentation of the mucus sheet in the
vortex. In the stolidobranch ascidians Asteromrpn and Cmrnidmmyu
there arc four branchid folds on eithrr side. The longitudinal bars are
not borric on pedicels but are only just raked off the surface of the
brrbnchittl sac : they are also coiwcntratcd towards the tips of the foldn
itnd are widely spaced in the troughs betwren folds. The bars are ciliated
frontally anti laterally, froritd cilia rtid in the passage of mucus, lateral