2
R. H. MILLdR
evident that physiological, chemical and biochemical studies accoiint
for a large proportion of the recent research on ascidians. These aspects
are not treated in the present review, which aims rather to survey advances in our knowledge ofthe animal as a whole living organism. Much
of the work on structure, evolution and genetics, and most physiological
studies are therefore also excluded. No way of selecting topics is entirely
satisfactory and the inclusion of some papers and the omission of others
may seem arbitrary. Another difficulty in preparing a review of this
nature is to decide how old a publication may be and yet be regarded as
an advance. I have not chosen a date, but have been guided by whether
the matter in a paper has already been dealt with in a review or major
contribution devoted to ascidians.
11. FEEDING
A. The feeding mechanism
It has long been known that ascidians feed by filtering organisms
and particles from water drawn into the branchial sac through the oral
siphon and expelled through the atrial siphon, and that the process involves mucus secreted by the endostyle. Early accounts differ somewhat in detail; thus Roule (1884) described the mucus as passing out
from the endostyle and across the inner faces of the branchial walls,
where food particles are trapped, but Herdman (1899) thought that food
was retained by mucus at the entrance to the branchial sac. Orton
(1913) and Hecht (1918) confirmed Roule’s account of the process. In
some particulars, however, Hecht’s description has been modified by
subsequent work. He believed that food particles are retained by the
stigmata (the openings in the branchial wall) and are trapped in mucus
only after being passed by cilia to the tips of the branchial papillae.
But MacGinitie (1939) found that continuous mucous sheets on the
branchial walls trap particles from the water before it passes out through
the stigmata. This was confirmed by Jmgensen (1949), Jmgensen and
Goldberg (1953), Millar (1953a), and Werner and Werner (1954). Although most observers seem to have assumed that ciliary action alone is
responsible for moving the mucous sheet towards the roof of the
branchial sac, it is possible that muscles play some part, and Hecht
(1918) described waves of contraction which bring adjacent rows of
papillae together so that the sheet is pushed or pulled across the
branchial wall. It is, however, by no means certain that such a process
is part of the normal pattern of feeding, neither Werner and Werner
(1954) nor Millar (1953a) having observed it in Ciona intestinalis (L.).
When it reaches the roof of the sac, the endless filter of mucus is
R. H. MILLdR
evident that physiological, chemical and biochemical studies accoiint
for a large proportion of the recent research on ascidians. These aspects
are not treated in the present review, which aims rather to survey advances in our knowledge ofthe animal as a whole living organism. Much
of the work on structure, evolution and genetics, and most physiological
studies are therefore also excluded. No way of selecting topics is entirely
satisfactory and the inclusion of some papers and the omission of others
may seem arbitrary. Another difficulty in preparing a review of this
nature is to decide how old a publication may be and yet be regarded as
an advance. I have not chosen a date, but have been guided by whether
the matter in a paper has already been dealt with in a review or major
contribution devoted to ascidians.
11. FEEDING
A. The feeding mechanism
It has long been known that ascidians feed by filtering organisms
and particles from water drawn into the branchial sac through the oral
siphon and expelled through the atrial siphon, and that the process involves mucus secreted by the endostyle. Early accounts differ somewhat in detail; thus Roule (1884) described the mucus as passing out
from the endostyle and across the inner faces of the branchial walls,
where food particles are trapped, but Herdman (1899) thought that food
was retained by mucus at the entrance to the branchial sac. Orton
(1913) and Hecht (1918) confirmed Roule’s account of the process. In
some particulars, however, Hecht’s description has been modified by
subsequent work. He believed that food particles are retained by the
stigmata (the openings in the branchial wall) and are trapped in mucus
only after being passed by cilia to the tips of the branchial papillae.
But MacGinitie (1939) found that continuous mucous sheets on the
branchial walls trap particles from the water before it passes out through
the stigmata. This was confirmed by Jmgensen (1949), Jmgensen and
Goldberg (1953), Millar (1953a), and Werner and Werner (1954). Although most observers seem to have assumed that ciliary action alone is
responsible for moving the mucous sheet towards the roof of the
branchial sac, it is possible that muscles play some part, and Hecht
(1918) described waves of contraction which bring adjacent rows of
papillae together so that the sheet is pushed or pulled across the
branchial wall. It is, however, by no means certain that such a process
is part of the normal pattern of feeding, neither Werner and Werner
(1954) nor Millar (1953a) having observed it in Ciona intestinalis (L.).
When it reaches the roof of the sac, the endless filter of mucus is