The overdl picture of iodiiicb binding and its significance is at first
sight confusing. Thr two prirwipal sites of bound iodine are the cuticle
of the test, and the zonc 7 cells of the endostyle, and in both cases
iodinated tyrosines me the principal compounds involved, but small
quantities of m i iodincited thyronine (thyroxine) can be detected in the
endostylc. On present cvidenoc thcrc is 110 reason to believe that there
is any functional connection between these two processes. It seems
likely that iodination in tho test is purely associated with the formation
of structural proteins for the cuticle. The secretion of iodinated compounds in thc eridost,yle is a specialization of protein metabolism to
produce the correct, type of filtration membrane passing over the
branchial wall (Barrington arid Thorpo, 1965b). The presence of
iodinatcd protrins ill the epidermal secretions of Saccoglossus lends
support to the view that the cndostylar compounds of ascidians are
concerned with the filtration membrane. In Saccoglossus and other
enteropneusts the epidermal secretions are used as an external food
collecting device.
If we accept these ideas as correct it then becomes easier to
understand how natural selection may have retained and developed
the secretion of iodinated proteins for use in filter feeding and subsequently turned them to advantage as hormones after the change to
raptorial feeding in the vertebrates. A discussion of the evolution
of the thyroid nntl its precursoru is given by Barrington (1964a,b).
VI. THE ALIMENTARY SYSTEM
A. Food transport
Througlioiit the wholc of the alimentary canal food is transported
by cilia and there is little evidence of muscular action taking part,
except for lateral transport in the bmnchial sac of certain species where
mucus is passed from one branchial papilla to another by muscular
twtion of the ppillae themselves (Hecht, 1918a ; Berrill, 1961)). Roule
( 1 884) has also described very fine bundles of longitudinal muscles in
the rectnl region and sphincter muscles in the a m s of Ciona intestinalis
which may asskt defaccation.
Cilia around the mouth of the oesophagus are responflible for
drawing the food cord from the branchial sac into the oesophagus where
other cilia impart( torsion (Millar, 1953a). The fact that the food cord
folds ripon itself i n the stomach (Plough and Jones, 1939; Berrill, 1950:
persoan1 obwrvstion) suggests t hiit the (:ilitL of the stomach may be
unimportunt3 in food transport. Millar ( I 96%) describes a ring of'
ciliated cells at the junction of stomach and oesophagus which he
sight confusing. Thr two prirwipal sites of bound iodine are the cuticle
of the test, and the zonc 7 cells of the endostyle, and in both cases
iodinated tyrosines me the principal compounds involved, but small
quantities of m i iodincited thyronine (thyroxine) can be detected in the
endostylc. On present cvidenoc thcrc is 110 reason to believe that there
is any functional connection between these two processes. It seems
likely that iodination in tho test is purely associated with the formation
of structural proteins for the cuticle. The secretion of iodinated compounds in thc eridost,yle is a specialization of protein metabolism to
produce the correct, type of filtration membrane passing over the
branchial wall (Barrington arid Thorpo, 1965b). The presence of
iodinatcd protrins ill the epidermal secretions of Saccoglossus lends
support to the view that the cndostylar compounds of ascidians are
concerned with the filtration membrane. In Saccoglossus and other
enteropneusts the epidermal secretions are used as an external food
collecting device.
If we accept these ideas as correct it then becomes easier to
understand how natural selection may have retained and developed
the secretion of iodinated proteins for use in filter feeding and subsequently turned them to advantage as hormones after the change to
raptorial feeding in the vertebrates. A discussion of the evolution
of the thyroid nntl its precursoru is given by Barrington (1964a,b).
VI. THE ALIMENTARY SYSTEM
A. Food transport
Througlioiit the wholc of the alimentary canal food is transported
by cilia and there is little evidence of muscular action taking part,
except for lateral transport in the bmnchial sac of certain species where
mucus is passed from one branchial papilla to another by muscular
twtion of the ppillae themselves (Hecht, 1918a ; Berrill, 1961)). Roule
( 1 884) has also described very fine bundles of longitudinal muscles in
the rectnl region and sphincter muscles in the a m s of Ciona intestinalis
which may asskt defaccation.
Cilia around the mouth of the oesophagus are responflible for
drawing the food cord from the branchial sac into the oesophagus where
other cilia impart( torsion (Millar, 1953a). The fact that the food cord
folds ripon itself i n the stomach (Plough and Jones, 1939; Berrill, 1950:
persoan1 obwrvstion) suggests t hiit the (:ilitL of the stomach may be
unimportunt3 in food transport. Millar ( I 96%) describes a ring of'
ciliated cells at the junction of stomach and oesophagus which he