The sub-cuticular portion ot’ t h e tcwt (Stitkenart’s “ fundamental
substance ”) contains both iIr:itl-rriucopolysttccharidex and neutral
polysaccharictcs I
N well as contctining about 20% of the total nitrogen
of the test. Some of this nitrogen is in the form of hexosamine which
elso forms a significant component of the test substance of H. aurantium
(Smith, 1970b). Stithenart suggests that “ bundles of the fundamental
substance are built up with cellulose fibres embedded by amorphous
mucopolysaccharides ”. However, the picture is probably not as
simple as this; neither StiOvcnart in H . papilloea nor Smith in H.
aurantium could obtain positivc reactions to P.A.S. in the test substance
and Smith suggests that if cellulose does exist in the tunic of Halocynthia
it must be heavily substituted.
A number of other studier?, have been made to investigate the nature
of the microfibrils in the test of various species and there is general
agreement that the fibrils arc composed of a polysaccharide related to
but not identical with cellulose and varying in diameter from 20 to
200 b according to species ( b’rey-Wyssling and Frey, 1950 ; Ranby,
1962; Parker and Leeper, 1969; Mishra and Colvin, 1970; Smith and
Dehnel, 1971).
From these recent studies it is possible to draw some general conclusions about the nature and formation of the test. The test is not an
inert structure like the molluscan shell but is an active living tissue
comparable to the fibrous connective tissue of the vertebrates. It
functions in part as a protective covering and in part as a supporting
skeleton. In some species it also appears to assist in restoring body
shape after muscular contraction (wide p. 99). There is usually a
hyaline ground substance formed of acid-mucopolysaccharides associated
with protein, and embedded in this are microfibrils of a polysaccharide
complex closely resembling cellulose. These fibrils are not visible in the
light microscope but their identity is apparent from electron micrographs and X-ray diffraction studieR (see purticularly Mark and
Susich, 1929 ; Meyer et aE., 1951 ; Runby, 1952; Hall and Sax], 198) ;
Deck et al., 1966; Mishra and Colvin, 1969, 1970; Wardrop, 1970).
The degrec to which fibrc development takes place varien from
species to species arid may have an important bearing on the mechnnical properties of the test. In the phlebobranchiate ascidianfi in pnrticulnr
(i.e. Ciona, Aecidiella, Phallusia) there is a tendency to produce large
gelatinou8 tests in which small fibrils are scattered throughout the
matrix. The stolidobranchiatc ascidiane on the other hand (i.e. Pyura
and Halocynthia) show a tendency to develop fibre bundles in a reduced
matrix. These latter tests, particularly Halocynthia and Molgula, tend
substance ”) contains both iIr:itl-rriucopolysttccharidex and neutral
polysaccharictcs I
N well as contctining about 20% of the total nitrogen
of the test. Some of this nitrogen is in the form of hexosamine which
elso forms a significant component of the test substance of H. aurantium
(Smith, 1970b). Stithenart suggests that “ bundles of the fundamental
substance are built up with cellulose fibres embedded by amorphous
mucopolysaccharides ”. However, the picture is probably not as
simple as this; neither StiOvcnart in H . papilloea nor Smith in H.
aurantium could obtain positivc reactions to P.A.S. in the test substance
and Smith suggests that if cellulose does exist in the tunic of Halocynthia
it must be heavily substituted.
A number of other studier?, have been made to investigate the nature
of the microfibrils in the test of various species and there is general
agreement that the fibrils arc composed of a polysaccharide related to
but not identical with cellulose and varying in diameter from 20 to
200 b according to species ( b’rey-Wyssling and Frey, 1950 ; Ranby,
1962; Parker and Leeper, 1969; Mishra and Colvin, 1970; Smith and
Dehnel, 1971).
From these recent studies it is possible to draw some general conclusions about the nature and formation of the test. The test is not an
inert structure like the molluscan shell but is an active living tissue
comparable to the fibrous connective tissue of the vertebrates. It
functions in part as a protective covering and in part as a supporting
skeleton. In some species it also appears to assist in restoring body
shape after muscular contraction (wide p. 99). There is usually a
hyaline ground substance formed of acid-mucopolysaccharides associated
with protein, and embedded in this are microfibrils of a polysaccharide
complex closely resembling cellulose. These fibrils are not visible in the
light microscope but their identity is apparent from electron micrographs and X-ray diffraction studieR (see purticularly Mark and
Susich, 1929 ; Meyer et aE., 1951 ; Runby, 1952; Hall and Sax], 198) ;
Deck et al., 1966; Mishra and Colvin, 1969, 1970; Wardrop, 1970).
The degrec to which fibrc development takes place varien from
species to species arid may have an important bearing on the mechnnical properties of the test. In the phlebobranchiate ascidianfi in pnrticulnr
(i.e. Ciona, Aecidiella, Phallusia) there is a tendency to produce large
gelatinou8 tests in which small fibrils are scattered throughout the
matrix. The stolidobranchiatc ascidiane on the other hand (i.e. Pyura
and Halocynthia) show a tendency to develop fibre bundles in a reduced
matrix. These latter tests, particularly Halocynthia and Molgula, tend