T H E PHYSJOLOOY OF ASCIDIANS
35
to bc two principal typos of* cell. absorptive and mucus secreting.
'l'hornas found that tho inucus secrcting cells are very variable in their
morphology wid that in gewral they produce a preponderance of
carbohydrate-protein complexes ii nlike the oesophagus where acidmucopolysaccharides predominate. Croxall could find no clear distinction between iibsorptivt cells nnd mucus secreting cells and found that
cells intmmrdiate in the degree of ciliation and extent of optical
staining frcquently o(-curr(>d. He considered that the " absorptive " eel1
might represent one phase of activity of the typical mucus secreting cell.
Croxall agrees that carbohydrlttt.-proteiii complexes predominate in the
intestinal secretions. Thomas also records that certain mucus cells in
the intestine with a holocrine mode of secretion have an infolding of the
plasma membrane at the base and sides of the cell. This resembles the
infolding of gautric cells noted by Degail and Levi (1964) in Pyuridae,
but in the intestinal cells of Ciona there are abundant mitochondria
between the folds of the membrane suggesting an active regulatory
function for these cells. According to Peters (1966) the intestine secretes
a peritrophic membrane around the food cord which is rich in chitin.
4. The rectum
The recturii conHist8 of low ciliated mucus cells with histochemical
reactions similar to oesophagcul rc4ls. Some of these cells contain
autophagic vacuoles which are voidvd with the mucus during secretion
(Thomas, 1970a).
('. I ) i g ~ ~ t i o ? ~
Digestion in tiwidions has lteeii Htudied by Yonge (1925) in Ciona,
Berrill (1 929) in Pyurn and Boltenin and Van M'eel (1940) in Ciona and
is reviewed by Barrington (1962). Digestive enzymes are secreted into
the stomach and digestion is entirely extracellular ; unlike many other
filter feeding organisms (i.c. hivaives, AmphioxuH) there is little
evidence for intracellular uptrhke of solid particles. Van Wee1 (lid
demonstriite a small uptake of iron saccharate into ceIls of the xtomach
epithelium and subsequent passage from them to blood cells, but tho
extent to which this occurred is insufficient to juutify the assumption
that it is II. normal part of the digestive process.
There is general agreement that tisaidians have a powerful amylaue,
rtn invertltse and a lipase, but only r~ wcuk protease. A maltase and a
lacttlse are reported by Rerrill for the two Pyurid ascidians. b u t Yongc
denies their existence in Ciom. Minor difFrrcwcc,s ofthiu sort Hhould he
further invtnstigated as they mtiy br important in the overall physiological-ecology of aacidians and the manner in which they can utilize the
35
to bc two principal typos of* cell. absorptive and mucus secreting.
'l'hornas found that tho inucus secrcting cells are very variable in their
morphology wid that in gewral they produce a preponderance of
carbohydrate-protein complexes ii nlike the oesophagus where acidmucopolysaccharides predominate. Croxall could find no clear distinction between iibsorptivt cells nnd mucus secreting cells and found that
cells intmmrdiate in the degree of ciliation and extent of optical
staining frcquently o(-curr(>d. He considered that the " absorptive " eel1
might represent one phase of activity of the typical mucus secreting cell.
Croxall agrees that carbohydrlttt.-proteiii complexes predominate in the
intestinal secretions. Thomas also records that certain mucus cells in
the intestine with a holocrine mode of secretion have an infolding of the
plasma membrane at the base and sides of the cell. This resembles the
infolding of gautric cells noted by Degail and Levi (1964) in Pyuridae,
but in the intestinal cells of Ciona there are abundant mitochondria
between the folds of the membrane suggesting an active regulatory
function for these cells. According to Peters (1966) the intestine secretes
a peritrophic membrane around the food cord which is rich in chitin.
4. The rectum
The recturii conHist8 of low ciliated mucus cells with histochemical
reactions similar to oesophagcul rc4ls. Some of these cells contain
autophagic vacuoles which are voidvd with the mucus during secretion
(Thomas, 1970a).
('. I ) i g ~ ~ t i o ? ~
Digestion in tiwidions has lteeii Htudied by Yonge (1925) in Ciona,
Berrill (1 929) in Pyurn and Boltenin and Van M'eel (1940) in Ciona and
is reviewed by Barrington (1962). Digestive enzymes are secreted into
the stomach and digestion is entirely extracellular ; unlike many other
filter feeding organisms (i.c. hivaives, AmphioxuH) there is little
evidence for intracellular uptrhke of solid particles. Van Wee1 (lid
demonstriite a small uptake of iron saccharate into ceIls of the xtomach
epithelium and subsequent passage from them to blood cells, but tho
extent to which this occurred is insufficient to juutify the assumption
that it is II. normal part of the digestive process.
There is general agreement that tisaidians have a powerful amylaue,
rtn invertltse and a lipase, but only r~ wcuk protease. A maltase and a
lacttlse are reported by Rerrill for the two Pyurid ascidians. b u t Yongc
denies their existence in Ciom. Minor difFrrcwcc,s ofthiu sort Hhould he
further invtnstigated as they mtiy br important in the overall physiological-ecology of aacidians and the manner in which they can utilize the