Thomas and (koxall nll found that ocxopl~agcal mucus is preponderantly
at1 acid mucopolysacchttridc, but Croxnll also idantified carbohydrated
protein complexes in smaller quantity.
2. The stomnc?~
Thore is a rcasonablv amount of rigreement among different authors
conoerning thc types of' cell found in the stomach of Ciona, although
slightly different interpretations are plared on their function. The
subject has been well reviewed by Thomas (1970b) who has added much
detail in the form of electron micrographs and histochemicd studies.
According to Thomas there are between 30 and 40 longitudinal ridges
or folds in the stomach and separated from each other by furrows, all of
them lined by a simple epithelium. One of these ridges forms a continuation of the ventral oc?sophageal groove and contains ciliated mucus
secreting cells similar to those found in the oesophagus. These are the
only mucus secreting cells found in t,he stomach. The remaining ridges of
the stomach possess three types of cell. ( 1) At the base of the ridge there
are undifferentiated cells which probably give rise to other cell types,
notably the glandular cells. (2) Gland cells found on the sides of the
ridges have very few cilia and produce sccretory granules at their apex
which are eventually shed into the stomach lumen. (3) Vacuolar cells
located at the apex of the ridgas contain a surcession of vacuoles near
the top of the cell each of which contains fragments of membrane and
small vesicles. These vacuoles may be shed separately into the lumen
of the stomach or sometimes the whole cell is shed. Miller (1953a)
also noted the apocrine secretion of vacuolar material from these cells,
but Yonge (19%) and Van Wee1 (I!)40) believe they were absorptive
cells.
In the stomach of members of the family Pyuridae there is a
prominent digestive gland discharging into the lumen of the stomach
by conspicuous ducts. Fouque (1959) and Degail and Levi ( 1964) have
described the histdogy of this gland in Pyura microwsmus (Savigny)
and M~crocosmus cluudhns (Savigny). Mucus secretory cells are
reported to occur at the proximal ends of the digestive diverticula and
in the lateral walls of the diverticula there are two types of cell. Secretory cells with globules are the principal cell type ; they have a bass1
nucleus and severhl large vesicles containing flat membranes and
granules of secretion. Mitochondria are evenly spread throughout the
cell and at the apex there are numerous small vesicles. Interspersed
amongst these cells are basophilic secretory cells which do not always
extend to reach the lumen of the diverticulum. According to Degail
and Levi these cells have a conspicuous endoplasmic reticulum and
at1 acid mucopolysacchttridc, but Croxnll also idantified carbohydrated
protein complexes in smaller quantity.
2. The stomnc?~
Thore is a rcasonablv amount of rigreement among different authors
conoerning thc types of' cell found in the stomach of Ciona, although
slightly different interpretations are plared on their function. The
subject has been well reviewed by Thomas (1970b) who has added much
detail in the form of electron micrographs and histochemicd studies.
According to Thomas there are between 30 and 40 longitudinal ridges
or folds in the stomach and separated from each other by furrows, all of
them lined by a simple epithelium. One of these ridges forms a continuation of the ventral oc?sophageal groove and contains ciliated mucus
secreting cells similar to those found in the oesophagus. These are the
only mucus secreting cells found in t,he stomach. The remaining ridges of
the stomach possess three types of cell. ( 1) At the base of the ridge there
are undifferentiated cells which probably give rise to other cell types,
notably the glandular cells. (2) Gland cells found on the sides of the
ridges have very few cilia and produce sccretory granules at their apex
which are eventually shed into the stomach lumen. (3) Vacuolar cells
located at the apex of the ridgas contain a surcession of vacuoles near
the top of the cell each of which contains fragments of membrane and
small vesicles. These vacuoles may be shed separately into the lumen
of the stomach or sometimes the whole cell is shed. Miller (1953a)
also noted the apocrine secretion of vacuolar material from these cells,
but Yonge (19%) and Van Wee1 (I!)40) believe they were absorptive
cells.
In the stomach of members of the family Pyuridae there is a
prominent digestive gland discharging into the lumen of the stomach
by conspicuous ducts. Fouque (1959) and Degail and Levi ( 1964) have
described the histdogy of this gland in Pyura microwsmus (Savigny)
and M~crocosmus cluudhns (Savigny). Mucus secretory cells are
reported to occur at the proximal ends of the digestive diverticula and
in the lateral walls of the diverticula there are two types of cell. Secretory cells with globules are the principal cell type ; they have a bass1
nucleus and severhl large vesicles containing flat membranes and
granules of secretion. Mitochondria are evenly spread throughout the
cell and at the apex there are numerous small vesicles. Interspersed
amongst these cells are basophilic secretory cells which do not always
extend to reach the lumen of the diverticulum. According to Degail
and Levi these cells have a conspicuous endoplasmic reticulum and