discarding thcm in faeces when they have passed the peak of their
activity.
Almost nothing is known about the detailed process of absorption
in the ascidiw alimentitry canal r~iid subsequent, metabolism of food.
, 411 authors cigrne on the oxietenoc of' trbsorpt
that the vacuolated cells in tht: stomach are also absorptive and would
take up iron iind olive oil, but later workers (Millar, 1953a ; Thomas,
1970b) believe they release thcir vacuoles from the apex, although the
precise function of these vaciiolated cells is still in doubt (cf. p. 32).
In this context it is of interest that lhvidson et al. (1971) report the
presence of goblet cells in the gut, of Giona i~teslinalis which give staining reactions similar to the insuliii producing " ,%cells " of mammalian
pancreas. They do not npecify t h c precise location of the cells in the gut,
but hio-assays of the stomach antl adjacent parts of the intestine yielded
5.7 m U of insiilin per gram wet weight of tissue.
The existiiig picturc of digestioii in ascidians then suggests that
food wrapped in " mucus " entcrs the stomach where a change of pH
to the iso-electric: point of the mucus softens it rmd permits penetration
of the enzymes to the food ; it is axairmod that digestion commences in
the stomach and continues throughout, the length of the alimentary
canal alongside tkbsorption of the products of digestion in cells of the
intestine, Then? are two difiiculties in the way of accepting such a
simple hypothesis. Rccent work 011 the endostyle suggests that the
material in which food is wrapped i s not. tt simple mucus, but is primarily
formed of an iodinated protein (sce p. 27). Examination of the food
cord in living transparent animals suggests that the food cord is not
softened in the st.omach but remains firm enough for it to fold back and
forth without disruption. Actual softening of the food cord appears to
take place a.t the pyloric junrt,ion of stomach and intestine. In those
species in which there arc folds in the wall of the intestine the groovex
or furrows lining the inside converge to a ciliated ring a t this point,
and this coincides with thci point at which the duct of the pyloric gland
enters t,he alimentary canal. In Pyurida,e with digestive diverticulae of
the stomach, the ducts from these enzyme-secreting diverticulae also
discharge toward the pyloric end of the stomach. Millar (1953a)
described the ciliated ring and concluded t h a t it wax uued to pull the
food cord through from the stomach to the intestine, but an alterntltivs
viewpoint, is that it is used to mix the food and digestive enEymea at a
point whcrt? tlre food cord is temporarily softened. Jx the function of
the pyloric #Ittiid to bring about this change by the Yecret,ion of appropriate substances?
activity.
Almost nothing is known about the detailed process of absorption
in the ascidiw alimentitry canal r~iid subsequent, metabolism of food.
, 411 authors cigrne on the oxietenoc of' trbsorpt
that the vacuolated cells in tht: stomach are also absorptive and would
take up iron iind olive oil, but later workers (Millar, 1953a ; Thomas,
1970b) believe they release thcir vacuoles from the apex, although the
precise function of these vaciiolated cells is still in doubt (cf. p. 32).
In this context it is of interest that lhvidson et al. (1971) report the
presence of goblet cells in the gut, of Giona i~teslinalis which give staining reactions similar to the insuliii producing " ,%cells " of mammalian
pancreas. They do not npecify t h c precise location of the cells in the gut,
but hio-assays of the stomach antl adjacent parts of the intestine yielded
5.7 m U of insiilin per gram wet weight of tissue.
The existiiig picturc of digestioii in ascidians then suggests that
food wrapped in " mucus " entcrs the stomach where a change of pH
to the iso-electric: point of the mucus softens it rmd permits penetration
of the enzymes to the food ; it is axairmod that digestion commences in
the stomach and continues throughout, the length of the alimentary
canal alongside tkbsorption of the products of digestion in cells of the
intestine, Then? are two difiiculties in the way of accepting such a
simple hypothesis. Rccent work 011 the endostyle suggests that the
material in which food is wrapped i s not. tt simple mucus, but is primarily
formed of an iodinated protein (sce p. 27). Examination of the food
cord in living transparent animals suggests that the food cord is not
softened in the st.omach but remains firm enough for it to fold back and
forth without disruption. Actual softening of the food cord appears to
take place a.t the pyloric junrt,ion of stomach and intestine. In those
species in which there arc folds in the wall of the intestine the groovex
or furrows lining the inside converge to a ciliated ring a t this point,
and this coincides with thci point at which the duct of the pyloric gland
enters t,he alimentary canal. In Pyurida,e with digestive diverticulae of
the stomach, the ducts from these enzyme-secreting diverticulae also
discharge toward the pyloric end of the stomach. Millar (1953a)
described the ciliated ring and concluded t h a t it wax uued to pull the
food cord through from the stomach to the intestine, but an alterntltivs
viewpoint, is that it is used to mix the food and digestive enEymea at a
point whcrt? tlre food cord is temporarily softened. Jx the function of
the pyloric #Ittiid to bring about this change by the Yecret,ion of appropriate substances?