1. The mouth is larger and more protrusible in Gobio and Cyprinus than in Rutilus, features which may be associated with the mud-grubbing, bottomfeeding habits of the two former fish, while the arrangement of the adductor muscles in Rutilus enable it to close its mouth more quickly than they and hence it can rapidly snap up its free-moving prey.
2. The selection of food is largely by taste in Gobio and Cyprinus, and hence both are richly supplied with taste-buds from lips to oesophagus, special concentrations being present on barbs around the mouth. Rutilus, on the other hand, augments taste by sight. Taste-buds are accordingly more restricted in distribution and less abundant where they do occur. They are fewer on the lips; there are no barbs.
3. The relative development of the pharyngeal masticatory apparatus (horny pad, pharyngeal teeth) bears a direct relationship to the amount of plant food in the diet, i.e. it increases in the order Gobio, Rutilus, Cyprinus.
4. The mucus-secreting cells attain their maximum development in the pharynx in all three fishes, as is usual amongst teleosts (cf. Al-Hussaini, 1947a, p. 278) and are thus effectively placed to lubricate the food, irrespective of its nature, at the very commencement of its journey through the gut.
5. Gill-rakers carrying taste-buds and mucus-secreting glands are present in all three fish, but they are exceptionally short in Gobio for a bottom-feeding fish. This may be compensated for to some extent by a pair of food-selecting palatal cushions.
6. All three fishes are stomachless, the short oesophagus joining the pharynx directly to the pyloric sphincter. The loss of the food-holding capacity of the stomach is compensated for by the swelling of the first limb of the intestine.
7. The intestinal tube is longest and its looping most complex in Cyprinus, and shortest and with the simplest looping in Gobio. Four parts are recognized in the intestinal tube, not by external features, but by their mucosal foldings and certain histological characters.
8. An estimation of the mucosal area shows that the absorptive area of the intestinal epithelium is practically equal in the three species when related to the weight of the fish. This ratio has here been called the ‘mucosal coefficient’.
9. The intestinal epithelium comprises two principal histological cell types, viz. the absorptive cell and the goblet cell.
10. The internal surface across which food substances pass from the cells to the tissue fluids is greater in Cyprinus and Rutilus than in Gobio, owing to the greater length of the absorptive cells.
11. In order to be valid, estimations of the relative efficiency of the fish intestine should take into account the relative length of the gut (R.L.G.), the mucosal coefficient (Q.M.), and the length of the absorptive cells.