Abstract
The present work was undertaken to study the morphology of the digestive system and the feeding habit of the Japanese flying fish, Cypselurus agoo. 117 adult specimens collected at many localities and in various seasons, as are shown in the Table I, were used for the study.
The mouth is small and situated terminal. Denticles on both jaws and on the tongue are merely of villiform structure, while those on the inner surface of the yharyngeal bone are rather well developed and pointed like small spear heads (Pl. 1 Fig. 4). The digestive tract pursues in a straight course from the oesopbagus to the anus through the body cavity (Pl. 1 Fig. 1); the stomach is not discernible as such macroscopically and even microscopically; pyloric appendages are entirely wanting. Such intestinal type as this is included in the category A-the most primitive type-of JACOBSSHAGEN1). However, the intestinal mucous membrane is well developed and its absorption area is so much increased by folds and ridges that the intestine may be efficacious several times as much as is considered merely by its length.
The food, as is shown by the Table 2, consists of macro-zoo plankton of tropical and oceanic nature, and it is not rare to find nothing of food remains except mucilage in the intestinal content. The fact may indicate that the food of the fish is quickly digested.
Those particular features, such as the straight course of the digestive tract, the absence of stomach and pyloric appendages and the quick digestion of food, may be counted as an ingenious device, as in the case of the bird, to lighten the body weight of the fish, and they together with the very long pectoral fins are structural adaptation in favour of its aerial flight.
