IV. Structure and function of the organs of feeding and digestion in the septibranchs, Cuspidaria and Poromya

Abstract

Previous work on the structure and function of the alimentary system in the Lamellibranchs (Yonge (1923, 1925, 1926a, 1926b)) showed that the many peculiarities which they exhibit appear to be correlated with the highly developed ciliary feeding mechanisms on the gills and palps, as a result of the action of which only the smallest particles are passed into the œsophagus and stomach. This latter organ is concerned chiefly with sorting the particles, the larger ones being passed directly into the mid-gut and the smaller ones entering the ducts of the digestive diverticula (“liver” or “hepatopancreas”), where they are digested intracellularly. The food is largely of a vegetable nature and the digestive processes are concerned especially with the disposal of carbohydrates. There are present, free in the lumen of the gut, in the epithelium and in the surrounding tissues, great numbers of phagocytes which actively ingest food particles. Their presence, also, appears to be correlated with the finely divided nature of the food and the fact that, but for the digestive action of these phagocytes, particles of food, unless sufficiently fine to enter the ducts of the digestive diverticula, can only be digested if composed of starch or glycogen. The only extracellular digestive enzymes in the gut of the Lamellibranchs, namely, those set free by the dissolution in the stomach of the head of the crystalline style, act exclusively on these two carbohydrates. Owing to their deep water habitat, the Septibranchs have been little studied, but Pelseneer (1891,1911) and Plate (1897) have reported, on the evidence of the stomach contents, that they are carnivorous, while all investigators who have worked upon them have shown that in structure both the food collecting and digestive organs of the Septibranchs are quite distinct from those of the other Lamellibranchs. Gills are absent, their place being taken by the muscular septum, the labial palps are very small and the gut is provided with a muscular coating of a thickness unknown in the other Lamellibranchs, where the finely divided food is carried through the gut exclusively by ciliary activity, and so muscle for peristalsis is unnecessary.