Studies on the Comparative Physiology of Digestion
Authors: Yonge C. M. 
Published: April 2021

The observations recorded in this paper on the feeding, the alimentary organs, and the digestive processes in Mya arenaria, may be epitomised as follows:—

1. The food consists of organic debris, sand particles, and micro-organisms suspended in food currents, which are created by the ciliary action of the gills, and conveyed to the mouth by ciliary currents on the surface of the gills and labial palps.

2. Particles are carried towards the ventral margin of the demibranchs, where they are caught in the currents created by the large cilia present in the region of the marginal food groove, and carried towards the mouth. A third anterior current, also created by large cilia, runs along the gill axis.

3. The direction of the ciliary currents on the labial palps has been described in detail, and the view expressed that the ciliary mechanism present on the inner face of the palps is devoted entirely to the separation of the food into large and small particles, the former being despatched to the tip of the palp, and the latter carried forward to the mouth.

4. Coarser particles which do not reach the gills, and other particles rejected by the gills and palps, are carried away by ciliary currents present on the visceral mass and mantle, and are finally expelled from the mantle chamber.

5. The anatomy and histology of the alimentary canal and the hepatopancreas have been described.

6. The presence of muscle fibres in the wall of the gut is practically restricted to the æsophagus and rectum, but the entire alimentary tract, with the exception of the area of the gastric shield, is ciliated and abundantly provided with mucussecreting glands.

7. The presence of ciliary currents in all parts of the gut has been demonstrated.

8. The ciliary currents present in the stomach have been described, and the definite separation of the food into larger and smaller particles, particularly by the mechanism of the grooved area, has been shown. The larger particles are carried straight into the intestine and the smaller particles to the base of the gastric shield, where they are caught in the substance of the tip of the style.

9. The universal presence of phagocytes or wandering cells throughout the gut has been noted, and their special activity in the grooved area of the stomach described. The balance of evidence has been shown to be in favour of the view that they are nutritive in function, although they may also have an excretory function, as they have been shown to be capable of ingesting matter of absolutely no food value.

10. The histology of the style-sac, the origin of the style, and the ciliary currents present in the sac have been described.

11. The mature style has been described and evidence brought forward to prove that it is an albuminoid mass saturated with an amylolytic enzyme, which is revolved, and, at the same time, pushed forward into the stomach by the action of the various cilia present on the epithelium of the style-sac. The tip bears against the gastric shield and is gradually worn down against this surface and through the action of the hepatopancreatic secretion.

12. The permanence of the style in Mya has been shown to be due to its protection from the corroding effects of the protease present in the hepatopancreatic secretion owing to the possession of a separate style-cæcum.

13. The importance of the style as a factor in the evolution of the higher Lamellibranchs, the presence of an homologous structure in certain of the Gastropods, and the question of its taxonomic importance have been touched upon.

14. The periodicity of digestion, as contrasted with the mechanical regularity of feeding, has been discussed.

15. The absence of digestive enzymes in the intestine, together with the evidence of previous workers, has been advanced in favour of the assertion that digestion takes place in the stomach only, and absorption in the stomach, mid-gut, and rectum.

16. The hepatopancreatic secretion has been shown to possess amylolytic, proteolytic, and lipolytic enzymes, and the action of these has been examined.

17. The style enzyme has been examined and found to reduce starch and glycogen but not sucrose. The optimum medium has been shown to be neutral, the optimum temperature to lie near 32°C, and the temperature of destruction at 51°C. Experiments in which the concentration of the enzyme and substrate were varied gave results which prove that the style enzyme has the typical properties of such a substance.

18. The presence of reserve supplies of glycogen and fat has been shown.