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

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Abstract
Living and preserved specimens of C. rostrata, C. cuspidata, C. obesa, and P. granulata were studied. The midgut and rectum are essentially the same as in other lamelli-branchs. A description is given of the septibranch muscular septum which replaces and practically fulfills the same function as the usual lamellibranch gill, but in addition to maintaining a circulation of water through the mantle cavity, it also functions in the drawing in of large food-masses by means of currents created by its sudden movements. The highly developed ciliary mechanism on the ridged palps of other lamellibranchs has been largely lost, due to the fact that septibranchs have converted the feeding mechanism and alimentary system, specialized in other lamellibranchs for dealing with small particles, into an apparatus for the disposal of large food masses. The foregut of the septibranchs is totally unlike that of lamellibranchs which feed on small particles. The oesophageal cilia are of little importance, food being carried along by the peristaltic movements of the thick surrounding muscle. The stomach acts as a crushing gizzard, for which purpose its ridged walls, cuticular lining, surrounding muscles, and freedom from the surrounding tissues fit it admirably. The crystalline style is small and probably vestigial. The ducts leading into the tubules of the digestive diverticula are exceptionally short and wide and have very wide lumina, probably due to the comparatively large fragments forced into them by the squeezing action of the stomach. There is a total absence of wandering phagocytes, which are so conspicuous in the gut tract of the ciliary feeding lamellibranchs. This difference is attributed to the absence in the food of particles small enough for them to ingest, and also to the ease with which particles can enter the digestive diverticula. These latter secrete no enzymes which are capable of digesting such animal food as coagulated masses of blood corpuscles, the evidence indicating that septibranchs have been incapable of modifying their digestive processes during the evolution of their special digestive system. Preliminary protein digestion is completely lacking, and it therefore seems probable (although experimental evidence is lacking on the point) that they must have developed a more powerful intracellular protease in the digestive diverticula than is found in those lamellibranchs where the digestion is primarily concerned with carbohydrates. There is no evidence of any storage of glycogen in C. obesa. The septibranchs have probably evolved in the deep water in which most of them still live. The balance of evidence is in favor of the view that the septum is of branchial origin.

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