The treatment of food by prosobranch veligers
Fretter, V. and Montgomery, M.C. (1968) The treatment of food by prosobranch veligers. Journal of the Marine Biological Association of the United Kingdom, 48 (2). pp. 499-520.
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A study of nineteen species of monotocardian veligers belonging to eight different superfamilies reveals the uniformity of structure of the alimentary tract up to the time of metamorphosis. Results of feeding experiments with nineteen species of unicellular algae show that there is also uniformity in the functioning of the gut. The velum gathers any particle, organic or inorganic, which its preoral cilia can manipulate, though not all particles impinging on these cilia are directed into the food groove and some which reach the mouth may, under certain circumstances, be rejected. In high concentrations of digestible food a larva, previously starved, will fill the stomach in a few minutes and then stop feeding until digestion of the meal is under way. Meanwhile some food particles may be directed to the mouth, and the occasional one is eaten as digestion proceeds. In low concentrations of food only the occasional cell can be gathered and feeding is more or less continuous. Food is passed rapidly into the digestive chamber of the stomach which is divisible into three functional units: the digestive diverticula, the vestibule into which they and the oesophagus open, and the area dominated by the gastric shield where the food accumulates. Digestible plant cells are retained in this chamber; the length of time is associated with the accessibility of the cell contents. The cells are subjected to mechanical and enzymic treatment. Except for some with thick cellulose walls (Chlamydomonas, Peridinium) the vigorous rotation of the alga against the gastric shield either weakens the cell wall so that gastric juices can penetrate it (Cricosphaera carterae, Exuviaella baltica, E. pusilla, Brachiomonas submarina, Dunaliella primolecta), or fragments it (Monochrysis lutheri, Isochrysis galbana, Pyramimonas grossii, Halosphaera minor). This mechanical treatment is accompanied by rhythmic pulsations of the digestive diverticula which are filled with the contents of the stomach, so that food is brought into direct contact with the ingesting cells, and then emptied to the stomach again with secretions and waste from the digestive epithelium. The contents of the plant cells, but not cell walls, are ingested. The plant pigments colour the ingesting cells, the intensity of the colour giving an indication of the amount of food ingested. The pigments are excreted later, in some cases differentially. The style sac elaborates waste passed in from the digestive chamber of the stomach and the digestive gland; otherwise it is empty except under abnormal circumstances. Acceleration in emptying the stomach may be the response to a bulky accumulation of waste or another large meal. When the velum collects only inorganic particles of no food value feeding is continuous. The particles are not retained in the stomach but pass rapidly to the intestine for excretion; an exception to this was found in some very young and also unhealthy veligers. The digestive diverticula do not suck in the stomach contents, for the sequence of events in the normal digestive process is inhibited. If the plant cells now enter the stomach they are retained in the vestibule adjacent to the openings of the digestive gland, while the rest of the stomach is emptied of the inorganic matter, and digestion of the cells proceeds. The cause of the initiation of the digestive processes is unknown. Activities of the stomach and the velum are under nervous control and some are closely linked.
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