Digestion in Ascidians and the Influence of Temperature

Abstract

1. The mechanism of feeding and digestion in the Pyurid Ascidians Tethyum pyriforme americanum and Boltenia ovifera is described. 2. The structure and histology of the "liver" is described and it is shown that it is primarily an organ of secretion. 3. It is found that the only digestive enzymes are those poured into the gut by the liver, and consist of a powerful amylase, a protease, a very weak lipase, and also an invertase, a maltase, and a lactase. 4. The brownish pigment of the liver gives reactions with acids somewhat like those of bile pigment. There is no trace of bile salts, however, nor of cholesterol. 5. The amylase has an activity range from pH 6.0 topic pH 8.5 with an optimum near pH 7.5. The protease is active from pH 6.0 to above pH 10.0. A similar protease is secreted by Molgula citrina and Ascidia prunum. 6. The relative strengths of the amylase and protease are compared, the amylase being very much the stronger. 7. While experiments of brief duration indicate an optimum temperature for enzyme activity above 40° C, the more prolonged the experiments the lower does the optimum become. Whatever the optimum may be after an experiment of 2 hours' duration, it falls about 20° C. during the next 45 hours, if the experiments be so prolonged. 8. At 15° C. and at 10° C. the food takes about 35 and 55 hours respectively to pass through the alimentary canal, and at 50 C. somewhere between 70 and 90 hours. These temperatures approximately cover the normal range in temperature of the environment, and therefore of the animal itself. 9. From experiments lasting 33 hours the optimum temperature for enzyme activity was found to be about 17° C.; that is, within one or two degrees of the body temperature. From experiments lasting 57 hours the optimum temperature was found to be about 13° C ; that is, within three degrees of the body temperature. 10. These temperature optima not only represent the relative amounts of substrate converted at different temperatures, but also represent the absolute amounts converted and convertible. 11. The enzymes, amylase and protease, are two-thirds to three-quarters destroyed during their period of activity within the alimentary canal of the animal, and in order to utilise the remainder the digestion mixture would have to be retained within the canal for twice as long a time. 12. Therefore it seems probable that the organism in making such a compromise between a high activity of the enzyme and its economical use is working to a maximum efficiency; and it is possible that a permanent increase in the stability of the digestive enzymes would be turned to advantage through a more prolonged retention of the food within the gut.