Problems of nitrogen catabolism in invertebrates

IN the preceding paper of this series [Baldwin and Needham, 1934] some figures were reported for the uric acid content of the nephridium or kidney of the large land-snail, Helix pomatia.It was found that even for a considerable time after the end of the hibernation period, no less than three-quarters of the dry weight of the organ could consist of uric acid.This remarkable retention stimulated a desire to investigate whether other gastropod nephridia showed a like accumulation, and the possibility arose of studying in this way the distribution of uri- cotelic metabolism.We are accustomed to the view that the use of uric acid as the main vehicle of waste nitrogen excretion may in some way or other be re- garded as an adaptation to terrestrial life, and the suggestion has been made [Needham, 1929; 1931; see also Hopkins, 1928] that terrestrial oviparity in particular, where development within the cleidoic egg occurs, is associated with uricotelic metabolism.Hitherto, however, such generalisations have had per- force to rest upon our knowledge of the nitrogen metabolism of vertebrates only.The urea production of fishes, amphibia and mammals has been compared with the uric acid production of reptiles and birds.But the invertebrates may per- haps provide a still more striking opportunity for the correlation of metabolism with habitat.In the insects, where the excretion of uric acid as the main end- product is widespread if not universal, such opportunities, it is true, are not promising, for the Subphylum Insecta is as a whole so typically terrestrial, and the aquatic forms are so obviously secondary, that the boundaries of habitat and metabolism will be the boundaries of the group.In the molluscs, however, we have a morphologically homogeneous population distributed through every conceivable environment; thus the gastropods are to be found in the sea and on its littoral, in brackish-water and fresh water, both rivers and lakes, and on land in surroundings showing every degree of humidity.The lamellibranchs, on the contrary, are mainly marine, with a few fresh-water species.Amphineura, Sca- phopoda, and Cephalopoda are wholly marine.With the probability that terrestrial snails are uricotelic organisms now raised almost a certainty [see Baldwin and Needham, 1934] we are evidently at liberty to enquire how far this form of metabolism extends and to correlate within one group of genera the environment and the metabolism.Strohl [1914, p. 527], in his valuable review of the excretory mechanisms of molluscs, concludes that in general the gastropods are characterised by positive findings of uric acid in their nephridia, but that in the lamellibranchs this substance is always absent, and its place seems to be taken by urea.This statement, it will be seen, is in rough agreement with the expectations which we derive from ( 238)