III. Uptake in relation to organic content of the Habitat

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Abstract
Experimental evidence shows that many soft-bodied marine invertebrates accumulate amino acids, glucose, and fatty acids from dilute solutions by direct uptake through the epidermis. Comparison with what is known of the occurrence of these compounds in marine deposits suggests that some bottom-living animals could take up enough to meet all their metabolic needs as calculated from oxygen consumption. The species with significant rates of uptake include some polychaet worms and small unitentaculate pogonophores. Pogonophores seem capable of obtaining useful quantities of amino acids and glucose from concentrations less than 10-5 Moles/l; for the same uptake polychaets require concentrations of the order of 10-5 to 10-4 Moles/l, whereas tapeworms living in the rich environment of the digestive tract of vertebrates function best at levels around 10-3 Moles/l. These concentrations may possibly indicate the levels of dissolved substances in the normal habitat, and illustrate the degree of adaptation of the uptake mechanism. It is not yet possible to draw any definite conclusions about the nutritive value of epidermal uptake in animals which posess a functioning alimentary canal, though in some forms it could be of more than supplementary use. In pogonophores, which have no internal digestive system, it is reasonable to suppose that the small species examined so far can and do exist on the total absorbable organic matter in the habitat.