INTRODUCTION Two principal and ecologically different groups of benthic marine animals wer-first recognized by C. G. Joh. Petersen (1913, p. 15): (1) The Epifauna (Fig. 1), come prising all animals living upon or associated with rocks, stones, shells, piling, and vegetation—i.e., sitting or crawling on a substratum (Examples: Mytilus beds, coral-reef fauna, animals inhabiting Zostera or kelp beds; and (2), The Infauna (Fig. 2), comprising all animals inhabiting the sandy or muddy surface layers of the sea bottom—i.e., living buried or digging in a substratum (all animals associated with the level bottom). A third group, including bottom-dwelling fishes and motile invertebrates, is intimately related to the animals of the benthos. The conditions under which these two types of bottom fauna live are, in many respects, quite different, and the factors governing their distribution, chances of survival, and even evolution are, therefore, not the same. DIFFERENCE IN LIFE CONDITIONS FOR EPIFAUNA AND INFAUNA The epifauna, occupying on an average less than 10 per cent of the total area of the sea bottom, seems to reach its absolute maximum in very shallow water, particularly in the intertidal zone. Here there are a great many micro-environments: reefs, rock pools, holes, and crevices exposed for a longer or shorter time to the air, in sunny or shady places, sheltered from or beaten by the waves, with bare surfaces or overgrown with different types of vegetation, and so on. This multitude of micro-environments with their different conditions for life supports a large number...