MARINE BIOLOGY is not merely the study of the kinds of living things that are found in the oceans and seas; it is con cerned with the ways in which the environment or salt water of varying temperature, salinity, depth, pressure and other physical conditions affects the life histories and abundance and nature of marine organisms. Many kinds of plants and examples of all the great sub-kingdoms of animals live in the sea, but in studying sys tematic botany and zoology we are concerned mainly with the classification, structure and development of these organisms, and the question of the environment does not interest us greatly. There are, however, groups of plants and animals that are ex clusively or predominantly marine. There are others that live drifting about in sea water; others, again, that live attached to the sea bottom; some that live at abyssal depths in the ocean, and others that live only in very shallow water—and so on ; and the study of these particular conditions has high interest. Then the salts in the sea, their varying concentration and their origins; the varying depths of the seas and oceans; the enormous pressures at great depths ; the darkness even at limited depths ; the great ocean currents; the tides which regularly cover and uncover the littoral zone; the interchange of materials between the sea and the land, or the sea and the atmosphere—all these are conditions that powerfully affect the kinds and the abundance of living things in the oceans and seas. Thus we may regard marine biology as the study of the forms of life that belong to a particular environment --that of the seas and oceans—and of the ways in which these forms of life have become adapted so as best to utilize that en vironment.
point of view, however, marine plant life is represented by the large, rooted Algae and by the planktonic Diatoms and Peridin ians. The marine vertebrate animals are the whales, seals, por poises, etc.—these are true mammals that evolved terrestrially and then assumed marine habitats. There are marine birds, but obvi ously the habitats of these animals are not exclusively sea water, as in the case of the marine mammals. There are no marine am phibians. Marine reptiles are represented by turtles and sea snakes. The fishes are, of course, exclusively aquatic, and the ma jority of the species are marine. The arthropods are the groups of animals commonly represented by the Crustacea, spiders, mites, millipedes and insects. There are a few insects that inhabit the foreshore or the shallow waters adjacent to this zone, but there are no truly marine mites, spiders or millipedes. The Crustacea are exclusively aquatic and they are abundant in the sea, having there much the same role as that of the insects on the land. They are quite ubiquitous and are represented by an immense variety of species. The molluscs are nearly all aquatic animals, only a few species of Gastropods (univalves) inhabiting the land. They are far more abundant in the sea than in fresh waters. The echinoderms (that is the starfishes, sea urchins, feather-stars and sea-cucumbers) are exclusively marine. The coelenterates (the zoophytes, medusae, "jelly-fishes," siphonophores, corals, etc.) are almost entirely marine. The mixed assemblage of worms, polyzoa, rotifers, etc., are mostly aquatic and they are about equally represented in the sea and in fresh water. Sponges are almost entirely confined to the sea. The protozoa are ubiquitous. Perhaps we had better use the term Protista, for this does not make the rather arbitrary distinction between animal and plant organisms. These unicellular forms of life inhabit all possible habitats, but their variety and abundance in the sea is far greater than on the land.