Certain kinds of shellfish, for example oysters, mussels, cockles, live in the sand or attached to the stones or seaweed on the seashore, generally between high and low watermarks. They obtain their food from the water as it streams over their gills. They require adequate room for growth and development, and constant irrigation by water containing sufficient floating food. When mussel beds or oyster beds become overcrowded, the fish are ill-nourished, their health is impaired and their growth is arrested. It has been shown that, if they are transferred to new beds, their condition rapidly improves and ultimately they increase considerably in size. All edible shellfish need systematic care and attention. Their cultivation by man affords the simplest instance of an attempt at a systematic aquiculture. =Food.= The surface water of the sea abounds in minute forms of vegetable and animal life. This vast floating population of microscopic organisms is called the “plankton.” Just as man and all land animals depend ultimately for their food supply upon grass and other green-leaved plants which, under the influence of sunlight, are able to transform the inorganic constituents of the atmosphere and the soil into organic foodstuffs—albumen, fat, carbohydrates—so the minute unicellular marine plants of the plankton are able, under the influence of sunlight, to convert the inorganic constituents of their environment into fat, albumen and carbohydrate. Upon these minute organisms, therefore, directly or indirectly, all marine life depends. In addition to these minute plants, the plankton contains nearly all forms of marine life at some stage or other of their life history. Fish are only found in it as eggs, or larvae. Crustacea of all kinds are present, and form one of its most important constituents. Crabs and lobsters spend their larval, free-swimming career among the plankton, until they reach the adult stage and settle down to the bottom. Various minute crustacea, known as “Copepoda” (lit., oar-footed) spend the whole of their lives drifting about in the surface water. They occur in incredibly large numbers, and are the most abundant of all forms of marine life. These copepoda form the main source of the food of pelagic fish, such as the herring, mackerel and sprat. The larvae of the edible molluscs, oyster, mussel, cockle, develop in the warm surface water until they settle to the bottom and begin their adult life. There are also many larval forms of marine worms and jellyfish, and many kinds of microscopic, unicellular organisms, some of which are vegetable and others are clearly animal. The chief animal forms belong either to the Infusoria, the Foraminifera or the Radiolaria. The shells of the two latter forms accumulate at the bottom of the sea, producing the deposits known as the Globigerina and Radiolarian oozes. In this way, chalk deposits were formed in primitive times.