RA'DIOLAIRIA (Neo-Lat. nom. p1., from Lat. radiolus, diminutive of radius, ray). A group of minute marine animals forming an order of rhizopodous Protozoa (q.v.). About 85 families, including many thousand species, are known, most of them microscopic. They live in the surface water of the ocean and their shells, after death, sink to the bottom and form siliceous deposits, known as radiolarian ooze. (See OozE.) They are distinguished from all other Protozoa by their complex and generally very beautiful shells, which are composed of silicon, except in a few cases where either the material is a horny substance called 'acanthin,' or the shell is entire ly wanting. The radiolarians are further distin guished by the presence of a peculiar, mem branous, inner capsule. This is either spherical and perforated by numerous small openings or it is ovoid with a single large opening. Within it is some clear transparent cytoplasm and the nucleus, while outside is a layer of protoplasm which is covered by a gelatinous envelope, known as the calymna. The skeleton or shell consists of one or more fenestrated spheres; when more than one are present they are concentric. They are con nected with each other by radiating rods and spi cules which are usually continued outside the outermost sphere as projecting spines, and may be continued inwardly to meet within the centre of the capsule. The variety of form and arrange
ment is very great and is the cause of the beauty of these animals under the miscroscope. The pseudopodia are usually very flexible and anasto muse freely, but in some cases they are stiff and not inclined to fuse. Contractile vacuoles are not present, but in most radiolarians are very small yellowish spheres, supposed to be parasitic algae; it is possible that these are concerned with the process of excretion. Reproduction takes place either by fission or by spore formation. In the latter ease both macrospores and miscrospores are formed, and it is supposed that one of each must fuse together to give rise to a new individual. Radiolarians play an important part in the economy of the ocean, furnishing food for count less hosts of minute crustaceans and other ani mals, which in turn supply the fishes. All re cent investigations into the biology of the ocean give an important place to this order.