The New International Encyclopædia/Rhizopoda
RHIZOP′ODA (Neo-Lat. nom. pl., from Gk. ῥίζα, rhiza, root + ποῦς, pous, foot). The first and lowest class of Protozoa (q.v.); a large assemblage of varying forms agreeing in the possession of projections of the body-protoplasm called ‘pseudopodia.’ These pseudopodia are used as organs of locomotion and also for obtaining food. They may be of very irregular and constantly changing shape or comparatively rigid, independent of each other, or forming a very complex network. The protoplasm or ‘sarcode’ of a rhizopod consists of an outer layer called ‘ectosarc,’ which is thin, transparent, and homogeneous, and an inner portion called ‘endosarc,’ which is granular and more opaque. Most of the numerous species are spherical with radiating pseudopodia, but the lowest forms have no constant shape. Most rhizopods are provided with some sort of shell, but the lowest forms have no such covering. The simplest shells are those made up of particles of dirt or foreign material of some sort, united together by some secretion of the ectosarc; in other cases the shell is formed of a horny material called ‘acanthin’ or of carbonate of lime or silica, often in a very remarkable and elaborate pattern. Generally rhizopods are freely moving animals, but some are attached in adult life by stalks. The individuals are usually distinct, but colonial rhizopods are known, and such colonies are sometimes half an inch across.
1. A radiolarian, showing the interior; 2, cross-section of the same through the processes.
The only internal organs of the Rhizopoda are the ‘vacuoles,’ those which contain more or less digested food and those which contain the waste matter or excreta of the body. The latter are the larger and more conspicuous, and, owing to the sudden collapse when the excreta are thrown out of the body, are known as ‘contractile’ vacuoles. Reproduction takes place by simple budding or fission, the two processes differing only in the relative size of the resulting individuals. The formation of spores, however, occurs in many cases, especially after a resting period. Such resting periods occur during unfavorable conditions such as prolonged cold or drought, or when an unusual amount of food has been taken. In most cases during such a resting period the rhizopod surrounds itself with a firm, impervious coat, called a ‘cyst.’ When the unfavorable conditions cease the sarcode divides up into several minute spores, each of which on the dissolution of the cyst becomes a new individual. The growth of these individuals is rapid when food is abundant. For further information, see Bütschli's Protozoa in Bronn's Klassen und Ordnungen des Thierreichs (Leipzig, 1887); concerning fresh-water forms, see Leidy Fresh Water Rhizopods of North America (Washington, 1879).