The geological importance of the Foraminifera, their intrinsic beauty, the prodigious variety of their forms, their incredible multitude, and the peculiarity of their structure, have given these microscopic organisms the highest place in the class of Rhizopods. The body of these animals consists of a perfectly homogeneous sarcode or semi-fluid protoplasm, showing no tendency whatever to any film or surface-layer. It is inclosed in a shell; and the only evidence of vitality that the creature gives, is a protrusion and retraction of slender threads of its sarcode, through the mouth or pores of the shell, or through both according to its structure. Fig. 97 shows some of their forms. By far the greater number of the Foraminifera are compound or many-chambered shells. When young, the shell has but one chamber, generally of a globular form; but as the animal grows, others are successively added by a kind of budding in a definite but different arrangement for each order and genus of the class. When the creature increases in size, a portion of its semi-fluid sarcode projects like a bud from the mouth of its shell. If it be of the one-chambered kind, the bud separates from its parent before the shelly matter which it secretes from its surface consolidates, and a new individual is thus produced. But if the primary shell be of the many-chambered kind, the shelly secretion consolidates over the sarcode projection which thus remains fixed, and the shell has then two chambers, the aperture in the last being the mouth, from which, by a protrusion of sarcode, a third chamber may be added, the new chamber being always placed upon the mouth of its predecessor, a process which may be continued indefinitely, the mouth of the last segment being the mouth of the whole shell. [Illustration: Fig. 97. Various forms of Foraminifera:—A, Oolina claxata; B, Nodosaria rugosa; C, Nodosaria spinicosta; D Cristellaria compressa; E, Polystomella crispa; F, Dendritina elegans; G, Globigerina bulloïdes; H, Textularia Mayeriana; I, Quinqueloculina Bronniana.]