We have already referred to the fact that the shells present a number of perforations on the exterior, in addition to those which pierce the partitions within, and it is this characteristic which has led to the application of the name Foraminifera (hole-bearing) to the little beings we are considering. [Illustration: FIG. 55.--A SPIRAL FORAMINIFER SHELL] [Illustration: FIG. 56.--A FORAMINIFER OUT OF ITS SHELL] The animal inhabiting the shell is exceedingly simple in structure, even more so than the amœba. It is merely a speck of protoplasm, exhibiting hardly any differentiation--nothing, in fact, save a contractile cavity (the _vacuole_), and numerous granules that probably represent the indigestible fragments of its food. The protoplasm fills the shell, and also forms a complete gelatinous covering on the outside, when the animal is alive; and the vacuole and granules circulate somewhat freely within the semi-solid mass. Further, the protoplasm itself is highly contractile, as may be proved by witnessing the rapidity with which the animal can change its form. When the foraminifer is alive, it floats freely in the sea, with a comparatively long and slender thread of its substance protruded through each hole in the shell. These threads correspond exactly in function with the blunt pseudopodia of the amœba. Should they come in contact with a particle of suitable food-material, they immediately surround it, and rapidly retracting, draw the particle to the surface of the body. The threads then completely envelop the food, coalescing as soon as they touch, thus bringing it within the animal. [Illustration: FIG. 57.--THE SAME FORAMINIFER (FIG. 56) AS SEEN WHEN ALIVE] [Illustration: FIG. 58.--SECTION OF THE SHELL OF A COMPOUND FORAMINIFER] The foraminifer multiplies by fission, or by a process of budding. In some species the division of the protoplasm is complete, as in the case of amœbæ, so that each animal has its own shell which encloses a single chamber, but in most cases the ‘bud’ remains attached to a parent cell, and develops a shell that is also fixed to the shell of its progenitor. The younger animal thus produced from the bud gives rise to another, which develops in the same manner; and this process continues, the new bud being always produced on the newest end, till, at last, a kind of colony of protozoons is formed, their shells remaining attached to one another, thus producing a compound shell, composed of several chambers, arranged in the form of a line or spiral, and communicating by means of their perforated partitions. It will now be seen that each ‘cell’ of the compound protozoon feeds not only for itself, but for all the members of its colony, since the nourishment imbibed by any one is capable of diffusion into the surrounding chambers, the protoplasm of the whole forming one continuous mass by means of the perforated partitions of the complex skeleton.