Each bryozoon consists externally of a single horny or calcareous cell, sometimes furnished with a valve-like lid that folds down when the animal withdraws itself. When danger is past, and the creature begins again to emerge, the upper parts, which were drawn in like the inverted finger of a glove, are pushed out until a series of tentacles, covered with minute hair-like bodies, called cilia, are expanded. The vibratile motion of these cilia causes a constant current in the direction of the mouth, which lies in the centre of the hollow whence the tentacles spring; animalcules are in this way brought in rapid succession within reach of the mouth, and form a never-failing source of nourishment. The interior is greatly more complex than that of the _polypi_. The stomach is connected above with a cavity like the gizzard of a bird, furnished with pointed sides, which serve to triturate the food before it passes into the stomach. There is also a distinct intestine. The muscular action for the expansion and retraction of the animal is highly developed, and the generative system is a greatly more complex one than that of the polyps already referred to. In short, however closely they might be thought to resemble the corals in outward form, their internal structure undoubtedly links them with a much higher type of organization, and justifies the naturalist in subjoining them as a sub-order to the mollusca. The cells are grouped at short intervals along a horny or calcareous substance, that sometimes encrusts sea-weed, or spreads out as a flat leaf-like membrane, or rises into cup-shaped or dendritic forms. A series of cells constituting a separate and independent colony, is termed a polypidom. The cells are further connected together by an external jelly-like integument, in which they are sunk, and which serves to secrete the calcareous particles from the sea. It is interesting to know that creatures so minute and yet so complexly organized, existed abundantly in the seas of the Carboniferous period. No less than fifty-four species are enumerated as having been obtained from the carboniferous strata of the British Islands, and scarcely a year passes without one or two new species being added to the list. The most frequent belong to the genus _Fenestella_, or little window, a name indicative of the reticulated grouping of the branches like the wooden framework of a window. Each of these branches, or interstices, as they are called, was more or less straight, being connected with that on either side by a row of transverse bars, just as the central mullion of an abbey window is connected with the flanking ones by means of cross-bars of stone. Not unfrequently some of the branches subdivide into two, as we saw to be the case among the cup-corals. [Illustration: Fig. 18.--_a_, Fenestella oculata (M'Coy), nat. size; _b_, magnified portion of the same.]