The segments of the animal in the exterior whorl have direct communication with the water by means of a shelly marginal cord, _a_, fig. 101, perforated by multitudes of minute tubes, less than the 1/10000 of an inch in diameter, through which threads of sarcode finer than those of a spider’s web can be protruded. These tubuli are so very fine and numerous, that they characterize the Nummuline family. [Illustration: Fig. 102. Interior of the Operculina.] Fig. 102 represents the interior of the Operculina, which is an existing representation of the Nummuline type. Every segment of the animal is enclosed in a shell of its own, but all the segments are connected in the spiral direction by narrow passages in the walls as in the Faujasina. Although each of the interior whorls has its perforated marginal band, the segments can have no direct access to the water; however, they are indirectly brought into contact with it by means of a system of branching shelly canals, radiating from the central chamber, ending in conspicuous pores in the external surface of the shell. During this course the canals send small tubes into the chambers on each side of them; through these the internal segments of the animal can fill the canals with cords of sarcode, and protrude them into the water, whence they are supplied with food. The genus Polystomella is distinguished by the high development of the intermediate skeleton and the canal system that maintains it. The Polystomella crispa (fig. 97, E), a beautiful species common on the British coasts and in other temperate seas, has a lenticular form, the 1/16 to the 1/12 of an inch in diameter. It consists of a small number of convolutions winding round the shorter axis of the lens, increasing rather rapidly in breadth, and each one almost entirely enclosing its predecessor, so that the shell is exactly alike on both sides, and only the last convolution is to be seen. At the extremities of the axis there is a mass of solid shell-substance, perforated by orifices which are the apertures of a set of straight, parallel canals. In the figure only the last convolution is visible, upon which the convex septal bands are very conspicuous, dividing the surface into well marked segments, upon the exterior edge of each of which there are strong transverse crenulations. The only communication which the chambers have with the exterior, is by means of a variable number of minute orifices near the inner margin of the sagittate partition-plane, close to its junction with the preceding convolution; a very high microscopic power is required to see them, as well as the minute tubercles with which the surface of the shell is crowded, more especially on the septal bands and in the rows of depressions between the segmental divisions.