In such many-chambered shells as are pierced by tubuli wide enough to permit particles of food to be drawn into the interior, each segment of the animal, being fed within its own chamber, has a life of its own, at the same time that it shares with all the others in a common life maintained by food taken in through the mouth of the shell. There are many instances of this individual life combined with a common life among the lowest tribes of animals.
Although the Perforated order contains types widely apart, they are always connected by intermediate forms; but there is no such connection between the two great natural orders, which are not only separated by the tubuli in the shell, but in many instances by the structure of the interior and the corresponding character of the animal.
In the Lagenidæ, which form the first family of the Perforated order, the vitreous shell possesses great hardness, and is pierced by numerous small tubuli. It is very thin, and of glossy transparency. The first four shells in fig. 97 represent some of its forms.
The genus Nodosaria has a very extensive range of forms, from the elongated structure to the nautiloid spiral, depending upon the relative proportions and arrangement of the segments. The segments are separated by constrictions transverse to the axis of growth, or by bands as in the Nodosaria rugosa, B, fig. 97. It frequently happens that parts of the shell are not perforated; and there are generally longitudinal ribs which sometimes have spines projecting from every part of the interior, as in Nodosaria spinicosta, C, fig. 97.
In the genus Nodosaria, the axis of growth changes from a straight line to that of a spiral, so that the septa or divisions between the segments cross the axis obliquely, and the aperture instead of being exactly central becomes excentric. Between these extremes there is a numerous series of gradations. The Cristallaria is the highest type; the form is a nautiloid spiral, more or less compressed (D, fig. 97), of which each whorl has its chambers extended by winged projections so as to reach the centre, and entirely encloses the preceding whorl. The number of chambers in each whorl is much smaller than in most of the nautiloid spirals, not being more than eight or nine. The divisions are always strongly marked externally by septal bands, varying in character according to the species. The margin of the shell runs into a keel, which is sometimes extended into a knife-edge. Nearly all the Lagena family are found in the North Atlantic and Mediterranean, especially in the Adriatic, which is rich in species. In the Nodosaria the cells which compose the shell have so little connection one with another that they may be easily detached; which gives reason to believe that the separation of the parts may be a means of reproduction and dispersion.
