On Molecular and Microscopic Science, Volume 2 (of 2)

by Mary Somerville

Nearly all the Foraminifera on the British coasts belong to the Vitreous or Perforated order, which consists of three natural families and many genera. Their shells are vitreous, hyaline, and generally colourless, even although the substance of the animal is deeply coloured; in some species both the animal and its shell are of a rich crimson. The glassy transparency of the shells would be perfect were they not perforated by numerous tubes running from the interior of the chambers straight through the shell, and ending in pores on its surface. According to microscopic measurement the tubes in the Rotalia, which are the largest, are on an average the ¹⁄₁₀₀₀ of an inch in diameter, and as they are somewhat more than that apart, the transparency of the shell appears between them and gives the surface a vitreous aspect. The pseudopodia of the animal have been seen to pass through every part of the wall of the chambers occupied by it; the apertures of the tubuli in this case are wide enough to permit particles of food to be drawn into the interior of the shell. But threads of sarcode of extreme tenuity alone could pass through the tubuli of the Operculina, which are not more than the ¹⁄₁₀₀₀₀ of an inch in diameter, and the distance between them not much greater, which gives the shell an opaque appearance. Particles of food can hardly be small enough to pass through such tubes into the interior to be digested. Dr. Carpenter, however, is almost certain, from the manner in which the animal repairs injuries done to its shell, that the semi-fluid sarcode extends itself at certain times, if not constantly, over the exterior of the shell, as in the Gromia; and therefore it is by no means impossible that the digestive process may really be performed in this external layer, so that only the products of digestion may have to pass into the portion of the sarcode occupying the body of the shell.

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.

The Globigerinidæ are the most numerous family of the perforated series, and the most remarkable in the history of the existing Foraminifera. They are distinguished by the coarseness of the perforations in their shells, and by the crescentic form of the aperture by which the chambers communicate with each other.

The genus Globigerina consists of a spiral aggregation of globose segments, which are nearly disconnected from each other although united by mutual cohesion. The segments are always somewhat flattened against one another in their planes or junctions, and sometimes the flattening extends over a pretty large surface as in G, [fig. 97]. The entire series of segments shows itself on the upper side, but on the lower side only the segments forming the latest convolution are prominent; they are usually four in number, and are arranged symmetrically round a deep depression or vestibule; the bottom of which is formed by the segments of the earlier convolutions. In this vestibule each segment opens by a large crescent-shaped orifice, the several chambers having no direct communication with each other. The entire shell of the ordinary type may attain the diameter of about ¹⁄₃₀ of an inch, but it is usually much smaller; the typical form, however, is subject to very considerable modifications. In newly formed segments of Globigerina, the hyaline shell substance is perforated by tubuli varying from ¹⁄₁₀₀₀₀ to ¹⁄₅₀₀₀ of an inch in diameter, arranged at pretty regular distances; but in deep seas the surface of the shell is raised by an external deposit into tubercles or ridges, the orifices of the pores appearing between them.

Fig. 100, p. 41.