Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/388

 374 FORAMINIFERA In regard to the propagation of Foraminifera, little is as yet certainly known. The growth of the individual may proceed, as will be presently shown, to an almost indefinite extent, by the gemmiparous multiplication of the segments or divisions of its body, with a, corresponding multiplication of the chambers of its shell ; but it more frequently happens that when the organism has attained a FIG. 4. Jlotalia, with pseudopodia extended through the pores of the shell. certain limited size, the new segments detach themselves, each one growing into the likeness of its parent ; and this is probably the ordinary way in which the continuance of the race is provided for. But it would also seem that under certain circumstances the sarcodic body of the parent breaks itself up into segments, each containing an &quot;endoplast&quot; or &quot; nucleus,&quot; and that around each of these a shelly covering is formed while it is still enclosed within the original shell, the offspring finally escaping by its rupture. Whether this mode of propagation is preceded by any process of &quot;conjugation,&quot; and is thus related to sexual generation, is still undetermined. By far the greater number of Foraminifera are composite fabrics, evolved from a simple protoplasmic body by a process of continuous gemmation, each bud remaining in connexion with the stock from which it was put forth; and according to the plan on which this gemination takes place, will be the configuration of the composite fabric thereby produced. Thus if the bud be put forth from the aperture of the monothalamous Lagena (9 in fig. 1) in the direction of the axis of its body, if a second chamber be formed around this bud in continuity with the first (receiving the neck of the latter into its own cavity), and if this process be successionally repeated, it is obvious that a straight rod-like shell (10 in fig. 1) will be produced, having a series of chambers communicating with each other by the apertures that originally constituted their mouths, and opening externally by the mouth or aperture of the last formed chamber. The successive segments may be all of the same size, or nearly so ; in which case the entire series may either resemble a string of beads, or may approach the cylindrical form, according to the shape of the chambers. But it more frequently happens that each segment some what exceeds its predecessor in size, so that the entire shell has a somewhat conical shape. If, on the other hand, the axis of growth should be slightly curved instead of straight, the resulting composite shell will be arcuate, while a more rapid deflection gives it a spiral curvature (5 and 1 1 in fig. 1). The form of the spire will depend in the first place upon whether its convolutions lie in the same plane, like those of a nautilus, so that the shell is equilateral (16 and 19 in fig. 1), or pass obliquely round a vertical axis, as in a snail, so that the shell becomes &quot; inequilateral &quot; (18 in fig. 1), having a more or less conical form, with the primordial chamber at its apex. In other cases, again, the vertical axis is greatly elongated, and the number of chambers forming each re volution around it is reduced to four, three, or even two (13 and 14 in fig. I). But further, the spiral plan of growth may give place to the cyclical, successive circles of new chambers beii.g formed around the interior growth, so as to give the shell a discoidal shape (6 and 17 in fig. 1). And sometimes all regularity of plan disappears in the later stages of growth, new chambers being added in various directions, so that the fabric becomes &quot; acervuline.&quot; In the older classifications of Foraminifera, these differences of plan of growth were adopted as characters of primary importance in the subdivision of the group. But it lias now become obvious that comparatively little value is to be attached to them. For to associate together all rectilineal, all spiral, and all cyclical Foraminifera, is not only to run counter to the indications of natural affinity that are furnished by the intimate structure of the shell, the conformation of the individual chambers, and the mode of their intercommunication, but to set at nought two general facts of fundamental significance, -first, that one plan of growth often graduates insensibly into another, as does the straight into the spiral in the group of which 10 and 11 of fig. 1 are extreme forms; and, second, that it is not at all uncommon for the plan to change during the growth of one and the same individual, the spire, for example, either straightening itself out, so as to revert to the rectilinear type, or returning into itself so as to make a complete circle, round which a succession of concentric annuli is then produced on the cyclical type, while the original regularity is sometimes lost altogether in the &quot; acervuline &quot; piling up of the later-formed chambers. On the other hand, the fundamental importance of the^/ora- tion or imperforateness of the external envelope, as affecting the physiological condition of the contained animal, is now universally admitted. For where (as in Peneroplis, 5 in fig. 1) that envelope, whether composed of calcareous shell or built up by the cementation of sand-grains, has no other communication with the exterior than by the single or multiple aperture of the last chamber through which the whole pseudopodial apparatus of the contained animal is put forth, the nutrition of the entire seginental body that occupies the previously formed chambers must be carried on by a continual interchange of protoplasmic substance extending through the entire mass, however great may be the multiplication of its segments. Where, on the other hand (as in Rotalia, fig. 4), the wall of each chamber is perforated for the passage of pseudopodia, the segment it contains is thereby placed in direct communication with the surrounding medium from which it derives its susten ance, so as to be independent of the remainder of the series. The &quot; imperforate &quot; calcareous shells of Foraminifera are also termed &quot; porcellauous,&quot; from the opaque-white aspect they present when viewed by reflected light; though, when thin, they show a rich brown hue by transmitted light. Even under % high magnifying power, their substance appears entirely homogeneous. The perforated shells, on the other hand, having an almost glassy transparence (except where this is interrupted by tabulation), are known as &quot; vitreous &quot; or &quot; hyaline.&quot; Where the shells are thin, the perforations are simply pores (fig. 4) ; but where they are thick, the perforations are tubules usually running straight and parallel from surface to surface (fig. 24)t