Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/389

 FORAMINIFERA 375 Thus, then, the Foraminifera are naturally divisible in the first instance into two groups, the Imperforata and the Ferforata ; the former of which is again divisible into the Arenacea, which build up &quot; tests &quot; by the aggregation of sand-grains, and the Porcellanea, which form porcellanous shells ; whilst the latter, Vitrea, includes all the perforate types. There is often a very curious parallelism between the forms contained in these three series respectively, still more between certain &quot;porcellanous&quot; and certain &quot;vitreous&quot; types ; but this arises in each case from similarity in plaii of growth, and does not indicate any real affinity. In fact the two calcareous-shelled series may be compared to two trees of different orders, which resemble each other in their mode of branching, but have dissimilar leaves and flowers. I. ARENACEA. The calcareous shells of certain types of Foraminifera, alike in the &quot; porcellanous &quot; and in the &quot; vitreous &quot; series, are not unfrequently covered with sand- grains, cemented into the surface-layer of the true shell- substance. But in the group now to be described, the casing of the body is entirely composed of foreign particles (usually grains of sand, but sometimes minute Foraminifera, sponge-spicules, &c.), the animal furnishing nothing save the cement that holds them together ; and in its lowest forms no special cement seems to be supplied, the sand- grains being simply imbedded in the sarcodic substance of the body. The increased attention which has of late been given to this group (our knowledge of which has been largely augmented by deep-sea dredging) seems to justify our regarding the &quot; arenaceous &quot; type as on the whole less advanced than either the &quot;porcellanous&quot; or the &quot;vitreous,&quot;- its lowest forms presenting a condition less specialized than even the simplest of the calcareous-shelled Foraminifera, whilst its highest, although sometimes attaining a compara tively gigantic size, build up their massive &quot; tests &quot; upon a very simple plan, exhibiting no approach to the complexity of structure to be found in the &quot;shells&quot; which some of them closely resemble in external form. A few of the more interesting types of the Arenaceous group will now be briefly noticed. In the sandy mud which covers certain parts of the bottom of the 500-fathoms channel between the north of Scotland and the Faroes, the large inoiiothalanious Astrorhizae are sp abundant as sometimes to form a great part of the contents of the dredge. Their typical shape (as the name implies) is discoidal with stellate radiations ; but they are sometimes globular, sometimes cervicorn, and present every intermediate gradation between these forms. The &quot;test&quot; is composed of loosely aggregated sand-grains, not held together by any mineral cement ; and it is destitute of any definite orifice, so that the sarcode-body (of a dark green colour) which occupies the cavity must put forth its pseudopodial extensions between the sand-grains of its test. But there is, in the writer s belief, a lower form even than this. For in the same dredgings there were found a number of little globular masses varying in size from a pin s head to a large pea, formed of aggregations of sand grains, foramiuiferal shells, &c., held together by a tenacious protoplasmic substance, without any definite structural arrangement. And although these might be supposed to be mere accidental agglomerations, yet there seems adequate reason for regarding them as living organisms of the simplest pos sible &quot; inouerozoic &quot; type. For just as a simple &quot;moner,&quot; by a differentiation of its homogeneous protoplasm, becomes an Amceba, so would one of these uridifferentiated mixtures of sand and pro toplasm, by the separation of its two components the sand being limited to the superficial layer, so as to form an investing &quot;test,&quot; of which the whole interior is occupied by the sarcode-body alone, become an Astrorhiza. The next degree of elevation is shown in the presence of a distinct aperture or mouth, the sand-grains surrounding which are generally held together firmly by a ferruginous cement, even when those of the remainder of the test are as loosely aggregated as in Astrorhiza. One of the most common forms of this type is a simple cylindrical tube, closed at one end, but having a conical neck with a circular orifice at the other. And in a still higher type, this cylinder is divided into a succession of chambers, each of which opens into the next by a conical orifice bordered by cemented sand-grains, while the last opens externally by a like orifice,- thus sketching out the &quot; uodosarme &quot; type of the &quot; vitreous &quot; Foraminifera, but on a much Larger scale. In the more advanced Arenaceous types, on the other hand, the sand-grains of the entire &quot;test&quot; are firmly united together by a cement composed of phosphate of iron, which must be exuded from the sarcodic body of the animal, its materials being originally de rived from the sea water. In some instances the sand-grains selected are of such minute size that, when worked up with the cement, they form a sort of &quot; plaster,&quot; the surface of which is quite smooth, both externally and internally. This is the case with the genus Trocham- mina, under which is ranged a whole series of forms ranging from a simple undivided tube to a perfect helical spire resembling a Catherine-wheel (whence the name given to the genus), and thence, by the alteration of its spire to the turbinoid, and by the subdivision of its cavity into chambers, to the &quot;rotaline&quot; type. In other instances the sand-grains are somewhat larger, but are apposed with such extraordinary regularity, and cemented together so artificially, as to form a most delicate but firm test of very uniform thickness, perfectly smooth both externally and internally. Tests of this kind present a singular series of adumbrations of the &quot;orbuline,&quot; &quot;globigcrinc,&quot; and &quot;nodosarine &quot; types of the vitreous series. In other cases, again, the sand-grains being larger, the &quot;test&quot; is con structed more coarsely, but still with remarkable symmetry. One of the most interesting of the simple coarse-grained forms is the little Saccammina sphcrica, whose flask-shaped tests (fig. 5, a, b) of the size FIG 5. Arenaceous Foraminifcru: a, exterior of Saccammina; 6, the samu laid open; &amp;lt;, portion of test more highly magnified; d, J ilulina ; e, portion of test more highly magnified. of minute seeds, with prolonged necks and circular apertures, now found living abundantly in particular localities, have been also dis tinctly recognized in Carboniferous Limestone. That the size of the sand-grains used in the construction of such tests is not accidental (depending on the fineness of division of the sandy bottom on which the animals live), but is the result of selection on the part of the animals that use them, is shown not only by the fact that coarse grained and fine-grained &quot;tests&quot; are brought up from the same bottom, but by the very curious difference in the materials used to form two kinds of tests nearly of the same size and of corresponding simplicity of type. The very same deep dredgings which yielded Saccammina brought up a number of other monothalamous spherical tests, filled with dark green sarcode, to which the name Pilulina has been given from their resemblance to homoeopathic globules (fig. 5, d). These, instead of being constructed by the cementation of sand-grains, are composed of a sort of felt (e) made by the regular &quot;laying&quot; of siliceous fibres (the fine-pointed ends of elongated sponge spicules) with very minute sand-grains dispersed among them; and the aperture, instead of being a round hole at the end of a short neck, is a sigmoid fissure with somewhat projecting lips. The constancy of these differences indicates a dissimilarity in the &quot; potentialities &quot; of the animals of the respective types, of which we find no indication in their apparently-homogeneous sarcodic bodies. The highest development of this type known to exist^at the present time is shown in thelarge polythalamous &quot; nautiloid &quot; forms which have been brought up in considerable abundance from depths mostly ranging between 200 and 500 fathoms. The test (fig. 6, a) is for the most part composed of coarse sand-grains firmly cemented to each other ; but it is generally smoothed over exter nally with a kind of &quot;plaster&quot; resembling that of which the &quot; trochammine &quot; tests are made up. On laying open the spire, it is found to be very regularly divided into chambers by partitions formed of cemented sand-grains (b), a communication between these chambers being maintained by a fissure left at the inner margin of the spire. So far, the plan of structure accords with that of the smaller. &quot; uonionine &quot; forms (resembling 19 of fig. 1), which we