Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/873

Rh EADIOLARIA.] PROTOZOA 849 filled with sea-water accumulated by endosmosis) and the stiff radiat ing pseudopodia are directly correlated with the floating pelagic life of the two organisms. All the Kadiolaria are pelagic, and many exhibit this vacuolation ; only a few of the Reticularia are so, and their struc tural correlation to that habit has only lately been ascertained. The Reticularia are almost exclusively known by their shells, which offer a most interesting field for study on account of the very great complexity of form attained by some of them, notwithstand ing the fact that the animal which produces them is a simple uni cellular Protozoon. Space does riot permit the exposition here of the results obtained by Carpenter in the study of the complex shells of Orbitolites, Operculina, Nummulites, &c. ; it is essential that his work Introduction to the Study of the Foraminifera (Hay Society, 1862) should be consulted, and in reference to the sandy-shelled forms the monograph by Brady, in the Challenger Reports, vol. ix., 1883 ; and it must be sufficient here to point out the general prin ciples of the shell-architecture of the Reticularia. Let us suppose that we have an ever-growing protoplasmic body which tends to produce a calcareous shell on its surface, leaving an aperture for the exit of its pseudopodia. It will grow too large for its shell and accumulate outside the shell. The accumulated external mass may then secrete a second chamber, resting on the first as chamber 1 rests on chamber in Fig. XII. 4. By further growth a new chamber is necessitated, and so is produced a series following one another in a straight line, each chamber communicating with the newer one in front of it by the narrow pseudopodial aperture (a, ft 1, a-, a s ). Now it is possible for these chambers to be very variously arranged instead of simply as in Fig. XII. 4. For instance, each new chamber may completely enclose the last, as in Fig. XII. 3, supposing the protoplasm to spread all over the outside of the old chamber before making a new deposit. Again the chambers need not succeed one another in a straight line, but may be dis posed in a spiral (Fig. XII. 1). And this spiral may be a flat coil, or it may be a helicine spiral with a rising axis ; further it may be close or open. All these forms in various degrees of elaboration are exhibited by Miliolidea and various Ferforata. But the Perforata in virtue of their perforate shell-walls introduce a new complication. The protoplasm issues not only from the mouth of the last-formed chamber, but from the numerous pores in the wall itself. This latter protoplasm exerts its lime-secreting functions ; it gathers itself into coarse branching threads which remain uncalcified, whilst all around a dense deposit of secondary or supplemental shell-substance is thrown down, thus producing a coarsely canalicular structure. The thickness and amount of this secondary shell and the position it may occupy between and around the chambers of primitive shell-substance vary necessarily in dif ferent genera according to the mode in which the primitive cham bers are arranged and connected with one another. Calcarina is a fairly typical instance of an abundant secondary shell-deposit (Fig. XII. 10), arid it is the existence of structure resembling the chambers of Calcarina with their surrounding primary and secondary shell- substances which has rendered it necessary to regard Eozoon (41) as the metamorphosed encrusting shell of a pro-Cambrian Reticularian. The division of the Reticularia into Imperforata and Perforata which is here maintained has no longer the significance which was once attributed to it. It appears, according to the researches of Brady, that it is not possible to draw a sharp line between these sub-classes, since there are sandy forms which it is difficult to separate from imperforate Lituolidea and are nevertheless perforate, in fact are &quot;sandy isomorphs of Lagena, Nodosaria, Globigerina, and llotalia. &quot; It does not appear to the present writer that there can be any insurmountable difficulty in separating the Lituolidea into two groups those which are sandy isomorphs of the porcel- lanous Miliolidea, and those which are sandy isomorphs of the hyaline Perforata. The two groups of Lituolidea thus formed might be placed in their natural association respectively with the Imperforata and the Perforata. The attempt to do this has not been made here, but the classifi cation of Brady has been adopted. In Btitschli s large work on the Protozoa (9) the breaking up of the Lituolidea is carried out to a logical conclusion, and its members dispersed among the Miliolidea on the one hand and the various orders of Perforata on the other hand. The calcareous shell-substance of the Miliolidea being opaque and white has led to their being called &quot; Porcellana,&quot; whilst the transparent calcareous shells of the smaller Perforata has gained for that group the synonym of &quot;Hyalina.&quot; The shells of the calcareous Reticularia and of some of the larger arenaceous forms are found in stratified rocks, from the Palaeozoic strata onwards. The Chalk is in places largely com posed of their shells, and the Eocene Nummulitic limestone is mainly a cemented mass of the shells of Nummulites often as large each as a shilling. The Atlantic ooze is a chalky deposit consisting largely of the shells of Globigerina, &c. CLASS VII. RADIOLARIA, Ilaeckcl, 1862 (63) (Polijcystina, Ehr.). Characters. Gymnomyxa in which the protoplasmic body of the dominant amceba phase has the form of a sphere or cone from the surface of winch radiate filamentous pseudopodia, occasionally anastomosing, and encloses a spherical (homaxonic) or cone-shaped (monaxonic) perforated shell of membranous consistence known as the central cnpsule, and probably homologous with the perforated shell of a Globigerina. The protoplasm within the capsule (intra- capsular protoplasm) is continuous through the pores or apertures of the capsule with the outer protoplasm. Embedded in the former lies the large and specialized nucleus (one or more). Gelatinous substance is frequently formed peripherally by the extracapsular protoplasm, constituting a kind of soft mantle which is penetrated by the pseudopodia. A contractile vacuole is never present. Usually an abundant skeleton, consisting of spicules of silica or of a peculiar substance called acanthin arranged radially ortangen- tially, loose or united into a basket-work, is present. Oil globules, pigment, and crystals are found in greater or less abundance in the protoplasm. In most but not all Radiolaria peculiar nucleated yellow cor puscles are abundantly present, usually regarded as parasitic Algse. Reproduction by fission has been observed, and also in some few species a peculiar formation of swarm-spores (flagelluke) within the central capsule, in which the nucleus takes an important part. All the Radiolaria are marine. The Radiolaria are divided into two sub-classes according to the chemical nature of their spicular skeleton, and into orders according to the nature and the disposi tion of the apertures in the wall of the central capsule. EP ps FIG. XIII. Thalassicolla pelagica, Haeckel ; x 25. CK, central capsule ; EP, extracapsular protoplasm ; al, .alveoli, liquid-holding vacuoles in the protoplasm similar to those of Heliozoa, E elomyxa, Hastigerina, &c.; ps, pseudopodia. The minute unlettered dots are the &quot;yellow cells.&quot; SUB-CLASS I. Silico-Skeleta, Lankestcr. Charade rs. A more or less elaborate basket-work of tangential and radial elements consisting of secreted silica is present ; in rare exceptions no skeleton is developed. ORDER 1. PERIPYL^EA, Hertwig. Characters. Silico-skeletal Radiolaria in which the central cap sule is uniformly perforated all over by fine pore-canals ; its form is that of a sphere (homaxonic), and to this form the siliceous skeleton primarily conforms, though it may become discoid, rhabdoid, or irregular. The nucleus is usually single, but numerous nuclei are present in each central capsule of the Polycyttaria. Fain. 1. SPHJERIDA, Haeck. Spherical Peripylsa with a spheri cal basket-work skeleton, sometimes surrounded by a spongy outer skeleton, sometimes simple, sometimes composed of many concentric spheres (never discoid, flattened, or irregular). The central capsule sometimes encloses a part of the spherical skeleton, and often is penetrated by radiating elements. Genera (selected). Ethmosph&ra, Haeck. ; Xiphosphxra, Haeck. ; Staurosphsera, Haeck. ; Hcliusphxra, Haeck. (Fig. XIV. 14) ; As- tromma, Haeck. ; Haliomma, Haeck. ; Actinomma, Haeck. (Fig. XIV. 17; note the sphere within sphere, the smallest lying in the nucleus, and the whole series of spherical shells connected by radial spines) ; Araclmospli&ra, Haeck. ; Plegmosphsera, Haeck. ; Spongo- sphfera, Haeck. (Fig. XVI. 8). Fam. 2. DISCIDA, Haeck. Discoid Peripylsea ; both skeleton and central capsule flattened. Genera (selected). Phseodiscw, Haeck. ; Hdiodiscus, Haeck, ; Spongodiscus, Haeck. ; Spongurus, Haeck. XIX. - - 107