64: THE MICROSCOPE AKD ITS REVELATIONS. 



CHAPTER XII. 

 FORAMINIFERA AND RADIOLARIA. 



455. RETUKNING now to the lowest or Rhizopod type of Animal life 

 (Chap, x)., we have to direct our attention to two very remarkable series 

 of forms, almost exclusively Marine, under which that type manifests 

 itself; all of them distinguished by skeletons so consolidated by Mineral 

 deposit, as to retain their form and intimate structure long after the 

 Animals to which they belonged have ceased to live, even for those un- 

 defined periods in which they have been imbedded as Fossils in strata of 

 various geological ages. In the first of these groups, the Foraminifera, the 

 skeleton usually consists of a calcareous many-chambered Shell, which 

 closely invests the sarcode-body, and which, in a large proportion of the 

 group, is perforated with numerous minute apertures; this shell, how- 

 ever, is sometimes replaced by a ' test/ formed of minute grains of sand 

 cemented together; and there are a few cases ( 397) in which the Ani- 

 rnal has no other protection than a membranous envelope. In the sec- 

 ond group, the Radiolaria, the skeleton is always siliceous ; and may 

 be either composed of disconnected spicules, or may consist of a symme- 

 trical open framework, or may have the form of a shell perforated by 

 numerous apertures, which more or less completely incloses the body. 

 The Foraminifera probably take, and always have taken, the largest 

 share of any Animal group in the maintenance of the solid carcareous 

 portion of the Earth's crust; by separating from its solution in Ocean- 

 water the Carbonate of Lime continually brought down by rivers from 

 the land. The Radiolaria do the same, though in far less measure, for 

 the Silex. And both extract from Sea-water the organic matter univer- 

 sally diffused through it, converting it into a form that serves for the 

 nutrition of higher Marine animals. 



SECTION I. EOKAMINIFERA. 



456. The animals of this group belong to that Reticularian form of 

 the Rhizopod type ( 397), in which, with a differentiation between the 

 containing and the contained sarcodic substance which is involved in the 

 formation of a definite. investment, a distinct nucleus (sometimes sin- 

 gle, in other cases multiple) is probably always present. 1 The Shells of 



1 The absence of a nucleus was long supposed to be a characteristic of the ani- 

 mal of the Foraminifera ; and its presence in Gromia (first detected by Dr. Wal- 

 lich) was regarded as differentiating that type from the Foraminifera proper. 

 But the researches of Hertwig and Lesser having established its presence in sev- 

 eral true Foraminifera, and the Author's own observations on other forms having 

 confirmed theirs, its general presence may be fairly assumed, until contradicted 

 by more extended observation. 



