795 



CHAPTER XIV 



FOEAMINIFEEA AND EADIOLABIA 



RETURNING now to the lowest or rhizopod type of animal life 

 (Chapter XII), we have to direct our attention to two very remarka bit- 

 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 undefined 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, however, is sometimes replaced by a 

 test,' formed of minute grains of sand cemented together ; and 

 there are a few cases in which the animal has 110 other protection 

 than a membranous envelope. In the second group, the fiadiolaria, 

 the skeleton is always silicious and may either be composed of dis- 

 connected spicules, or may consist of a symmetrical open framework, 

 or may have the form of a shell perforated by numerous apertures, 

 which more or less completely incloses the body. The Forctminifera 

 probably take, and always have taken, the largest share of any animal 

 group in the maintenance of the solid calcareous 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 

 Radwlaria do the same, though in far less measure, for the silex. 

 And both extract from sea-water the organic matter uiiiversallv dif- 

 fused through it, converting it into a form that serves for the nutri- 

 tion of higher marine animals. 



Sl'( TION I. FORAMINIFERA. 1 



The animals of this group belong to that r<;tienl:ii-ii< n form of 

 the rhizopod type in which with a differentiation between the 

 containing and the contained protoplasm which is involved 

 in the formation of a definite investment a distinct nucleus 

 (sometimes single, in other cases multiple) is probably alwavs 



Fora t 



1 For the earlier literature yonsult Jlr. C. D. Sherbom's 'Bibliography of the 

 rauiinifern, recent and fossil^ from 1565 to 1888,' London, 1888. 



