FOIIA.MIXIFERA 



21 



as tine pores (foramina), are either all equal in diameter, being extremely 

 HUM 11 and closely set together (Fig. 7, A), or an; larger in calibre and more 

 widely separated from one another (Fig. 7, B}. Certain vitreous, perforate 

 1'oiaminifera are still further provided with coarser, anastomosing canals, 

 which are restricted to definite parts of the shell, such as the septa, or the. 

 median plane of the 

 spiral, in forms which 

 are symmetrically con- 

 voluted. In the livin- 

 state, these larger canals 

 are occupied by bands or 

 stolons of sarcode, but 

 there is no connection 

 1 >et ween them and the 

 other foramina or radial 

 tubules. In many forms 

 exhibiting more com- 

 plicated structure, cal- 

 careous deposits are ob- 

 served, which are dis- 

 tributed partly over the 

 periphery, and partly in 

 cavities and depressions 

 within the shell itself. 

 This is the so-called in- 

 termediate or supplemental 

 skeleton, which often 

 gives rise to peculiar 

 excrescences on the peri- 

 phery (Fig. 9), and in 

 some cases is also pierced 

 by tubules. 



Reproduction in the 

 Foraminifera takes place 

 either by means of fission, 

 or by means of small 

 nuclei which originate i,,.,.,,., 



within thp rarfnt inrli &> Individual Discoliths and Cyatholiths ; c, Coccospheres ; d, Globigerina; 

 [ ~ e, Globi,,,;-;,;, with bnrsted test; /, Textitlaria; g, g', Radlolaria; h, i, 

 Victual, invest themselves Diatoms ; k, I, Sponge spicules ; m, Mineral fragment. 



with unilocular or multi- 



locular tests, and forthwith abandon the parent shell, whereupon the latter 



falls to pieces. 



The vast majority of Foraminifera are marine in habit. They occur in 

 shallow water bordering the coasts, sometimes attached to algae, sometimes 

 creeping on the bottom. A few genera are extraordinarily abundant in the 

 open sea, occurring at different depths as free-swimming forms, and also on the 

 floor of the ocean. Enormous quantities of their remains are spread over large 

 tracts of the sea-bottom, and down to a depth of 2300 fathoms they remain an 

 essential constituent of the chalk-like deep-sea ooze. This is a finely divided 

 agglomeration of decomposed calcareous substances, such as the shells of 



FIG. 10. 

 ooze magnified 700 diameters. 



with Coccoliths ; 



