138 



FORAMINIFERA AND POLYCYSTINA. 



being empty and deserted, each chamber of the Rotalia, or 

 any other Foraminiferous shell, is occupied by a segment of 

 sarcode, which is to a great degree independent of the rest, 

 and is only connected with those on either side of it by 

 delicate threads of the same substance ; and the extension of 

 the shell is due to the formation of an additional segment of 

 sarcode on the outside of the last-formed chamber. Each 

 segment has usually the power of putting forth its own 

 " pseudopodia " through minute apertures in the shell, and 

 thus of drawing in its own nourishment through these ; but 

 even when (as sometimes happens) these * food-collecting 

 threads are put forth from the last chamber alone, the nutri- 

 ment there obtained is transmitted to the segments within by 

 percolation through the substance of the sarcode, and not 

 through any tubular canal. The accumulation of the shells 

 of Foraniinifera in some parts of the existing sea-bottom is 

 very remarkable; and similar accumulations in past ages 

 have formed no unimportant part of the crust of the earth 

 a large part of the Chalk-formation having had its origin in 

 them, as well as nearly the whole of the Nummulitic limestone 

 by which it was succeeded. 



132. But animals whose essen- 

 tial structure seems to be nearly 

 the same, may form siliceous in- 

 stead of calcareous shells; and 

 thus are produced those beautiful 

 organisms, known tinder the 

 name of Polycystina (fig. 79), 

 which are occasionally found in 

 the existing seas, but whose re- 

 mains are met with under a far 

 greater variety of forms in certain 

 of the newer marine deposits. 

 There is not in these the same 



t -i i J* '. Fig. 79. POIYCYSTIKA. 



tendency to lorrn composite A _, 



i ji TI- v ,. A, Podocyrtis ; B, Rhopalocanium. 



structures by the multiplication 



of segments, as in the Foraminifera ; but the complication of 

 the individual form is often much greater. Yet, however 

 complex the form, the essential composition of these crea- 

 tures seems to retain the same attribute of simplicity, which 

 cannot be conceived capable of further reduction. 



