FORAMINIFERA. 



I I 



for the emission of pseudopodial filaments. In all the porcellanous 

 For ami nif era, therefore, the shell is " imperforate" and the pseudo- 

 podia are emitted from a general " oral aperture." There is also 

 reason to believe that the shell in these types is largely composed 

 of aragonite. Though normally calcareous, the test may be en- 

 crusted by sand-grains, or it may be composed of siliceous particles 

 embedded in a chitinous envelope, or, as shown by Dr H. B. Brady 

 to sometimes occur in examples of Miiiola dredged from great 

 depths, it may even be genuinely siliceous. On the other hand, 

 in the so-called "hyaline" or "vitreous" types of the Foraminifera, 

 the calcareous test (composed principally of calcite) is glassy and 

 transparent, and its walls are perforated by numerous minute tubes 

 through which the pseudopodia are emitted. In the series of the 

 hyaline Foraminifera, therefore, the test is said to be "perforate" 

 Hence, in vertical sections of the shell of any such type {e.g., Globi- 

 gerina), the test is seen under the microscope to be traversed by 

 closely set tubules, running at right angles to the outer and inner 



Fig. 20. — a, Vertical section of 

 the test of Globigeri?ia bulloides, 

 highly magnified, showing the 

 pseudopodial tubes ; b, Tangen- 

 tial section of the same, showing 

 the tubules in transverse section. 

 (Original.) 



Fig. 21.— Section of the shell of Rotalia Sch?-cctcri- 

 ana, magnified, showing pseudopodial tubes in the 

 walls (d). The test is also traversed by a series of 

 radiating and transverse canals (a, b, c). (After 

 Williamson and Carpenter.) 



surfaces of the shell ; while in tangential sections (i.e., sections 

 parallel with the surface) these tubules are transversely divided, and 

 appear as dark spots or clear pores (see figs. 20 and 21). 



The presence, or absence, of pseudopodial tubes in the walls of 

 the test has been employed as a character whereby the entire group 

 of the Foraminifera might be divided into the two sections of the 

 Perforata and Imperforata ; the former including the hyaline or 



VOL. I. H 



