12 



PROTOZOA. 



compared with the internal capsule of the Radiolaria. The shell 

 also often possesses long, delicate, spiny processes, which are broken 

 off unless the animal be very carefully handled (Fig. 10). 



The Foraminifera are mainly bottom organisms, but a few genera 

 of the hyaline forms are pelagic. The pelagic genera are important 

 because of their extraordinary abundance. Whether these pelagic 

 forms have the power of supporting life on the surface of the bottom- 

 ooze is unknown. 



The definition of species and genera in this order is rendered 

 difficult by the large number of intermediate varieties, which, 

 indeed, often constitute a complete series. Carpenter* on this 

 subject writes as follows: "The ordinary notion of species as 

 assemblages of individuals marked out from each other by definite 



FIG. 7. EuglypJia glnliosn- (after 

 Hertwig and Lesser). 



FIG. 8. Difflvgia oblonga 

 (after Stein). 



characters that have been genetically transmitted from original 

 prototypes similarly distinguished, is quite inapplicable to this 

 group ; since, even if the limits of such assemblages were extended so 

 as to include what would elsewhere be accounted genera, they would 

 still be found so intimately connected by gradational links, that 

 definite lines of demarcation could not be drawn between them." 



Fain. 1. Arcellina. Shell watch-glass shape, membranous ; with more than 

 one nucleus ; with contractile vacuoles ; gas vacuoles with a hydrostatic 

 function may be secreted by the protoplasm. Fresh water. Cocliliopod ium 

 Hertw. and Lesser, Pyxidicula Ehrbg., Arcclla Ehrbg., Hyalosphenia Stein, 

 Quadrula F. E. Schulze, Difflurjia Leclerc, shell incrusted by foreign particles, 

 usually pyriform in shape (fig. 8). 



* W. R. Carpenter, " Introduction to the Study of the Foraminifera," preface, 

 p. x. 



