THE LAURENTIAN AND HURONIAN PERIODS. 73 



to considerable distances, and can be again retracted into the 

 soft mass of the general body-substance, and they are the 

 agents by which the animal obtains its food. The soft bodies 

 of the Foraminifera are protected by a shell, which is usually 



Fig. 26 Shells of living Foraminifera. a, Orbulina universa, In Its perfect condi- 

 tion, showing the tubular spines which radiate from the surface of the shell ; b, Globi- 

 gerina bulloides, in Its ordinary condition, the thin hollow spines which are attached 

 to the shell when perfect having been broken off; c, Textularia variabilis ; d, Pe.ne.r- 

 oplis planatus; e, Rotalia concamerata; /, Cristellaria subarcuatula. [Fig. a la 

 after Wyville Thomson; the others are after Williamson. All the figures are greatly 

 enlarged.] 



calcareous, but may be composed of sand-grains cemented 

 together; and it may consist of a single chamber (fig. 26, a), 

 or of many chambers arranged in different ways (fig. 26, b-f). 

 Sometimes the shell has but one large opening into it the 

 mouth ; and then it is from this aperture that the animal pro- 

 trudes the delicate net of filaments with which it seeks its 

 food. In other cases the entire shell is perforated with 

 minute pores (fig. 26, e}, through which the soft body-substance 

 gains the exterior, covering the whole shell with a gelatinous 

 film of animal matter, from which filaments can be emitted at 

 any point. When the shell consists of many chambers, all of 



