112 



PROTOZOA. 



An advance upon the chitinous shell is that presented by the 

 so-called "arenaceous" Foraminifera (fig. 19), which are among the 

 largest of the living types, the test being sometimes half an inch or 

 more in length. In some cases the " arenaceous " test is nothing 

 more than a chitinous envelope, protected by a layer of mud, or 

 having sand-grains more or less largely embedded in its substance 

 (H. B. Brady). Typically, however, the test of the arenaceous 

 Foraminifera consists of sand-grains or other foreign particles united 



Fig. 19.— Shells of Arenaceous Foraminifera. a, Test of Astrorhiza, greatly enlarged ; B, 

 Test of Trockaminina ringens, enlarged thirty times ; c, Test of Trochammiua lituiformis, 

 enlarged eighteen times. (After W. B. Carpenter and Brady.) 



together by a variable amount of cementing material, this latter con- 

 taining from two to three per cent of carbonate of lime together 

 with a notable proportion of peroxide of iron. The arenaceous 

 Foi-aminifera have, as a rule, no pseudopodial apertures or "fora- 

 mina " in the walls of the test, the pseudopodia being emitted from 

 a single general or " oral " aperture, or from a number of apertures 

 in the wall of the last chamber. Though normally " imperforate," 

 there are, however, certain forms of the arenaceous or sub-arenaceous 

 Foraminifera (such as Va/zw/ina, Nodosinelia, and Endothyra) in 

 which the walls are pierced by pseudopodial foramina. Indeed, 

 some degree of porosity appears to be commonly present, and in 

 some forms there is no " oral aperture " to the test at all. 



The great majority of the Fora?ninifera possess a shell composed 

 essentially of carbonate of lime, with or without variable quantities 

 of other constituents. Two chief varieties of these calcareous tests 

 are known, termed respectively the " porcellanous " and the " hya- 

 line " or " vitreous " types. In the so-called " porcellanous " types, 

 the test is quite homogeneous in its composition, opaque-white 

 when seen by reflected light, and destitute of pores or foramina 



