10 PROTOZOA. 



are at first arranged spirally, but in the later stages of growth in a 

 rectilinear series. Such forms are called dimorphous. 



Conjugation has been observed to take place in a few cases 

 (Arcella, Difflugia, EuglypJia, etc.), but details concerning it are 

 unknown. In the Polythalamous forms, it is possible, as hinted 

 above, that conjugation takes place between the free-swimming 

 zoospores. 



The Foraminifera present four main varieties of shell : (1), the 

 chitinous, e.g., Gromia, imperforate ; (2), the porcellaneous, e.g., 

 Miliola, imperforate, and characterised by their opaque white colour 

 and abundant organic basis; (3), the hyaline, e.g., Gloligerina, 

 perforate, and with but little organic basis ; and (4), the arenaceous, 

 perforate, as in Psammosphcera, but generally imperforate. The 

 last are formed of small foreign particles united by a cementing 

 substance. It is the fact that perforate and imperforate arenaceous 



forms are found within the limits 

 of the same family, which has ren- 

 dered necessary the abandonment 

 of the old division of the order 

 into Perforata and Imperforata. 

 Specimens of Biloculina ringen* 

 living in the red clay at 3000 

 fathoms, a depth at which calca- 

 FIO. 6. Nummuiitic Limestone, with reous organisms are generally absent, 



horizontal section of N. distans (after , , . , 



Zitteii). were lound by Brady to have a shell 



composed of silica. 



Analysis of the calcareous shells shows that the mineral con- 

 stituents consist of carbonate of lime and carbonate of magnesia, 

 the latter varying from five to ten per cent. There is besides a 

 trace (generally under 0-5 per cent.) of silica. 



In spite of their small size, the shells of our simple organisms 

 may lay claim to no small consequence, since they not only 

 accumulate in enormous quantity in the sea sand (M. Schultze 

 calculated their number for an ounce of sea sand from Molo di 

 Gaeta at about one and a half millions), but are also found as 

 fossils in different formations (the cretaceous and tertiary), and 

 have yielded an essential material to the construction of rocks. 

 Silicious nodules of Polythalamia are even found in Silurian deposits. 

 The most remarkable, on account of their considerable size, are the 

 Nummulites (Fig. 6) in the thick formation of the so-called Num- 

 mulite limestone (Pyrenees and elsewhere). A coarse chalk of the 



