PKOTOZOA — FOEAMINIFERA. 



17 



are preserved in the rocks. This shell encloses the main ^^H^^y 

 mass of protoplasm, but the pseuclopodia stretch out through 

 certain openings. Either they are confined to the aperture 

 of the shell, or the whole shell-wall is pierced by minute 

 tubes through which the protoplasm can pass (Fig. 1). Shells 

 of the first kind are called Imperforate, and may be chitinous, 

 arenaceous, or calcareous. Shells of the second kind are 

 called Perforate, and are generally calcareous. The Imper- 

 forate calcareous shells are chalky-white and rather like 

 porcelain in appearance, whence they are called Porcellanous. 

 The Perforate calcareous shells are of more glassy appearance, 

 and are therefore called Hyaline. Both types of calcareous 

 shell are made of calcite, but in porcellanous shells this is 

 less pure. Attempts to classify Poraminifera according to 

 the composition of the shell have not proved satisfactory, 

 since genera or species which in other respects appear closely 

 allied differ in this respect. 



The form of the shell varies greatly. It may be simply 

 flask-shaped, without any divisions, e.g. Lagena ; or the 

 animal may grow out of the opening of such a single 

 chamber and fashion for itself another chamber adjoining 

 the first, and as it grows it may build chamber after chamber 

 (Fig. 2 a). Shells of the former type have been called 



Fig. 2. — A Foraminifer from the Upper Chalk, Anomalina ammonoides. 

 a, the shell ; 6, flint cast of the chambers of a similar shell, showing the 

 form of the protoplasmic body. Highly magnified. (From Chapman's 

 " Foraminifera," after Eley. By permission of Messrs. Longmans.) 



Monothalamia (single-chambered), those of the latter type, 

 Polythalamia (many-chambered). A hundred years ago, 



