SYSTEMATIC SURVEY. 



109 



families are abundantly represented on the surface. The shell is 

 usually calcareous, more rarely arenaceous or chitinous. There is 

 sometimes dimorphism. Multiplication occurs by fission, or by the 

 formation of swarm-spores (amoeboid or flagellate). Foraminifera are 

 common as fossils from Silurian rocks onwards, and at the present day 

 are very important in the formation of calcareous ooze ; in this respect 

 Globigerina^ with a chambered shell, is especially important. Species 

 of Gromia are found in. both fresh and salt water ; Haliphysema, a 

 form utilising sponge-spicules to cover itself, was once mistaken for a 

 minute sponge. 



Most kinds of chalk consist mainly of the shells of Foraminifera 



FIG. 50. A Foraminifer (Poly:>toiiieUa} showing shell and 

 pseudopodia. After Schultze. 



accumulated on the floor of ancient seas; Nummulites (Fig. 17) and 

 related fossil forms were as large as shillings or half-crowns. 



More complex are the (7) Radiolaria, which are divided by a chitinoid 

 membrane into an inner central capsule (with one or more nuclei), and 

 an outer portion, gelatinous and vacuolated, giving off radiating thread- 

 like pseudopodia, which very rarely interlace. There is usually a 

 skeleton in the form of a siliceous lattice-work or regularly disposed 

 spicules outside the central capsule, but in some cases the shell is 

 formed of a horn-like substance called acanthin, which is probably a 

 complex silicate. Radiolarians multiply by fission, which sometimes 

 includes a halving of the skeleton, and by spores, which in some, cases 

 are dimorphic. Most Radiolarians include unicellular Algce (yellow 



