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; Halphysema, 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 
Fic. 50.—A Foraminifer (Polystomella) showing shell and 
pseudopodia.—After Schultze. 
accumulated on the floor of ancient seas; Mummudlztes (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 Algz (yellow 
