CC@ELENTERATA. 2338 
A Foraminifer differs from an Ameeba in having an 
apparently simpler body, the protoplasm being without 
layers or cavity; but it has the property of secreting an 
envelope, usually of carbonate of lime. The shell thus 
formed is sometimes of extraordinary complexity and sin- 
cular beauty. It is generally perforated by innumerable 
minute orifices (foramina) through which the animal pro- 
trudes its myriad of glairy, thread-like arms. The major- 
ity are compound, resembling chambered shells, formed 
by a process of budding, each new individual being add- 
ed so as to make a straight series, a spiral, or a flat coil. 
Fic. 181.—Rhizopods: a, a monothalamous, or single-chambered, Foraminifer (La- 
gena striata) ; b, a polythalamous, or many-chambered, Foraminifer (Polystomella 
crispa, with pseudopodia extended); c, a Radiolarian, one of the Polycystines 
(Podocystis Schomburgkit). 
As a rule, the many-chambered species have calcareous, 
perforated shells; and the one-chambered have an imper- 
forated membranous, porcelanous, or arenaceous envel- 
ope. The former are marine. There are few parts of the . 
ocean where these microscopic shells do not occur, and in 
astounding numbers. A single ounce of sand from the 
Antilles was calculated to contain over three millions. 
Their remains constitute a great proportion of the so- 
called sand-banks which block up many harbors. Yet they 
are the descendants of an ancestry still more prolific; for 
the chalk- cliffs of England, the building-stone of Paris, 
