PROTOZOA. 
241 
A Foraminifer differs from an Amoeba in having an 
apparently simpler body, the protoplasm being without 
layers or cavity; its pseudopodia are long and thread-like, 
and may unite where they touch each other. It has the 
property of secreting an envelope, usually of carbonate of 
Fig. 185.— Rliizopods: a, shell of a moiiothalamous, or single-chambered, Foramini¬ 
fer (Lagena striata); b, shell of a polythalamous, or many-chambered, Foramini¬ 
fer (Polystomella crispa ), with pseudopodia extended; c, shell of a Radiolarian, 
one of the Polycystines (Podocyrtis SchomburgJcii ). 
lime. The shell thus formed is sometimes of extraordi¬ 
nary complexity and singular beauty. It is generally per¬ 
forated by innumerable minute orifices {foramina) through 
which the animal protrudes its myriad of glairy, thread¬ 
like arms. The majority are compound, resembling cham¬ 
bered cells, formed by a process of budding, the new 
cells being added so as to make a straight series, a spiral, 
or a flat coil. As a rule, the many-chambered species 
have calcareous, perforated shells; and the one-chambered 
have an imperforated membranous, porcelaneous, or are¬ 
naceous envelope. 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. The bottom of the ocean, up to about 50° 
on each side of the Equator, and at depths not greater than 
2400 fathoms, is covered with the skeletons of these ani- 
16 
