242 
COMPAKATIVE ZOOLOGY. 
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 shells, 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¬ 
mals, which are constantly falling upon it ( globigerina - 
ooze). 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 Foraminifera are among the most important rock¬ 
building animals. The chalk-cliffs of England, the build¬ 
ing-stone of Paris, and the blocks in the Pyramids of 
Egypt are largely composed of extinct Foraminifers. Fo¬ 
raminifera are both marine and fresh-water, chiefly marine. 
A Polycystine differs from a Foraminifer in secreting 
a siliceous, instead of a calcareous, shell, studded with 
spines; and the central part of the body is made up of 
many cells, and surrounded by a strong membrane. They 
are also more minute, but as widely diffused. They enter 
largely into the formation of some strata of the earth’s 
crust, and abound especially in the rocks of Barbadoes and 
at Richmond, Ya. The living forms are mostly marine, 
but some are fresh-water. 
