242 



COMPARATIVE 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, Va. The living forms are mostly marine, 

 but some are fresh-water. 



