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 



a 



FIG. 185. Rhizopods: a, shell of a monothalamons, or single-chambered, Foramini- 

 fer (Laiiena striata) ; 6, shell of a polythalamous, or many-chambered, Foramini- 

 fer (Polystomella crispa), with pseudopodia extended; c, shell of a Radiolarian, 

 one of the Polycj>tiues (Podocyrtis Schomburgkii). 



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 



