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. 186.— Rliizopods: a, shell of a monothalamous, ov single-chambered, Foramini- 

 fer {Lofjena striata) : b, shell of a polythalamous, or many-chainl)ered, Foramini- 

 fer {Polyetomelta crispa)^ with pseudopodia extended ; c, shell of a Radiolarian, 

 one of the Polycystiues {Podocyrtis Sehomburgkii). 



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 



