PROTOZOA. FORAMINIFERA 17 



shell has a glassy appearance, and is perforated by in- 

 numerable tubes for the passage of the pseudopodia; in 

 some forms {e.g. Rotalia) these tubes are ^^ of an inch 

 in diameter, but in others (e.g. Operculina) only 16 ^ 00 of 

 an inch. In the porcellanous forms the shell, when 

 viewed by reflected light, is opaque and white, having the 

 appearance of porcelain ; it is not perforated by tubes, but 

 possesses one or two large apertures through which most 

 of the pseudopodia pass out — some, however, are given off 

 from the layer of protoplasm which covers the surface of 

 the shell. In these porcellanous Foraminifera the shell 

 is sometimes pitted, producing at first sight the appearance 

 of perforation. 



In the arenaceous forms the shell consists of foreign 

 particles joined together by a cement. The particles are 

 usually grains of sand (commonly quartz), but sometimes 

 sponge spicules, or the shells of other Foraminifera. 

 The cement may be formed of chitinous, calcareous, or 

 ferruginous material. The shell is often imperforate. 



The chitinous forms (e.g. Gromia) do not occur as 

 fossils. 



The shell of the Foraminifera varies considerably in 

 form and structure ; in some genera it consists of a single 

 chamber, when it is said to be unilocular, as for example 

 in Orbulina, where it is spherical, and in Lagena (fig. 3, 

 F), where it is generally flask-shaped. In other cases it 

 consists of several chambers communicating with one 

 another, either by perforations in the walls (septa) 

 between them, or by larger openings. In these multi- 

 locular forms the shell grows by the addition of a new 

 chamber at the end of the one last formed ; this takes 

 place by the protrusion, through the aperture or mouth 

 of the shell, of a mass of protoplasm, at the surface 



w. p. 2 



