

i PHYLUM PROTOZOA 23 



of protoplasm enclosed in a single shell, there is formed a 

 composite structure, made up of a number of particles of 

 protoplasm, each with its nucleus, and each enclosed in a 

 shell, the whole of the shells being firmly united together, 

 and the whole of the particles of protoplasm being in con- 

 tinuity through the apertures of communication. The 

 several parts of such a compound shell, which are known 

 as the chambers^ are variously arranged in different Fora- 

 minifera (Fig. 5), according to the way in which the suc- 

 cessive buds have been given off. In some the buds suc- 

 ceed one another in a straight line, and the compound shell 

 which results (j) has consequently its chambers arranged 

 in a straight row. Or the chambers may be developed 

 alternately on opposite sides of the original shell (5), or with 

 the new chambers entirely overlapping their predecessors (4). 

 In other cases the development of the buds takes a winding 

 course, the resulting shell having its chambers arranged in 

 some form of spiral, like the spiral of a watch-spring or of 

 a corkscrew. Such a spiral shell (6 //) assumes a great 

 variety of forms in different Foraminifera, owing to differ- 

 ences, not only in the shape of the chambers themselves, but 

 also in the nature of the spiral in which they are arranged. 



In many cases the shell is further complicated by the 

 development of what is termed the supplemental shell (Fig. 

 5, 8 b\ a deposit of carbonate of lime outside the original 

 shell, traversed by a complex system of fine canals contain- 

 ing protoplasm, and sometimes produced into a number of 

 relatively large spines. 



Though the great majority of Foraminifera have dense 

 shells composed of carbonate of lime, there are many in 

 which the shell resembles that of Difflugia in being 

 composed of foreign particles, such as sand-grains, cemented 

 together : these are termed the arenaceous Foraminifera ; 



