PEOTOZOA. 



81 



Fig. 25. Transverse section of a Foraminifer 



(Alveolina Quoii); the arrangement of the separate 



chambers in relation to one another can be seen 



(after W. Carpenter) . 



organisms ; tliey vary greatly in complexity, and this complexity is 

 sometimes in inverse proportion to tliat of tlie body. Simple sliells, 

 generally oval in form, and 

 provided with an orifice, 

 obtain in one division of 

 the Amoeba3 (Difflugia, 

 Arcella). The shell is 

 sometimes soft ; but some- 

 times it is more firm, and 

 this firmness is increased 

 by the addition of foreign 

 bodies. They sometimes 

 seem to be internal shells, 

 owing to the extension of 

 the protoplasm over them. 

 The shells of the Foramini- 

 fera are more complicated 

 in form, owing to the for- 

 mation of new parts around 

 a simple rounded test, which 

 then form separate cham- 

 bers communicating with 

 one another by orifices, and 

 with the exterior by pores 



(Figs. 23, 25). These many-chambered shells become very firm by 

 the addition of chalk, or, though more rarely, of silica (Polymorphina 

 Nonionina) ; owing to the variations in the relative position, size, 

 and mode of connection of the chambers, these structures vie in 

 wealth of form with the more lightly constructed internal supports 

 of the Radiolaria. 



When the chambers are ranged along a straight line, rod-shaped 

 shells, often swollen into beads, are formed ; the separate portions 

 of which, known as " chambers,^' may be all of the same size, or may 

 increase in size from one end to the other (Nodosarid^e). A spiral 

 arrangement of the chambers which lie in the same, or in different 

 planes, gives rise to structures like those of the shells of the Nautilus 

 (Fig. 23). Special modifications are due to the superposition of the 

 spiral coils, the elongation or abbreviation of the spiral axis, and so 

 on. The planorbis-like shells of the Milliolidae, in which partial 

 constrictions give the first sign of the formation of chambers, repre- 

 sent the simplest condition of these forms. By the unequal addition 

 of fresh chambers the spiral form is completely effaced (Acervuliuae), 

 and can only be seen in the earliest-formed chambers. These tests 

 are usually confounded with external shells. But this only holds for 

 a few. The shell seems rather to be an internal support, in those 

 cases especially where the partition- walls of the so-called chambers are 

 frequently broken through, and the pore-canals at the same time pass 

 through the shell to the exterior, so that the protoplasm of the pseudo- 

 podia is able to cover the outside of the shell. When the partitions 



G 



