56 THE FORAMINIFERA 



assumed its final form. The small pocket-like " retral processes," which 

 are characteristic of this genus (see Fig. 7), were not formed until some 

 time after a continuous wall had been secreted, so that a partial absorption 

 and re-deposition of the lime salts must here occur (64, p. 30). 



Structure and Mode of Growth of the Shell Wall. After their first 

 formation by secretion at the surface of a mass of protoplasm, the 

 walls increase in thickness by the addition of shell material to the 

 outer surface. The anterior wall of each chamber is soon covered 

 by the addition of a new chamber in front of it, and it then forms 

 the whole, or half (as the septa in the species concerned may be 

 single or double), of the septum dividing the chambers from one 

 another. On the septa of the perforate forms the pores may be 

 limited to the peripheral parts or absent altogether. 



The thickness attained by the septa is not great, but the part of 

 the wall turned to the outer world continues to grow, and may 

 attain considerable thickness. 



The thickening results from the addition of successive layers of 

 material on the outer surface, and thus a laminated structure is 

 produced ; but though laminated tangentially, the shell is built up, 

 where it is perforated, of hexagonal prisms disposed radially to the 

 surface, and each traversed by a pore transmitting a pseudopodium. 



It appears that this result may be explained as follows. The shell 

 material is deposited by the protoplasm traversing each of the pores which 

 perforate the wall, on the area about its orifice. It would appear that at 

 the limits between the areas influenced by neighbouring pseudopodia there 

 is some slight difference in the character of the material secreted, and the 

 result is that the deposit is not quite uniform, but marked out into small 

 hexagonal areas, with a pore at the centre of each. If this is the case, 

 the prismatic structure results from the observance of the same limits in 

 successive layers throughout the thickness of the shell. 



In the more complicated perforate Foraminifera a system of 

 sinuses or canals is present, the main channels of which run in the 

 substance of the shell, and are distinct from the cavities of the 

 chambers, though communicating freely with them by branches. 

 This is known as the canal system. It is, of course, wholly distinct 

 from the radial pores leading direct from the chambers to the 

 exterior. The details of its distribution in PolystomeUa are given 

 below (cp. Fig. 9). It will be seen that in this form two main 

 "spiral canals" run on either side of the test, parallel with the 

 series of chambers, and give off branches, some of which run in 

 the thickness of the septa between the chambers, while others pass 

 direct to the exterior in the axial regions of the test. About the 

 ultimate branches of the canal system a deposit of shell substance 

 is laid down, which may be called the canalicular skeleton. In the 

 test of Polystomella this skeleton is mainly limited to the axial regions, 



