THE FORAMINIFERA 55 



chambered tests, the way in which the growth of the protoplasm is 

 accommodated is less obvious. 



It is probable, however, that here also, the shell, though at any 

 particular moment rigid, is slowly moulded and expanded under the 

 influence of the protoplasm. The small tests hitherto classified as 

 Psammosphaera fusca, but regarded by Rhumbler (33) as the young 

 of Saccammina sphaerica, with which he finds them to be connected 

 by all intermediate forms, are built up of fragments of sand placed 

 together irregularly, so that the contour of the young test is rough 

 and uneven. In the full-grown Saccammina test the fragments are 

 placed, as in a well-constructed stone wall, to form an even contour. 

 If, as these facts imply, a change in position of the constituents 

 has occurred during growth, there is no difficulty in accepting the 

 conclusion at which Rhumbler arrives, that it has also undergone 

 expansion as a whole. The alternate hypothesis, that in the course 

 of growth to its full size (3*5 mm.) the protoplasm periodically 

 discards its old shell and builds a new one, appears improbable, and 

 the student of the growth of bone will find no a priori difficulty in 

 admitting that a rigid structure may be the seat of profound inter- 

 stitial changes of substance. 



The growth of the tests of the cylindrical forms of the Astro- 

 rhizidea is effected by extension in a linear direction, fresh arenaceous 

 material being incorporated at their ends. They form simple or 

 branched (many Hyperamminae, Rhizammina), but usually un- 

 segmented tubes. In Hyperammina subnodosa (Fig. 1 7, d) the tubes 

 are constricted at irregular intervals, and thus present a transitional 

 condition of structure to the definitely segmented, chambered shells 

 of the great majority of Foraminifera. In the latter, while the 

 growth of the protoplasm as the result of the assimilation of food is 

 continuous, the growth of the shell is not continuous but periodic. 

 When a new chamber is to be formed, a mass of protoplasm is 

 protruded from the mouth of the shell, and at the surface of this 

 the new wall is formed, by secretion in the case of calcareous 

 shells, by cementing together of foreign elements in the arenaceous 

 forms. In some genera the secretion of shell substance takes place 

 only on the free surface of the protoplasm, but in others it occurs also 

 where the protoplasm rests against the previously formed test. In 

 such cases the septa dividing the chambers are double, and the new 

 chamber is complete on all sides with the exception of the aperture 

 or apertures left for communication with the exterior, or with its 

 successor, when a new chamber shall be added. In either case the 

 result of this periodic shell formation is the building up of a 

 segmented test, the segments of which, the chambers, are sharply 

 marked off from one another. 



Max Schultze found that in the formation of a new chamber by 

 Polystomella, the deposit of calcareous salts began before the chamber had 



