THE FORAMINIFERA 73 



of its life Polystomella crawls over the surface of submerged objects, 

 by means of its pseudopodia, but is incapable of swimming freely ; 

 and as the depth of 2 cm. far exceeds the distance to which the 

 pseudopodia of so young a specimen are likely to extend, the 

 colonisation of the coverslips by the microspheric form under 

 these conditions points to the existence of a free swimming 

 stage prior to the vegetative stage in which the young forms 

 were found. Such a stage is supplied by the free swimming 

 zoospore. 



The results furnished by direct observations on the life-history 

 of Polystomella may be summarised in two statements : 



The microspheric form terminates its existence by becoming trans- 

 formed into a brood of megalospheric young. 



The megalospheric form terminates its existence by becoming trans- 

 formed into minute zoospores of uniform size. 



Before discussing the bearing of these results on the relation- 

 ship of the microspheric and megalospheric forms, it will be con- 

 venient to consider some facts in another life-history, that of 

 Orbitolites complanata. 



LIFE -HISTORY OF ORBITOLITES COMPLANATA. The main 

 features of the structure of this species are described below (p. 

 104 and Figs. 36 and 37). For the present purpose it will be 

 sufficient to point out that the mode of growth is, except in the 

 early stages, cyclical, concentric rings of small chambers being 

 added at the margin of the disc-shaped test, and that the tests 

 are biconcave. In the microspheric form the central region is 

 thin, being built up of the microsphere and the small chambers or 

 " chamberlets " which succeed it. 



In the megalospheric form the centre of the shell is thicker 

 owing to the presence of the " primitive disc " (Figs. 37, A, and 38). 

 This consists of the megalosphere, a spiral passage, and of the 

 very large crescentic chamber, which nearly surrounds the other 

 constituents of the primitive disc. The outer margin of the 

 crescentic chamber is perforated by pores, by which it opens into 

 the innermost ring of chamberlets. 



Reproduction. The production of megalospheric young by a 

 microspheric parent has been repeatedly observed (4 and 20, and 

 Fig. 36, b). In the later stages of the growth of the parent 

 spacious brood chambers are formed at the periphery of the disc, 

 and in the reproductive phase the protoplasm is withdrawn from 

 the small chambers internal to them and massed in the brood 

 chambers, where it breaks up to form the brood of young. On 

 their escape, which is effected by the breaking down of the outer 

 walls of the brood chambers, presumably under the dissolving 

 action of the protoplasm of the young, the test of each young 

 individual has already the structure of the " primitive disc " 



