58 THE FORAMINIFERA 



in which it is interesting to note that the centre of symmetry of 

 the growth which occurred after the injury is entirely different 

 from the original one. 



Verworn (67, p. 455) finds that when a specimen of Polystomella 

 crispa (his observations were doubtless made on megalospheric 

 forms, see below) is broken into fragments, several of the larger 

 pieces remain alive and extend pseudopodia, but new shell is 

 secreted o/er the broken surfaces by only one, and this is found on 

 examination to be the fragment in which the nucleus is contained. 



The Form of the Test. The principal forms of test met with 

 among the Foraminifera will be considered later. We shall see 

 that in :.ome genera a particular mode of growth, in relation to 

 some simple symmetrical plan, whether rectilinear, piano -spiral, 

 helicoid, o annular, is observed with perfect regularity, while in 

 others i- symmetrical plan is only loosely followed. Many species 

 are adherent to other objects, and in them the chambers may be 

 " heaped " together irregularly, forming what are known as the 

 " acervuline " tests. 1 Some of the adherent forms take on an 

 arborescent shape. 



Multiform Tests. A remarkable phenomenon is presented by 

 many genera, and that is that the plan on which the chambers are 

 arranged in the growth of the test changes in the course of growth. 

 In such tests the chambers which succeed the central one are 

 arranged on a particular plan, whether piano-spiral, helicoid, or 

 some other, and after growth has progressed on this plan for some 

 time a change occurs, and a new plan of growth is with greater 

 or less abruptness adopted (Figs. 24, 39, 40, 44, etc.). In some 

 cases the plan of growth may be changed more than once before 

 the test is completed. Thus two or more types of arrangement of 

 the chambers are, in these genera, presented by the same test at 

 different stages of its growth. The genus Spiropleda is an example 

 in which the chambers are at first uniserial and arranged in a 

 piano-spiral, and later biserial and in a rectilinear series (Fig. 44, 

 A and B). To tests exhibiting such different modes of growth the 

 terms dimorphic, trimorphic, and polymorphic (according to the 

 number of forms of growth present) were originally applied, and 

 the phenomenon of their occurrence was spoken of as dimorphism, 

 trimorphism, or polymorphism. But it has since been discovered 

 that two kinds of individuals occur in the life-cycle of many For- 

 aminifera, and for this, which is, of course, an entirely distinct 

 phenomenon, the term dimorphism has, in accordance with 

 customary biological usage, been adopted. It has thus become 

 necessary to find other terms to characterise tests displaying 

 two or more modes of growth, and the adjectives biformed 

 and informed may, as proposed by Rhumbler (36, p. 63), be con- 



1 Actrvus, a heap. 



