308 M. F. Dreyer on the 



HaviDg now briefly indicated the relation of the poly- 

 thah'imoiis to the monothalanious forms, the question naturally 

 occurs to us which of the latter, the shells without secondary 

 growth, are to be regarded as the most primitive. A careful 

 investigation of the conditions coming under consideration 

 shows us that a positive answer to this question cannot be 

 given. The perforate, more or less homaxonic Monothalamia 

 in almost all cases show a primitive character ; but this may 

 also be assumed with a very high degree of ]n-obability for 

 many pylomatic Monothalamia. On the other hand, it is 

 exceedingly probable that a great part of the pylomatic 

 Monothalamia have only arisen secondarily from perforate 

 spherical forms. This view is supported especially by some 

 important transition-forms which occasionally occur. Thus 

 the number of pores in the spherical shell of Microcometes 

 varies from 5 to 1, so that in the latter case we have already 

 the indication of a monaxon-pylomatic development ; and in 

 Thurammina and OrhuHna one shell-pore is sometimes dis- 

 tinguished from the rest by its greater size. In Radiolaria 

 the secondary origin of a pylom occurs very widely, and 

 with regard to this I may refer to the detailed treatment of 

 the point in my * Radiolarienstudien.' 



Whilst, therefore, one form-type may pass over into the 

 other, this is by no means the case with the growth-types. 

 It never happens that a form which has grown terminally for 

 a time afterwards adopts the concentric growth, or the reverse. 

 According to extant observations at least it may pass as an 

 unexceptional rule that the same form always remains true to 

 the growth-type which has once been adopted. The beha- 

 viour of the pylomatic ^pumellaria is particularly instructive 

 upon this point. Not only in many single-shelled Spumel- 

 laria, but also in many in which several concentric spherical 

 or annular systems are already present, a pylom is developed ; 

 but nevertheless these forms continue witliout disturbance to 

 grow concentrically, the influence of the pylom not being of 

 sufficient importance to suppress the concentric growth and 

 cause the shell to continue its growth terminally. The 

 Ehizopoda in question are able to change their form-type, 

 but not their growth-type. 



In what has been said mention has several times been made 

 of developmental or transformational processes in the Rhizo- 

 pod skeleton ; with regard to these the following must also 

 be brought to mind. For the genetic explanation of the 

 innumerable phenomena of differentiation three possibilities 

 have been given in accordance with the different particular 

 results. A great number of structures are referable to simple 



