310 M. EF. Dreyer on the 
initial parts of differentiated skeletons with adult primitive 
forms. With the shells of Thalamophora this has been 
carried out in several special cases; and in the case of the 
Radiolaria, from their much greater differentiation it is pos- 
sible toa much greater extent and with more profit. In these 
cases comparative anatomy and ontogeny coincide, an advan- 
tage in morphological investigation which cannot be too 
highly appreciated, but which, unfortunately, like the compa- 
rative treatment of the Rhizopoda in general, has hitherto by 
no means received sufficient attention. 
Having now become acquainted with some of the most 
important points in the structure of the shells of the Rhizo- 
poda, it remains for us to give an explanation of these 
phenomena. Here, of course, we can only have to do with a 
preliminary attempt to throw some light upon the etiology of 
the enormous form-labyrinth of the Rhizopoda, for even an 
approximately complete solution of this difficult problem still 
lies in the far distance. 
The chief cause of the form-types of the soft body and of 
the shell is to be sought in the mode of life of the Rhizopoda 
under consideration. Rhizopoda with shells belonging to the 
perforate form-type and with pseudopodia radiating uniformly 
on all sides will live free and rotating in the water. The 
monaxonic and amphitect shells of the pylomatic form-type 
will belong to Rhizopoda which, in swimming or creeping, 
maintain a definite, perpendicular principal axis. ‘The eudi- 
pleural development, lastly, owes its origin to creeping in a 
particular direction, Just in the same way as in the example 
of the Polyclada already adduced in this connexion. 
The morphological evolution or the specific character of 
the form-types recurs, as has already been mentioned, in 
exactly analogous development throughout, independently of 
conditions of relationship and shell-material. With regard 
to the perforate form-type, on account of its undifferentiated 
character, there is not much to be said in this respect; and 
here we have chiefly to consider the above-mentioned asso- 
ciated phenomena ot the formation of the pylom, such as oral 
marginal ornaments of the pylom, apical spinosity, &c. The 
specific evolution of the form-type once selected is, as has 
been said, independent of the shell-maierial ; in the selection 
of the form-type liself, however, the latter plays an important 
part, and this applies in a still higher degree to the growth- 
“ype, inasmuch as the structural material plays a positively 
determinant part with respect to the mode of growth of the 
Rhizopod shell. 
The most important materials here coming under con- 
