Structure of Rhizopod Shells. 307 
whereas in the Rhizopoda all the chambers are filled by the 
sarcode-body. 
While the whole of the Rhizopod shells may be brought 
under the two form-types, this is not the case with the growth- 
types, for the simple reason that in many Rhizopoda no sup- 
plementary growth of the shell takes place. ‘hese are the 
one-shelled or single-chambered forms without secondary 
growth *, which are to be recognized in considerable numbers 
both in the perforate and the pylomatic types and both in 
Radiolaria and Thalamophora ; these aiid in a certain oppo- 
sition to the shells with secondary further growth occurring 
in one or other of the two growth-types. It is interesting to 
see that, apparently, there is a physiological difference to be 
placed side by side with this morphological distinction. 
Verworn + has observed that artificial injuries to the shell of 
a monothalamous Rhizopod (Difflugia urceolata, Carter) were 
not repaired, while in the polythalamous Rhizopods this takes 
place to the fullest extent, as shown by that author’s investi- 
gations upon Polystomella crispa and Carpenter’s on Orbito- 
lites tenuissima and O. complanata. From these results we 
may conclude with Verworn that the faculty of the soft body 
of secreting shell-material only continues as long as the nor- 
mal growth of the shell itself, from which then the above- 
mentioned different behaviour of the mono- and polythalamous 
Rhizopoda may be explained. 
As already mentioned, the two form- and growth-types are 
associated in this way :—the shells of the perforate type are 
further developed in accordance with the concentric growth- 
type, and the pylomatic shells, on the contrary, after the 
terminal growth-type. ‘To this rule, so far as I know, only 
one exception is known, namely that of the Pheodarian 
family Canospherida. ‘The members of this interesting 
group possess a small, pylomatic-monaxonic, central shell, 
surrounded at a considerable distance by a large, spherical, 
homaxonic, latticed ball, the two shells being held together 
by long radial beams. Here, certainly, the sarcodic stream 
in one direction which existed at the time of the secretion of 
the central shell is suppressed during the course of the 
succeeding development, to give place to a uniformly radial 
arrangement. 
* Even some monothalamous Thalamophora show a secondary shell- 
growth, such as, especially, the Cornuspirida. These pylomatic forms of 
course belong to the terminal growth-type, and are therefore to be ex- 
cepted here. ; 
+ “Biologische Protistenstudien,” in Zeitschr. f. wiss. Zool. Bd. xlvi. 
pp. 455-470, Taf. xxxii. Translated in ‘ Annals,’ ser. 6, vol. il. p. 155, 
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