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. These are the 

 one-shelled or single-chambered forms without secondary 

 growth*, which are to be recognized inconsiderable numbers 

 both in the perforate and the pylomatic types and both in 

 Radiolaria and Thalamophora ; these stand 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 f has observed that artificial injuries to the shell of 

 a monothalamous Rhizopod [Difjlugia 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 Orhito- 

 lites tenuissima and 0. 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 Phaiodarian 

 family Canosphasrida. The members of this interesting 

 group possess a small, pylomatic-monaxouic, central shell, 

 surrounded at a considerable distance by a large, spherical, 

 homaxonic, latticed ball, the two shells being held togetlier 

 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. 



t " Biologischo Protistenstudien," in Zeitschr. f. wiss. Zool. Bd. xlvi. 

 pp. 456-470, Taf. xxxii. Translated in ' Annals,' ser. 6, vol. ii. p. 165. 



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