;^00 M. F. Dreyer on the 



The last of them, which bears the furca, is also ufisymmetri- 

 cally developed. In correspondence with the relative sizes of 

 the two halves of the furca, the left side of the segment in 

 question is considerably broader than the right. The serra- 

 tion characteristic of the furca of this species terminates regu- 

 larly on each side just above the lateral seta, but on the right 

 side it commences, not as on the left immediately beneath the 

 hinder margin of the last segment, but somewhat further back. 

 This malformation may be original, but is more probably pro- 

 duced by a subsequent accident (perhaps in change of skin). 

 In the reproduction of the lost part of the furca it was not 

 again completed of the normal size ; but in this way certainly 

 the displacement of the above-mentioned seta is not easy to 

 explain. 



XL. — Considerations on the Structure of Rhizopod Shells. 

 By Fkiedrich Dkeyek *. 



In the course of my investigations upon Rhizopoda, and 

 especially upon the Radiolaria, various considerations of a 

 o-eneral character have impressed themselves upon me. In 

 part similar ideas have been already touched upon by previous 

 authors and occur scattered in the most various parts of the 

 copious literature ; several points to be referred to in what 

 follows I have already incorporated with the special investi- 

 gations in the first part of my ' Kadiolarienstudien ' f ; never- 

 theless I regard it as a not unprofitable task to reproduce in 

 the following pages in a connected torm the complete train of 

 thought of my considerations upon the structure of the Rhizo- 

 pod shells, as I hope that it will be of interest even for many 

 who do not occupy themselves specially with the Protistan 

 group in question. 



Even on a superficial consideration of the enormous num- 

 ber of forms of the Rhizopoda we may recognize in them an 

 essential difference in the general habit of the shell, and, in 

 accordance therewith, distinguish two groups of forms. One 



* Translated from the ' Biologisclies Centralblatt,' Bd, ix. pp. 333-352 

 (1st August, 1889). 



t F. Dreyer, ' Morphologisclie Kadiolarienstudien,' Heft I. " Die 

 Pylombildungen in vergleichend-anatomischer und entwicklungsgeschicht- 

 licher Beziehuug bei Kadiolarien und Protisten iiberbaupt, nebst System 

 und Beschreibuug neuer uud der bis jetzt bekannten pylomatischen Spu- 

 mellarien" (Jena, 188'J). 



