12 



PKOTOZOA. 



partments, by numerous apertures through which the pseudopodia are 

 protruded. When the segments are all placed in a straight line, or 

 when they are rolled in a spiral upon the same plane, or when they are 

 alternate and the shell is inequilateral, their texture is very generally 

 transparent like glass *. 



(27.) Nearly related to the Foraminifera, or at least apparently be- 

 longing to the same general type of structure, are the Polycystina, an 

 extensive group of very interesting microscopic bodies possessing great 

 beauty and variety of form and structure. These are minute silicious 

 shells, which appear, from the recent observations of Professor MUller 't, 

 to contain in the living state an olive-brown sarcode extending itself 

 into pseudopodial prolongations that pass through the large apertures by 

 which the shells are perforated. The sarcode does not seem always to 

 fill the shell, but only its upper part or vault, and to be very regularly 

 divided into four lobes. It is a peculiar feature in these Polycystina that 

 their shells are often prolonged into spines or other projections, which 



Fig. 4. 



are sometimes arranged in 

 such a manner as to give them 

 a very singular aspect (fig. 4). 

 It seems probable that these 

 creatures are at the present 

 time as widely diffused as the 

 Foraminifera, although, from 

 their extreme minuteness, they 

 have not been so often recog- 

 nized. They were first disco- 

 vered by Professor Ehrenberg 

 at Cuxhaven, on the North 

 Sea; they were afterwards 

 found by him in collections 

 made in the Antarctic seas; 

 and have been recently de- 

 scribed by Professor Bailey as 

 presenting themselves (with Foraminifera and Diatomaceaa) in the de- 



1. Podocystis Schomburgkii. 



2. Bhopalocanium ornatum. 



* The following table indicates the proportions of genera and species of Forami- 



niferous shells which have been met with in various geological epochs : 



Carboniferous series 1 genus, 1 species; 



Jurassic series 4 genera, 20 



Cretaceous series 30 250 



Tertiary series 55 460 



Now existing 68 900 



so that 1631 species of these minute organisms have already been distinguished by 



classical naturalists. 



t Vide Miiller, liber die Thalassicollen, Polycystineen und Acanthometren des 



Mittelmeeres, in Monatsbericht der Konigl. Akademie der Wissenschaften zu Berlin, 



13 Nov. 1856. 



