and their relation to existing Animals. 119 



Polythalamia, which contain in the subdivision Nodosarina forms 

 very similar to Polycystina solitaria, and in Sorita, Pavonina, 

 Melonia, a striking similarity to Polycystina composita, Haliom- 

 matina and Lithocyclidina. The cruciform and radiated struc- 

 ture of Siderolina and Siderospira is likewise present ; nay, even 

 the structure of recent oceanic forms may be traced in some. 

 Nevertheless the minute shells of the Polycystina possess phy- 

 siological characters, which, independent of their being siliceous, 

 separate them entirely from Polythalamia ; such characters namely 

 as the absence of real cavities, the presence of which contributes 

 to a different structure in the Polythalamia, and from which 

 peculiarity their name has been derived. It is likewise evident 

 in the greater number of instances, that the articulation of the 

 body does not increase in number with age, as is the case in Nodo- 

 saria and Rotalia, but is individually definite, which is a very 

 important character. Otherwise, the more easily-closed trans- 

 verse articulation of the Polycystina is a character entirely want- 

 ing in the Bacillaria, which possess a longitudinal structure in 

 their skeleton and development. 



These considerations have determined Professor Ehrenberg to 

 relinquish his former opinion that the Polycystina constitute a fa- 

 mily of Polygastric animalcules, or that they belong to Arcellina of 

 compound structure, and he arrives at the conclusion, that they 

 form like Bryozoa a subdivision of Tubulata, but in this instance 

 with siliceous shields and individual organized forms. They ap- 

 proach most nearly in systematic arrangement to Polythalamia, 

 and would occupy a separate group among animals possessed of 

 vessels but without a heart and pulsation, and provided with a 

 simple tubular intestinal canal. The forms developed in the 

 highest degree in that division would be Holothurice and Echi- 

 noidea. 



Professor Ehrenberg, in his former paper on Polycystina, de- 

 scribed five species ; two of which belong to the genus Haliomma, 

 and are found near Cuxhaven in the North Sea (they are other- 

 wise widely distributed in the ocean) : three species also, two of 

 which belong to the genus Eucyrtidium, and one to the genus 

 Lithopera, have been found at the bottom of the sea near the 

 South Pole. These are — Haliomma ovatum, H. radians, Litho- 

 pera denticulata, Eucyrtidium antarcticum, E. australe. The two 

 first forms, which he himself examined, belong to the Polycystina 

 composita, but in consequence of their soft nature their structure 

 remained doubtful. The cells appear to be filled with an olive- 

 brown substance. The other three forms, of which only the 

 empty shells were examined, were found in large numbers among 

 living infusoria at the Antarctic Pole ; hence Ehrenberg con- 

 cludes that they belong to the organic beings of the present 

 period. 



