POLYCYSTINA. 



383 



or root-footed animalcules, that form the lowest order of the 

 Protozoa, the lowest class of the animal world. 



It is a peculiar feature of these beautiful little shells (whose 

 delicate sculpture frequently reminds the ohserver of the finest 

 specimens of the hollow ivory balls carved by the Chinese) that 

 they are usually surmounted by a number of spine-like pro- 

 jections, very frequently having a radiate disposition. Some 

 have an oblong shape (Podocyrtis), others a discoid form (Ha- 



Polycystina. 

 a. Podocyrtis Schamburgkii. b, Halfomma Humboldtii. 



liomma), from the circumference of which the silicious spines 

 project at regular intervals, so as to give them a star-like aspect. 

 They are generally of a smaller size than even the Foraminifera, 

 appear to be almost as widely diffused, and have also largely 

 contributed to the structure of the earth-rind. They were first 

 discovered 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 brought up by the sounding 

 lead from the bottom of the Atlantic at depths of from 1,000 to 

 2,000 fathoms. 



The term Infusoria, which formerly comprised a most 

 heterogeneous assemblage of minute plants and animals, is now 

 confined to the highest order of the Protozoa, distinguished 

 from the Ehizopods by the possession of a mouth and of 

 ciliary filaments, whose vibrations serve them both for pro- 



