KHIZOPODA. 



35 



AH the Polycystina are microscopic, and they are all inhabitants of 

 the sea. They are best known to students of the microscope as 

 the " Fossil Infusoria of Barba- 

 does" (fig. 14), as they occur in 

 incalculable numbers in a sand- 

 stone in that island. 



They are, however, in no way 

 related to the " Infusoria " pro- 

 perly so called, having much 

 stronger affinities with the For- 

 aminifera. We know, also, now, 

 tliat deposits of the flinty shells 

 of these beautiful microscopic 

 organisms are in process of for- 

 mation in our oceans at gi'eat 

 depths. 



In the third family {Collozoa) 

 are included a number of singu- 

 lar gelatinous organisms which 



may be as large as an ordinary marble, but are often hai'dly visible 

 to the unassisted eye. They are found floating passively at the sur- 

 face of most seas ; and they may either have no skeleton, or may 

 secrete a flinty framework, which may give rise to a fenestrated 



¥\'^. U.— Shells of Polycystiva from the 

 " Biiuhadoes Earth," greatly inagiiitied. 

 (Original.) 



Fig. 15. — a Siliceous shell of CoUosph<rra ; h Sjilurrozniitn, sho\viiig the radiating 

 pseiidopudia and groups of siliceous spicula (after Muller). 



shell, as in CoUos-plivera (fig. 15, «), or may remain in the form of 

 scattered spicules, as in Spluerozomn (fig. l.'j, h). In TlialassicoUa 

 the skeleton is wanting. 



A fourth family has been constituted under the name of Heliozoa, 

 for a number of singular and beautiful microscopic organisms, which 

 mostly occur in fresh water, and which cannot be regarded as ty7:)ical 

 Eadiolarians, though more closely aUied to this group of the Pro- 



