KHIZOPODA. 161 



CLASS RHIZOPODA. 



The members of this typical class are characterized by the 

 absence of a mouth, and the possession of pseudopodia. The 

 latter are thrown out at will as long, delicate, contractile fila- 

 ments, resembling roots (whence the name Ehizopod), and are 

 used like the tentacles of the Polyp for locomotion, and for the 

 introduction of food. The majority have the power of secreting 

 a testaceous envelope, either siliceous or calcareous; a few are 

 naked ; while fewer still are strengthened like the Sponge by 

 spicules. 



The class is divided into the Amcebea, which are naked, with 

 blunt finger-like pseudopodia, and the Foraminifera and Radio- 

 laria, which secrete a test and have thread-like and anastomosing 

 pseudopodia. The latter order has siliceous tests or spiculge, and 

 includes Polycystina and other marine groups, and the fresh-water 

 Heliozoa. 



The Protozoans with calcareous shells constitute the interest- 

 ing group of Foraminifera. The existence of these microzoa 

 was first made known to naturalists by Beccarius nearly 150 years 

 ago. They were first ranked as minute forms of Nautili, and 

 even d'Orbigny, in his first memoir (1825) described them as 

 Cephalopods. It was reserved for Dujardin, ten years after, to 

 demonstrate their Rhizopod type of structure. Since then our 

 knowledge has been greatly extended by the elaborate researches 

 of Williamson and Carpenter. 



The Foraminifer is a marine animal, dwelling in shells of ex- 

 treme beauty, sometimes simple, but usually consisting of an 

 aggregate of chambers which intercommunicate by minute aper- 

 tures, whence the name. These chambers grow by successive 

 gemmation from a primordial segment, sometimes in a straight 

 line, more commonly in a spiral or discoidal form. D'Orbigny's 

 classification, founded on the numerical increase of the chambers, 

 has been set aside. Carpenter divides the order into two primary 

 groups according as the envelope is jperf orated or imperforated. 



