THE MICROSCOPE IN ZOOLOGY. 159 



Amoeba prince ps already described. Some of this latter 

 order, as Arcella and Difflugia^&re testaceous. In Arcella 

 the test is a horny membrane, analogous to the chitine 

 which hardens the integuments of insects. In Difflugia 

 the test is made up of minute particles of gravel, shell, 

 etc., cemented together. From the opening the amosboid 

 body puts forth its pseudopodia (Plate XII, Fig. 117). 

 Connected with Rhizopods are three remarkable series of 

 forms, generally marine, and distinguished by skeletons 

 of greater or less density, which afford many objects of 

 interest to the microscopist. These are the Foraminifera^ 

 the Polycystina, and the Sponges or Porifera. The shells 

 of the Foraminifera are calcareous, and those of Polycys- 

 tina siliceous ; both are perforated with numerous aper- 

 tures, which in Polycystina are often large. We have 

 previously referred to these forms as occurring in a fossil 

 state. 



Some Foraminifera have porcellanous, and others vitre- 

 ous or hyaline shells, usually many-chambered, and of every 

 shape between rectilinear and spinal. Most of them are 

 microscopic, but some are of considerable size, as the Or- 

 bitolites, which are found in tertiary limestones in Malabar. 

 The Nummulitic limestone, which extends over large areas 

 of both hemispheres, and of which the pyramids of Egypt 

 are built, is composed of the remains of the genus Num- 

 mulina; and the Eozoon Canadense has been shown by 

 Drs. Dawson and Carpenter to belong to the Foramini- 

 feral type. 



In some Foraminifera the true shell is replaced by a 

 sandy envelope, whose particles are often cemented by 

 phosphate of iron. Dr. Carpenter, whose researches have 

 largely extended our knowledge of this group, pertinently 

 remarks that "there is nothing more wonderful in nature 

 than the building up of these elaborate and symmetrical 

 structures by mere jelly specks, presenting no trace what- 

 ever of that definite 'organization' which we are accus- 



