INTRODUCTION. 



17 



chitinous envelope, fortify and ornament it externally 

 with sand-grains, diatom-tests, and other extraneous 

 substances. 



The tests of Hyalosphenia are either plane-surfaced, 

 or ornamented with variously-disposed pits or depres- 

 sions. Those of Nebeln and JQuglyplia are built up of 

 circular, oval, or hexagonal plates, placed edge to 

 edge, or with the edges overlapping. 



The development of the Rhizopodous tests has been 

 closely studied by Gruber, Verworn, and others. In 



FIG. 7. Test of Hyalosphenia papilio. x about 250. FIG. 8. Test 

 of Difflugia pyri/o-rmis (a common form) encrusted with sand- 

 grains, x 200. 



Euglypha alveolata Gruber concluded that there was a 

 development, within the protoplasmic body, of the 

 necessary materials chitin, cellulose, or silica by 

 chemical agency. Diaphanous plates of varied pattern, 

 circular or oval (Nebela, etc.). polygonal (some 

 Euglyphx); quadrangular (Quadrula); or of no definite 

 geometrical figure, are secreted; and during mitosis 

 (reproductive fission) these so arrange themselves in 

 the newly-formed cell as ultimately to form an external 

 covering, in all respects like that of the parent.* It is 



* ' Zeitschr. fur Wiss. Zool.,' xxxv (1881), 431. See also Schewiakoff in 

 'Morphol. Jahrb./ xiii (1888). 



