172 DESCRIPTION OF [Polygastrica. 



FAMILY X. BACILLARIA. 



This family is very extensive. It is deeply interesting 

 to the geologist, from the recent discovery of the remains 

 of several genera in various portions of the earth's struc- 

 ture, either forming by themselves, or with argillaceous 

 earths, strata of great extent ; or, when imbedded in various 

 stony formations, as flint, agate, semi-opal, &c., clearly de- 

 monstrating their production subsequent to the existence 

 of those interesting creatures. 



The first observers of this family considered its members 

 as animals; but by far the greater number of modern 

 naturalists regard them as plants, and place them among 

 the minute Algae ; hence it is we stand mainly indebted 

 to the botanist for our knowledge of their forms and 

 localities. 



A few of the genera are considered by some philosophers 

 as forming connecting links of the animal, vegetable, and 

 mineral kingdoms. Ehrenberg (whose skill and practice 

 in the use of the microscope has been very great) affirms 

 they are decidedly animal, and characterizes the family as 

 comprehending all animalcules, distinctly or apparently 

 polygastric, destitute of alimentary canal, the body fur- 

 nished with variable undivided processes, and covered by 

 a lorica or shell. While undergoing self- division they 

 appear connected together, as it were, by a percurrent 

 thread, so that they form chain-like or tabulated groups. 

 The lorica of each animalcule has one or more openings, 



