POLYGASTRICA. 53 



figure, and again shooting out processes of its substance in various 

 directions, so as to assume all kinds of shapes with the greatest 

 facility. 



The Flask animalcule, (Enchelis,) fig. 16, 3; the Trichoda 

 sol, fig. 16, 4 ; the Euglena viridis, fig. 16, 5 ; the Gonium 

 pectorale, fig. 16, 6 ; the Trachelius anas, fig. 16, 7 ; the Para- 

 mecium aurelia, fig. 16, 8 ; the Navicula, fig. 16, 9 ; the Vibrio 

 Spirillum, fig. 16, 10 ; and the Vorticella Stentor, fig. 1 6, 11, will 

 give the reader an idea of the most common species of these crea- 

 tures, the structure of which we shall now proceed to investigate. 



(74.) With regard to their external covering, thePolygastrica may 

 be divided into two parallel groups, in one of which the body is en- 

 tirely soft, whilst in the other the animals are enclosed in a delicate 

 transparent shell : the former are termed nuda, or naked ; the 

 latter loricata, or loricated animalcules. The shells of the lori- 

 cated division vary much in form ; sometimes being mere transparent 

 shields covering the back, as in Euplsea Charon (Jig. 17, 4) ; at 

 others they would seem to be capable of opening, like the bivalve 

 shells of mollusca, as in the minute Naviculse^g. 16, 9. Delicate 

 as these shells are, and requiring the most accurate examination, 

 even with a good microscope, to detect their presence, we shall be 

 surprised to find that they play an important part in nature, mak- 

 ing up by their immense accumulation for their diminutive size. 

 We have before us, while writing this, a specimen of pulverulent 

 matter collected upon the shores of Lake Lettnaggsjon, two 

 miles and a half from Urnea in Sweden, which from its extreme 

 fineness resembles flour : this has long been known by the natives 

 of the region where it is plentiful, under the name of Bergmehl or 

 mountain meal, and is used by them, mixed up with flour, as an 

 article of food ; experience having taught them that it is highly 

 nutritive. On examination with the microscope, the Bergmehl is 

 found to consist entirely of the shells of loricated infusoria, which, 

 having been accumulating from age to age at the bottom of the waters 

 in which the living animals are found, form a stratum of considera- 

 ble thickness. Nor is this all : for, when agglomerated and mixed 

 up with siliceous and calcareous particles, these exuviae become con- 

 solidated by time into masses of flint and marble, in which the 

 shape and characters of the shells are perfectly distinguishable, so 

 that even the species of the animalcules to which they originally 

 belonged is easily made out. 



(75.) The movements of the polygastrica, when seen under the 



