116 SKETCH OF THE INFUSORIA 



oncUa, Pyxidicula, Isthmia, &c. ;) at other times they appear under 

 the form of a tube enveloping the stomachs and other organs, ( Xan- 

 thidium, Euastrum, Micrasterias.) The genera Micrasterias, Ailhro- 

 desmus, Tessararthra and Xanthidium, have organs which may be 

 compared to seminal vesicles. Spontaneous division produces much 

 variety in the forms of the polypidoms. It sometimes takes place 

 longitudinally, sometimes transversely." 



The living species of this family may be found in almost 

 every situation where water occurs upon the surface of the earth. 

 Some genera are exclusively marine, others are exclusively 

 fiuviatile, while some genera, as Navicula, Gomphonema, &c., 

 include both salt and fresh water species. The marine species 

 may often be found in great quantities among the filiform Algse, 

 which they often invest completely with their crystalline cara- 

 paces. 



The fiuviatile species may be found in every pond, stream, 

 rivulet, bog, or pool, either nestling among Conferva?, parasitic 

 on aquatic plants, or living in the sedimentary matter at the bot- 

 tom. They often occur in such vast quantities as to cover hun- 

 dreds of square yards, to which they give a peculiar color — 

 green, yellowish or ferruginous, according to the peculiar internal 

 coloring matter of the individuals. JMost of the species are ex- 

 ceedingly minute, many are entirely invisible to the naked eye ; 

 others, however, are quite perceptible without the aid of the 

 microscope. Notwithstanding their extreme minuteness, it is 

 evident, from their vast abundance, that they have some im- 

 portant offices to perform in the economy of nature; and 

 like the coral insect, although the individuals are minute, the 

 result of their united labors is on a scale by no means insig- 

 nificant. 



Few organic bodies exceed in beauty the symmetrical, ele- 

 gantly sculptured forms of many of the species. Their beauty, 

 the singular phenomena they present, and the interest they have 

 lately received from being detected in a fossil state in Europe 

 and America, will be sufficient inducement for aU lovers of mi- 

 croscopic research to study this family. 



