72 NATURAL HISTORY 



numbers in some clear springs, are very minute, pel- 

 lucid, and without any apparent internal organization ; 

 their posterior extremity is globose, and the anterior 

 assumes the form of the letter T when at rest, and the 

 letter V when in motion, as exhibited in the two magni- 

 fied figures, group 47. It is difficult to examine them ac- 

 curately, as they are incessantly changing their position. 



107. Vibrio acus, {Euglena acus, E.) The needle- 

 shaped Vibrio. — This animalcule resembles the next 

 species {F, sagitta), shewn in figure 48; but is very 

 slender, the body not being a quarter the diameter of the 

 latter, and the tail drawn out into a delicate thread : 

 it is also distinguished from it by a bright red dot in 

 the head, called by Ehrenberg the eye. (See Cercaria 

 viridis, which has a similar spot ; also Book 111.) The 

 nature of its alimentary organs has not been domon- 

 strated. A bright line is sometimes seen down the 

 middle of the body, which makes its form apparently tri- 

 angular ; this, however, is an optical illusion. In pro- 

 gressing through the water, it seldom inflects itself. It 

 propagates by a division lengthways. Found near the 

 surface of stagnant water. Length, l-480th to 1 -200th 

 of an inch. 



108. Vibrio sagitta. The arrow Vibrio. — This species, 

 which is shewn, magnified, at figure 48, is flexible, and 

 progresses by contracting and elongating its neck : the 

 head is generally dark, and the body appears filled with 

 green molecules. Found in sea- water. 



