OF ANIMALCULES. 59 



them ; the second and third vary so much in size that 

 some do not require half that power, and a few even 

 less, being just discerned by the naked eye. The de- 

 finition given by MuUer to the animalcules under this 

 name, is, an invisible worm, verj/ simple, round, and rather 

 long. 



First Division —BACILLARIA. 



These minute and inactive animalcules are covered 

 with a hard shell- like coat, and appear closely allied to 

 some of the small fresh-water Algae, and are not very 

 appropriately classed with the Vibrio ; but as their in- 

 ternal structure has not been demonstrated, and Muller 

 has included some in this genus, I here introduce the group. 



{A.) Animalcules covered with a quadrangular bivalve 

 shelly divided across the back, 

 (a.) Distinct individuals, single or gregarious. 



Smooth : 



71. Navicula (shuttle-shaped) /M/t?fl. The yellow Na- 

 vicula.^{See p. 37).— Size, l-400th to l-720th of an inch. 



72. gracilis. The slender Navicula. — Size, 



l-400th to 1- 1500th ; new species. 



73. nlna. The arm Navicula. — Size, l-120th. 



Striated: 



74. fusiformis. The spindle-shaped Navicula. 



— l-360th to l-430th; new species. 



