Bacillaria.] THE INFUSORIA. 203 



vided with eight internal partitions. Found fossil with the 

 preceding; not so common. Size l-576th. 



Genus LVIII. NAVICULA. The little ship Animalcules 

 derived their generic name from the resemblance in form 

 of the many species to a weaver's shuttle : the English 

 name here given is from the German. It comprehends 

 those members of the family Bacillaria who are unattached 

 by a pedicle (free), and have a simple bivalved or multi- 

 valved siliceous lorica. They occur single, or in pairs, 

 but are never united in the form of a chain. The lorica 

 of the Navicula is a closed, mostly four-sided, hard, 

 and glass -like little case (testula bivalvis), which, in dry- 

 ing, often separates ; when lightly pressed, it breaks or 

 divides either into two or four longitudinal parts ; some- 

 times the angles are provided with a short rib, distinctly 

 furrowed, the lorica then separates into four equal parts ; 

 but in some cases the two rows of ribs are not visible, the 

 two halves of the lorica being obliquely furrowed ; it then 

 separates into two parts. By heating the body upon 

 platina-leaf, the animal is consumed, and the siliceous 

 lorica left clear and free. The gelatinous and diaphanous 

 body of these animalcules occupies the whole of the 

 interior of the lorica, and has, near the centre, a sharply 

 circumscribed colourless bright spot. In N. fulva, an 

 organ of locomotion has been seen by Ehrenberg, which 

 he describes as a fleshy, undivided, sole-like foot, proceed- 

 ing from the central opening, and similar in appearance 

 to the locomotive organ of snails. The side of the body 

 where this foot-like process emanates is called the ventral 

 surface of the animalcule. This foot not only answers the 

 purpose of allowing it to creep, but the animalcule, when 

 at rest, can draw objects to it, and push things away by it. 



