DIATOMACE.E I CEvETOCEREiE '. RHIZOSOLENIA. 



303 



and often of great length (Fig. 135), by the interlacing of which 

 the frustules are united into filaments, whose continuity, however, 

 is easily broken. In the Grenus Bacteriastrum (Fig. 136) there 

 are sometimes as many as twelve of these awns, radiating from 

 each frustule like the spokes of a wheel, and in some instances 

 regularly bifurcating. "With this group is associated the Genus 

 Rhizosolenia, of which several species are distinguished by the 

 extraordinary length of the frustule (which may be from 6 to 20 

 times its breadth), giving it the aspect of a filament (Fig. 137), 

 by a transverse annulation that imparts to this filament a jointed 

 appearance, and by the termination of the frustule at each end in 

 a cone from the apex of which a straight awn proceeds. It is 

 not a little remarkable that the greater number of the examples 



Fig. 135. 



Fig. 136. 



Bacteriastrum furcatum. 



Chcetoceros Wighamii :—a, front view, and b, side view of frustule; 

 c, side view of connecting hoop and awns ; d, entire filament. 



of this curious family are obtained from the stomachs of Asci- 

 dians, Salpse, Holothurise, and other Marine animals.* 



232. The second principal division (B) of the Diatomacea? consists, 

 it will be remembered, of those in which the frustules have a 

 median longitudinal line and a central nodule. In the first of the 

 Families which it includes, that of Cocconeidece, the central nodule 

 is obscure or altogether wanting on one of the valves, which is dis- 



* See Brightwell in "Quart. Journ. of Microsc. Science," Vol. iv. 

 (1856), p. 105, Vol. vi. (1858), p. 93 ; Wallichin "Trans, of Microsc. Soc," 

 N.S., Vol. viii. (1860), p. 48 ; and West in the same, p. 151. 



