296 



THE MICROSCOPE A.ND ITS REVELATIONS. 



tinction of this group consists in the presence of tubular t awns/ fre- 

 quently proceeding from the connecting hoop, sometimes spinous and 

 serrated, and often of great length (Fig. 182); by the interlacing of which 

 the frusbules are united into filaments, whose continuity, however, is 

 easily broken. In the genus Bacteriastrum (Fig. 183) there are some- 

 times 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. 184), by a transverse annulation that imparts to this fila- 

 ment 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 of this curi- 

 ous family are obtained from the stomachs of Ascidians, Salpse, Holo- 

 thuriae, and other Marine animals. 1 



1S3 



Bacteriastrum furcatum. 



Chcetoceros Wighamii:a, front view, and 6, side view of frustule; c, side view of connecting 

 hoop and awns; d, entire filament. 



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

 will be remembered, of those in which the frustules have a median longi- 

 tudinal 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 distinguished as the inferior. 

 This family consists of but a single genus Cocconeis, which includes, 

 however, a 'great number of species, some or other of them occurring in 

 every part of the globe. Their form is usually that of ellipsoidal disks, 

 with surfaces more or less exactly parallel, plane, or slightly curved; and 

 they are very commonly found adherent to each other. The frustules in 

 this genus are frequently invested by a membranous envelope which forms 

 a border to them; but this seems to belong to the immature state, subse- 

 quently disappearing more or less completely. Another Family in which 

 there is a dissimilarity in the two lateral surfaces, is that of Achnanthece; 

 the frustules of which are remarkable for the bend they show in the direc- 



1 See Brightwell in " Quart. Journ. of Microsc. Science," Vol. iv (1856), p. 105; 

 Vol. vi. (1858), p. 93; Wallich in "Trans, of Microsc. Soc.," N.S., Vol. vm. (1850), 

 p. 48; and West in the same, p. 151. 



