6 14 MICROSCOPIC FOKMS OF VEGETABLE LIFE THALLOPHYTES 



Amphitetras, which is chiefly characterised by the cubiform shape of 

 its frustules. In the latter the frustules cohere at their angles, so 

 as to form zigzag filaments, whilst in the former the frustules are 

 usually free, though they have occasionally been found in chains. 



Another group that seems allied to the Biddidpkiece is the curious 

 assemblage of forms brought together in the family Chcetocerece, some 

 of the filamentous types of which seem also allied to the Melosirece. 

 The peculiar distinction of this group consists in the presence of 

 tubular ' awns,' frequently proceeding from the connecting hoop, 

 sometimes spinous and serrated, and often of great length (fig. 458) ; 

 by the interlacing of which the frustules are united into filaments 

 whose continuity, however, is easily broken. In the genus Bacterias- 

 trum (fig. 459) 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 ex- 

 traordinary length of the frustule 

 (which may be from six to twenty 



J 



FIG. 458; Chcetoceros Wighamii: , front 

 view, and &, side view of frustule ; c, side 

 view of connecting hoop and awns ; 

 d, entire filament. 



FIG. 459. Bacteriastrum 

 furcatum 



times its breadth), giving it the aspect of a filament (fig. 460), 

 by a transverse annulatioii 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 of this 

 curious family are obtained from the stomachs of Ascidians, Salpa?, 

 Holothurise, and other marine animals. 1 



The second principal division (B) of the Diatomacece 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 distinguished as 



1 See Brightwell in Quart. Journ. Microsc. Science, vol. iv. 1856, p. 105 ; vol. vi. 

 1858, p. 93 ; Wallich in Trans. Microsc. Soc. n.s. vol. viii. 1860, p. 48 ; and West, in 

 the same, p. 151. 



