The 



93 



the central nodule joining together both halves of the inner fissure (fig. 65 

 2 schl). 



The inner fissure terminates at each end of the valve in a funnel-shaped 

 structure which projects into the cavity of the terminal nodule (fig. 65 3tk). 

 The outer fissure terminates in a definite polar cleft, which is a curved slit 

 in the terminal nodule (fig. 65 3 psp). The polar clefts (often called terminal 

 fissures) at the extremities of the same valve are usually curved in the same 

 direction, but a number of species are known in which they are curved in 

 opposite directions. The corresponding clefts of the epivalve and hypovalve 

 of a single individual are invariably curved in opposite directions (fig. 65 4)- 



Fig. 66. Surirella Capronii\~Br&o. var. calcarata (Pfitzer) Hustedt. 1, transverse section across 

 the cell showing the four wing-like expansions (fl), each with its raphe-canal (r). 2, trans- 

 verse section of wing, showing raphe-canal and outer cleft. 3, longitudinal section of wing. 

 4, wing viewed from the edge, qk, cross-canals ; zw, thin intercalary pieces of wing; Ik, raphe- 

 canal (or plasma-canal) ; sp, cleft of raphe. From Oltmanus (after Lauterborn). 



The somewhat complex system of clefts and canals described above 

 is concerned with streaming movements of the cytoplasm, the latter being 

 brought into direct contact with the surrounding medium by means of 

 the outer fissure of the raphe. This is stated by 0. Miiller to be the direct 

 cause of the curious movements of those diatoms which possess a true raphe. 

 (Consult page 102.) 



The raphe of other pennate diatoms is essentially different. In Surirella 

 the lateral margins of each valve are produced into wing-like expansions, 

 of which there are therefore four to each cell. Near the free edge of each 

 wing is a fine raphe-canal, with a longitudinal fissure extending its whole 



