94 



COMPARA TIVE ANA TOMY 



fuse with each other behind the foot in the middle line of the mantle cavity, they form 

 a septum which, uniting with the septum formed by the mantle between theinhalent 

 and exhalent siphons, divides the cavity into an upper and a lower chamber. The 

 ■water flows through the lower (inhalent) siphon into the large lower chamber, bathes 

 the gills, and, streaming forward, conveys the particles of food it contains to the 

 mouth. It then flows back along each side of the foot in the upper chamber of the 

 mantle cavity (which is itself divided into two canals by the line of insertion of the 



ft ff.. 



Fig. 80.— Part Of a transverse section of the outer branchial leaf of Dreissensia polymorpha 

 (after Peck). /, The separate lilaiiients ; /, sub-epithelial fibres ; ch, sullpllrtiIl,^' substance of tlie 

 filaments ; lac, lacunar or alveolar tissue ; pig, pigment cells ; he, blood corpuscles ; fi, epithelium 

 of the free edge of the branchial filaments ; Jfii, lfe-2, two rows of lateral epithelial cells of the 

 branchial filaments, carrying long cilia (ciliated tufts) ; Irf, tissue of the interttlamentar junctions. 

 Two interfoliar junctions are shown in the figure. 



gill) into the single posterior and upper chamber behind the foot, and escapes through 

 the upper (exhalent) siphon (Fig. 26, \\ 18). 



((') Septibranchia (Fig. 31 A and B, p. 21 ; and Fig. 88 D, p. 92).— These Mussels 

 were formerly erroneously considered to be gill-less. As a matter of fact, the 

 branchial septum just described has in them been much modified in structure, and 

 has become a muscular septum, running across the mantle cavity in a horizontal 

 direction and joining the siphonal septum posteriorly, while anteriorly it passes 

 round the foot. This septum is broken through by various perforations and slits, 

 which allow of communication between the upper and lower chambers of the mantle 

 cavity, and vary in the different genera. 



