704 



ZOOLOGY 



already familiar with in Anodonta. In all the higher forms the 

 outer lamella of the outer lamina unites with the mantle and 

 the inner lamella of the inner lamina with the visceral mass, 

 while, posterior to the latter, the inner lamellce of the right and 

 left inner laminae unite with one another. The blood-vessels, 

 which are confined to the filaments in the simpler types, occur 

 also in the interfilamentar and interlamellar junctions in the 



Fni. 5S7. — Half transverse sections of various Pelecypoda to sliow tlie chief kinds of gill. 

 A, Nncula ; B, Amusiuin ; C, Area ; D, mytiluB ; B, Anodonta ; F, Foromya. 



a. aperture in branchial septum ; b, v, blood-vessel ; ft. foot ; i. f. inner row of filaments ; 

 i. g. inner lamina ; i. I. inner lamella ; L l.j. interlamellar junctions ; m. mantle ; o. f. outer 

 row of filaments; o.g. outer lamina; o. I. outer lamella; sep, branchial septum. (Modified 

 from Korschelt and Heider, and Lang.) 



more complex forms of gills. In the Septibranchia the gills are 

 degenerate, being represented by a horizontal muscular partition 

 or septum (Fig. 587, F, and Fig. 589 IX), which divides the 

 inhalant and exhalant chambers from one another. Respiration 

 in this case is performed entirely by the internal face of the 

 mantle. 



Digestive Organs. — The mouth is anterior ; in forms with two 

 adductor muscles it is always placed immediately behind the 



