704 



ZOOLOGY 



SECT 



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

 outer lamella of the outer lamina unites with the mantle an 

 the inner lamella of the inner lamina with the visceral mas 

 while, posterior to the latter, the inner lamella of the right an 

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

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

 also in the interfilamentar and interlamellar junctions in th 



FIG. 587. Half transverse sections of various Pelecypoda to show the chief kinds of gill. 

 A, Nucula; B, Amusium; C, Area ; D,Mytilus; E, Anodonta; F, Poromya. 



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

 i. ff. inner lamina ; i. 1. inner lamella ; i. l.j. interlamellar junctions ; m. mantle; o. f. outer 

 row of filaments ; o. (i. 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 



