XII 



PHYLUM MOLLUSCA 



655 



(Fig. 549, F, sep. and Fig. 551 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 

 anterior adductor. It is bounded by two pairs of labial palps 



FIG. 541*. 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. g. inner gill ; i. 1. inner lamella ; i. 1. j. inter-lamellar junctions ; m. mantle ; o. f. outer row 

 of filaments; o. ;/. outer gill; o. 1. outer lamella; sep. branchial septum. (Modified from 

 Korschelt and Heider and Lang.) 



which sometimes attain an immense size (Fig. 548) ; there is never 

 any trace of jaws or other masticatory apparatus. The convolutions 

 of the intestine are sometimes very complex. The crystalline 

 style either lies freely in the stomach and anterior part of the 

 intestine, or is contained in a coecal pouch of the stomach (Fig. 

 552), which may be prolonged into one of the lobes of the 

 mantle. The anterior end of the style, which projects into the 

 stomach, appears to be slowly dissolved by the digestive juice, 



