XII 



PHYLUM MOLLUSCA 



687 



between the two limbs of each filament. In Lucina these 

 junctions are large and provided with blood-vessels ; and vascular 

 bars of tissue, the interfilamentar junctions, replace the ciliary 

 junctions of the lower forms. Thus by a regular series of grada- 

 tions the ctenidium is replaced by the complex double gill we are 

 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 lamellae of the right and left 

 inner laminse 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 more complex 

 forms of gills. In the Septibranchia the gills are degenerate, 



VIII 



IX 



MI 



FIG. 600. Sagittal section of part of enteric canal of Donax. I, lower lip ; II, Inte3tine ; III, 

 pyloric caecum ; IV, crystalline style ; V, cuticle ; VI, stomach ; VII, gullet ; VIII, upper 

 lip ; IX, mouth. (From Pelseneer.) 



being represented by a horizontal muscular partition or septum 

 (Fig. 597, F, and Fig. 599, 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 usually bounded by two pairs of labial palps 

 which sometimes attain a relatively immense size (Fig. 596) ; 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 caecal pouch of the stomach 

 (Fig. 600), which may be prolonged into one of the lobes of the 



