THE LAMELLIBRANCHIA 



22? 



individual filament and consequently the two lamellae of each gill- 

 plate are joined together by bridges or interlamellar junctions, 

 which are formed of connective tissue only in the Pectinidae, but 

 are vascular in the Aviculidae. Finally, the different elements of 

 the branchial apparatus are much more intimately connected in the 

 various groups of Eulamellibranchs, in which there are always 

 vascular interfilamentar and interlamellar junctions (Fig. 237). 

 Thus the blood brought to the gill by the afferent vessel is conducted 

 by vessels which run between the lamellae and communicate with 



Fio. 207. 



Adac.narca nitens, Pels., transverse section, br, right internal gill-plate; br 1 , left internal 

 Bill-plate (without reflected lamina) : br", external gill-plates (with reflected lamina) ; ca.b, 

 byssus cavity ; com.v, visceral commissure ; hep, liver ; in, intestine ; pa, mantle ; per, peri- 

 cardium ; r, kidney ; st, stomach ; tcs, testis. 



the filaments on either hand, forming in this manner the inter- 

 lamellar junctions. 



Each gill -plate may be thrown into a very regular series of 

 transverse folds, each fold involving a fixed number of filaments ; 

 this is the case in the Pectinacea, the Ostraeacea, and the more 

 specialised forms of Eulamellibranchia. In the last-named the 

 folding is still but slightly marked in the Veneridae, but becomes 

 much more so in the Cardiacea (in Tridacna a single fold may 

 contain as many as seventy filaments), the Myacea, etc. In the 

 Pectinacea and Ostraeacea the filament forming the junction between 

 two successive folds becomes thicker and more important than the 



