340 MOLLUSCA. 



the mantle (Fig. 264), and the inner lamella of the inner gill with 

 its fellow across the middle line behind the foot. In this way a 

 septum is formed which divides the mantle cavity into a dorsal part 

 the suprabranchial cavity, and a ventral the general mantle cavity. 

 These two cavities communicate by the pores in the gill lamellae, and 

 in many forms along the sides of the foot where the inner lamella 

 of the inner gill may end freely, not having undergone concrescence 

 with the foot. In some forms the inner lamella of the inner 

 gill is joined to the base of the foot in front, but not behind 

 The part of the suprabranchial cavity within the gill lamellae is 

 broken up by vascular strands which connect the lamellae, and 

 which result from the interlamellar concrescences already referred 

 to. In the siphoned forms the median septum resulting from 

 the . fusion of the inner lamellae of the inner gills across the 

 middle line is continued into the siphon, and causes the division 

 of that tube into a dorsal or exhalent channel, which communicates 

 with the suprabranchial chamber, and a ventral or inhalent which 

 communicates with the general mantle-cavity. 



In the Septibranchiata (Fig. 264 D) the gills are represented by 

 a muscular septum which is perforated by a certain number of pores, 

 and which, being fused with the mantle and foot and being con- 

 tinuous across the middle line behind the foot, completely divides 

 the mantle-cavity into a dorsal and ventral part. 



The filaments of the gills are always clothed with an epithelium, 

 which is in part at any rate ciliated. The cilia are specially long on 

 the so-called latero-frontal cells (Fig. 265), and in the Filibranchs and 

 Pseudo-lamellibranchs on the ciliated discs of the ciliary junctions. 

 The filaments are moreover stiffened by a dense chitin-like connective 

 tissue, which in the Filibranchs forms a tube (Fig. 265) lying just 

 beneath the epithelium and surrounding the central blood space 

 (which is often divided by a septum) ; and in the Eulamellibranchs, 

 on the other hand, this chitinous supporting substance has the form 

 of two stout rods lying side by side in the filament part of the gill 

 (Fig. 267 (7), the vascular tissue being mainly contained in the 

 internal outgrowths of the filaments, which have brought about 

 the interfilamentous and interlamellar concrescences (Fig. 267, of, 

 il, lac, and ol). 



Amongst the Eulamellibranchs we find some peculiar modifications of the 

 outer gill. In Tellina it is directed dorsally ; in the Anatinacea it is directed 

 dorsal ly and the outer lamella is absent ; in Lucina it is absent altogether. 



In Pseudolamellibranchs and some Eulamellibranchs the gills are folded along 



