120 CONCHOLOGICAL NOTES. 



Each Lamellibranch possesses two ctenidia attached 

 to the ventral surface of the body, one on either side of 

 the foot. The ctenidia are of variable lengths. In 

 the PBOTOBRANCHIATA, fig. 3, they are very short and 

 are attached to the surface of the body by a slender 

 portion. The next stage, fig. 4, shows them more or less 

 parallel, but diverging at their distal ends. The 

 FILIBRANCHIATA, fig. 1, show a further stage in the 

 prolongation of the ctenidia, in which they become bent 

 upon themselves, each reflected portion taking the form of 

 an ascending limb terminating with a free end. These 

 ascending limbs in the ETTLAMELLIBRANCHIATA, fig. 2, are 

 united at certain points by interfilamentary vascular 

 concrescences, fig. 7., and the end of the ascending, or 

 outer ctenidium becomes fused to the mantle, while that of 

 the inner ctenidium undergoes concrescence with the 

 corresponding one at the base of the foot. The 

 interlaminar junctions produce cavities, communication 

 with each other being effected by means of pores. In 

 some members of this division, the gills are folded along 

 dorsoventral lines, fig. 5, each fold including a certain 

 number of filaments.* In fig. 6, it may be seen that the 

 SEPTIBRANCHIATA are distinguished by a muscular septum 

 on either side of the foot. Each septum, although 

 perforated with pores, is continuous from mantle to foot, 

 and completely divides the mantle-cavity into a dorsal and 

 ventral part. 



The filaments of the ctenidia, figs. 8, 9, 10, are 

 composed of a vascular tissue enclosed within a chitinous 

 layer, over which is spread an epithelium, which, here and 

 there, develops epithelial prominences, from which 

 project at right-angles to the axis of the filament, rows 

 of long cilia. These cilia interlock with others emanating 

 from neighbouring filaments, as represented diagrammati- 

 cally in fig. 1 1 . 



*Sedgwick. Text Book of Zoology, p. 341, 



