THE LAMELLIBRANCHIA 



229 



The result of these multiple unions is that the gills form a 

 partition, extending as far as the division between the two posterior, 

 anal and branchial, pallial orifices (Fig. 209, D), and dividing the 

 pallial cavity into two chambers, called respectively the supra- 

 branchial or cloacal and the infra-branchial chamber (Fig. 209, i, i). 

 The respiratory water generally enters the pallial cavity by the 

 postero-ventral side (by the branchial orifice or by the branchial 

 siphon, if the latter is differentiated) ; thence it passes, as through 



Fio. 209. 



Diagrams of transverse sections of a Lamellibranch to show the adhesion, by concrescence, 

 of the gill-lamellae to the mantle-flaps, to the foot, and to one another. A shows two conditions 

 with free gill-axis ; B, condition at foremost region in Anodonta ; C, hind region of foot in 

 Anodonta; D, region altogether posterior to the foot in Anodonta. a, visceral mass ; b, foot ; 

 c, mantle-flap ; d, axis of gill or ctenidium ; e, adaxial lamella of outer gill-plate ; er, reflected 

 lamella of outer gill-plate ; /, adaxial lamella of inner gill-plate ; fr, reflected lamella of inner 

 gill-plate ; g, line of concrescence of the reflected lamellae of the two inner gill-plates ; h, 

 rectum ; i, supra-branchial space of the sub-pallial chamber. (After Lankester.) 



a filter, through the trellis-work of the branchial filaments con- 

 stituting the partition in question, and is expelled from the supra- 

 branchial chamber by the anal orifice of the mantle or by the anal 

 siphon. 



In one whole group, the Septibranchia, this branchial partition 

 loses its normal structure in consequence of a predominant develop- 

 ment of its contractile elements, by which it is converted into a 

 muscular septum perforated by apertures leading ventro-dorsally 

 (Fig. 211). The water passes through these apertures from the 

 infra-branchial into the supra-branchial or cloacal chamber. Respira- 



