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, 4, 7). 
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 
Fic. 209. 
Diagrams of transverse sections of a Lamellibranch to show the adhesion, by conerescence, 
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 ; ¢, adaxial lamella of outer gill-plate; e7, reflected 
lamella of outer gill-plate; f, 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 ; 7, 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- 
