ANODONTA — EXTERNAL FEATURES. 
1G5 
folds, which completely envelop the entire body of the animal to which 
they are dorsally and laterally attached, but open ventrally, and en- 
closing between them the pallial cavity, which, by the fusion of the 
gill lamella!, is divided into an upper or supra-branchial and a lower 
or infra-branchial chamber; in the latter are situate the foot, the gills, 
the palps, the oral orifice, etc., while the rectum, the renal and the 
genital organs open into the supra-branchial or exhalent chamber. 
The mantle is formed of interlaced contractile muscidar fibres and 
connective tissue, enclosing large and important blood spaces, which 
seem to be in correlation with the largely develo2)ed foot. 
The margins of the mantle were probably i)rimitively entirely open, 
but in our type are fused together on the posterior margin, forming 
the branchial and the anal orifices, and hence such species are ilis- 
tinguished as biforate. This fusion 
separates the pure in-coming infra- 
branchial stream from the excurrent 
supra-branchial one laden with 
effete substances and excreta. 
The branchial is the largest and 
most ventral opening and is con- 
tinuous with the general mantle 
cleft, but may be functionally 
limited by the close apposition of 
the mantle margins. 
The anal opening exhibits a sub- 
sidiary fusion by which two out- 
wardly distinct, but internally con- 
nected apertures, are formed, the 
one adjoining the branchial siphon and opposite the anus is the true 
anal siphon ; the more dorsal opening is known as the renal and also 
as the dorsal orifice, but the significance of its separation or the duty 
it perforins is not as yet understood. 
The local modification of the pallial margin, to form the branchial and 
anal siphons with definitely localized sensitive processes is in a large 
degree correlated with the activity of the animal, the circumscribed area 
they occupy showing that a part of the body is usually concealed in the 
mud ; while the greater development of the sensitive processes around 
the inhalent opening enables the mollusk to detect the presence and 
guard against the inhalation of injurious or distasteful substances. 
Fig. 331.— Posterior aspect of Anodonta 
cygnca to show the relative positions of the 
external orifices and the mode' of closure of 
the branchial siphon by the infolding of the 
digitate filaments (after Howes). 
a.s. anal siphon ; br.s, branchial siphon \ d.o. 
dorsal orifice ; 1. ligament of shell ; m. mantle. 
