232 THE LAMELLIBRANCHIA 
outside the ctenidial branchiae, in the form of little secondary pallial 
outgrowths, constituting the folded organs called “ frills.” 
3. Hzxcretory Organs.—The principal excretory organs are the 
kidneys, which are symmetrical organs situated below the peri- 
cardium in the postero-dorsal region of the body: they generally 
extend back to the posterior adductor muscle, but not so far in 
the Protobranchia (Fig. 231, &). They have the form of two sacs, 
with glandular walls, irrigated by the venous blood passing to the 
gills and opening on the one hand into the pallial cavity by 
orifices situated at their anterior ends and external to the visceral 
commissure, and on the other hand into the pericardium. This 
latter organ is a median dorsal pouch, situated in the posterior 
part of the visceral mass and enclosing the heart (Fig. 204, I; 
205, p). In the Anomidae alone of all the Lamellibranchia the 
pericardium is greatly reduced, does not contain the heart, and con- 
sists of two small ramified spaces. The 
pericardium communicates with the kidneys 
by two symmetrical ventral reno-pericardial 
or internal renal orifices (Fig. 212, 7p), and 
in Anonia the kidneys communicate with 
the two small ramified spaces mentioned 
above. The tubes of the gland of Keber, 
which ramify in the mantle, also open into 
the pericardium. The pericardium is always 
completely shut off from the circulatory 
system, a fact which can readily be demon- 
strated in Lamellibranchs with red blood. 
The structure of the kidneys is simplest 
in the Protobranchia. In this group each 
Fia. 
212. 
Diagram of the pericardial 
organs in the Lamellibranchia, 
dorsal aspect. a.a, anterior 
adductor; aw, auricle; g, right 
genital gland; g.o, genital 
orifice; h.a, posterior adduc- 
tor; k, right kidney; p, peri- 
cardium ; pe.g, pericardial 
gland; re, rectum; 7.0, renal 
opening; 7p, renopericardial 
orifice; st, stomach; v, ven- 
tricle. 
kidney has the form of a more or less 
cylindrical sac folded on itself in such a 
manner that the pericardial and external 
orifices are both anterior. The lumen of 
the sac is large, its wall is glandular and 
uniform throughout its extent, and the 
two kidneys do not communicate with 
one another (Solenumya, Fig. 213). This 
essential character of a tube folded into a U-shape, so that 
one branch is antero-posterior and the other postero-anterior, 
is preserved throughout the Lamellibranchia, but as a result 
of specialisation the interior surfaces of the renal sacs are more 
and more increased by the formation of multiple folds, giving 
a spongy appearance to the organs. Sometimes, also, the 
terminal part or postero-anterior branch loses its excretory 
character and is modified to form a simple duct, which more 
or less surrounds the other branch (Unionidae, Fig. 242 (6), an, ar). 
