232 



THE LAMELLIBRANCHIA 



outside the ctenidial branchiae, in the form of little secondary pallial 

 outgrowths, constituting the folded organs called " frills." 



3. Excretory 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, k). 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,^?). 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, i'i>\ and 

 in Anoinia 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 

 kidney has the form of a more or less 



Fio. 21'J. 



dorsal aspect, a.a, anterior cylindrical sac folded on itself in such a 



adductor; an, auricle; </, right , A .r j- i i i 



genital gland; g.o, genital manner that the pcncardial and external 



."^iiS" orifices are both anterior. The lumen of 



cardinm ; ;v.f7, pericardia} the sac is large, its wall is glandular and 



gland; re, rectum; r.o, ronal ., . 



opening; rp, renopericardiul Uniform throughout its extent, and the 



oriflee; K, stomach; *, ven- two _ fctfneys j o not communicate with 

 one another (Solenomya, 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 (C), an, or). 



