34 
RENAL ORGANS. 
and excretory oritices of each gland being placed anteriorly, the 
excretory opening being confluent with or near the genital aperture 
and the excreta, especially in the more archaic genera, being expelled 
in a Huid form. 
In the more advanced genera, as in the Unionidiv, etc., the secretory 
walls become more complicated and the glandular surface augmented 
in the proximal or ventral limb, to which it becomes chiefly restricted, 
Fig. 620. Fig. 621. 
Figs. 620 and 621. — Transverse sections through two regions of the renal organs of Anodonta 
cys^nea (L.), highly magnified (after Rankin). 
k. secretory section of kidneys ; u. ureter ; h.s. blood sinus ; /, foot ; c.pl.v.c. section of cerebro- 
pleuro-visceral commissure. 
the distal or superior limb forming the duct to convey the secretions 
to the exterior and opening into the mantle cavity, always above the 
cerebro-pleuro-visceral commissure ; but sometimes, as in Sphevrium, 
etc., within a common urogenital cloaca; in most groups, however, the 
renal and genital apertures are distinct, although the kidneys them- 
selves, as well as their ureters, communicate freely with each other, 
especially in the more specialized forms. 
In the Streptoneura the primitively paired character of the kidneys 
still persists in a more or less distinct form in the Diotocardia, yet 
the right kidney, though still connected with the pericardium and 
mantle-chamber, has entirely lost its renal function and has become 
utilized as a channel for the conveyance 
to the exterior of tlie genital products ; 
the left kidney, however, .still retaining 
its exclusively renal and secretory 
character. 
In other Streptoneures the left kidney 
is alone present in adults iu correlation 
with the retention of the moieties of 
other paired organs, and is situate to 
the left of the rectum, though actually 
traversed b)' that organ in some genera. The right kidney has, how- 
ever, also been detected to be transitorily present in Vivipara, etc., 
during embryonal development, its ureter, however, persisting in the 
adult, but acting solely as a genital duct. 
Fig. 622 . — Sttcctnea putris (L.). 
X 3, showing position of kidney or 
renal organ. 
r. renal organ or kidney ; h. pul- 
monary plexus ; i. intestine; 4. heart. 
