178 
ANODONTA — GLANDULAR SYSTEM. 
outer gills are long vertical ridges of solid lacunar tissue placed at 
intervals of about every seventh filament, and supporting a blood 
vessel at each side. The interlamellar junctions of the inner gills are 
hollow and vascular, and placed at intervals of about twenty filaments 
apart, alternating in position on the primary and secondary limbs of 
the gill. 
The transverse filaments are wavy transverse rods crossing the 
straight vertical filaments at regular intervals, and thus forming a 
regularly trellised pattern and enclosing vertically oblong interfila- 
mentary spaces, within which are the openings of undulatory rows 
of minute, irregular and somewhat ohliipie fenestra' or water passages, 
varying in size from to yoVc of an inch in diameter, which pass 
through the suhfilamentar lacunar tissue of the gill cavity. 
The (tLandular system in Anndontd is practically identical, in its 
broad features, with that described in lleli.r, the mantle, and tissues 
generally, presenting a very glandular structure, the glands being 
formed of differentiated epithelial cells, with calcareous, pigmented 
or mucous contents. Lime cells, capable of secreting nacreous or 
shelly matter, are so plentifully distributed over almost every part of 
the surface of the animal, that the stimulus or irritation of its delicate 
tissues liy the accidental presence of any extraneous inorganic or 
organic particles, immeiliately leads to the formation of pearls or 
l)early concretions l)y the concentric deposition around the intruding 
object, of calcareous substances similar in structure to the inner layer 
of the shell itself, but varying somewhat in character according to the 
j)osition in which the intruding i)articles may have become lodged. 
The Nephridia, Renal organs, or organs of Bojanus are symmetrically 
l)aired, tubular and sac-like, glandular or excretory organs, which are 
each folded ipjon themselves at the posterior end, the ventral or 
secretory limbs having their walls thrown into thick si)ongy brown 
folds, clothed with blackish ciliated epithelium. Each dorsal limb, or 
Ureter, is a thin-walled tube, very wide posteriorly, whose upper walls 
are continuous with, and inseparable from, the floor of the pericardium, 
while its floor is continuous with the roof of the glandular section 
which it overlays. 
The organs are placed side by side beneath the pericardium and in 
front of the i)Osterior adductor, but, although separated by the Vena- 
cava or groat central blood vessel, the excretory i)ortion or ureter of 
each kidney is united by a wide inter-renal oval space near the anterior 
