THE LAMELLIBRANCHIA 233 
In other cases the differentiation into two branches folded one over 
the other disappears through the reduction of the antero-posterior 
branch ; the kidney is then a simple sac into which the reno- 
pericardial duct opens. This may be seen in some Filibranchia, 
The two kidneys do not communicate with one another in 
Solenomya and in many Filibranchia, but a communication between 
their anterior ends is already present in Yoldia and Leda, and 
a communication, often of considerable extent, between their 
posterior ends is found in the majority of the more specialised 
forms, ¢g. Modiolarca, Lasaea, Donax, Tapes, Pseudokellya, and 
especially in the Myacea, Pholadidae, and Anati- 
nacea. The renal organs are excessively ramified 
and extend over the whole surface of the visceral 
mass in Ostraea, and both in this genus and in 
Pholas they surround the posterior adductor 
muscle. The kidneys similarly extend very far 
forward in Mytilus and in the majority of the | 
Anatinacea, penetrating into the mantle itself ny : 
in Lyonsiella. Finally, in the Septibranchia the Fie. 213. 
kidneys are almost wholly immersed in the Left kidney of Solenc- 
=, S = 5) Ne Tr mya mediterranea, ventral 
pallial sinus (Fig. 21 I, XIV ). aspect. I, posterior part 
In the most archaic Lamellibranchia the Saini oes aU ete 
renal secretion is passed out of the body in pericardial orifice’; IV, 
= : : 5 duct of the gonad. 
a liquid form, but in others in the form of 
solid concretions, exhibiting concentric layers of growth, and in 
normal conditions containing only urea. 
The excretory function is carried out by the pericardial glands 
as well as by the kidneys. These glands are differentiations of 
the epithelial wall of the pericardium, and may be localised on 
and impart a brownish tint to the auricles, as in many Filibranchia 
(Arcidae, Mytilidae, Pectinidae) and the Ostraeacea, or they may 
be near the auricles as in the Aviculidae. In this condition 
they are less well developed in the more specialised groups, but 
they reappear in Pholas and Sazxicava. The glands frequently 
exist in the form of a glandular lining of the anterior part of 
the pericardium or of the glandular diverticula which pass from the 
pericardium into the mantle (Fig. 212, peg). This arrangement 
may be seen in the Unionidae—the diverticula constituting the 
“organ of Keber ”—in certain Lucinidae, Veneridae (Fig. 202, pe.g), 
Tellinidae, Solen, Pholas, and Aspergillum. In the last-named 
genus these diverticula used to be called pericardial veins at a 
time when the pericardium was thought to be a blood sinus. The 
pericardial glands excrete hippuric acid, which is poured into the 
pericardium, and from thence passes into the kidneys. 
4. Nervous System and Sense-Organs.—The different pairs of nerve- 
centres are always placed at some distance from one another in the 
e 
