414 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
renal organ, the pallial vein (fig. 11, V. pall.), which brings 
back to the heart, directly, the blood from the mantle lobes. 
_ The mantle lobes have an extremely large system of 
vessels (fig. 11), which are usually injected along with 
the arterial system. The Pallial Vein can be first traced 
at a point just within the circle of attachment of the 
pallial muscles, posteriorly (fig. 11). From this point it 
proceeds anteriorly, gradually approaching the adductor 
muscle on its way, until finally it reaches and opens into 
the efferent branchial vessels. On both sides, but 
principally on the ventral, it gives off a series of branches 
which divide and re-divide, ramifying in the thickness of 
the mantle and forming a complete network which abuts 
on the circle of pallial muscles, and is connected by a 
series of fine passages between these to the cireumpalhal 
arteries (fig. 11, A.c.) already described. The mantle 
lobes are thin, and comparatively little metabolism goes 
on there, especially when one considers the great area of 
blood spaces, and it must be assumed that the mantle 
lobes play the most important part in the respiration of 
the animal. The blood from the mantle reaches the 
heart without having passed through the renal organ, so 
that the heart contains mixed blood—completely aerated 
blood from the mantle, and probably incompletely 
aerated blood from the gills. This mixed blood will pass 
both to the palhal respiratory surface and to the body 
generally, from whence it is collected and taken to the 
renal organs, then to the gills, and so back to the heart. 
NERVOUS SYSTEM. 
The nervous system of Pecten is of the typical 
Lamellibranch type. The usual three pairs of ganglia— 
cerebral, pedal and visceral—are present, though much 
modified in shape and position, In addition to these, 
