82 



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 circumpallial 

 arteries (fig. 11, A. c.) already described. The mantle 

 lobes are thin, and comparatively little metabolism goes 

 on there, especially when oue 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 Idood — -completely aerated 

 blood from the mantle, and probably incompletely 

 aerated blood from the gills. This mixed blood will pass 

 both to the pallial 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, thoiigh much 

 modified in shape and position. In addition to these, 



