



Microns 



50 



Figure 263. — Transverse section of branchial nerve of C. virginica. Gill muscles are above the nerve. Kahle, hematoxy- 



lin-eosin. 



Microns 



Figure 264. — Radial nerve in the mantle of C. virginica. 

 The sheathing of connective tissue forms the periphery. 

 Here the nerve consists of three distinct nerve trunks. 

 Blood cells are scattered in the connective tissue of the 

 mantle. Kahle, hematoxylin-eosin. 



bodies and long processes which extend I'roni tlie 

 surface of the organ deep into the underlying con- 

 nective tissues. The nuclei are elongated, and the 

 cells are deeply stained with iron and alum hema- 



toxylin. A large nerve trunk, shown in figure 

 265, sends its branches to the surface of the pallial 

 organ. The nerve entering the organ is a branch 

 of one of the posterior pallial nerves which emerge 

 from the visceral ganglion. Typical vesicular 

 connective tissue forms the core of the organ. 



There has been no experimental study of the 

 pallial organ, and its physiology is not well under- 

 stood. Elsej^ (1935) expressed the opinion that 

 its primary function is the regulation of respiratory 

 current. Haas (1935) thought that in Lima the 

 pallial organ is primarily concerned with chemical 

 testing of water, and this \aew is repeated by 

 Franc (1960) in his review of Bivalvia. 



The long, stiff cilia seem to be more suitable for 

 detecting mechanical disturbances than for 

 responding to chemical stimuli. The organs of 

 chemical taste in the oyster are the tentacles of 

 the mantle. Because of their location at the edge 

 of the mantle they are the first ones to come in 

 contact with the irritating or poisonous substances 

 in water and give the oyster a signal to prevent 

 its access into the pallial cavity. It appears 

 unreasonable that an organ of chemical taste 

 should be located inside the water transporting 

 system near its end. Dahmen (1923) and Awati 

 and Rai (1931) think that the pallial organ in 

 bivalves is primarily the organ for detection of 



THE NERVOUS SYSTEM 



291 



