CHAPTER XIII 

 THE NERVOUS SYSTEM 



Page 



Methods __ 281 



Anatomy 283 



Microscopic structure 285 



The pallia! organ 289 



Sensory stimulation 293 



Bibliography __. 295 



The nervous system of the oyster is rehitively 

 simple. The visceral and cerebral gangha are 

 joined by the cerebro-visceral connectives; the 

 U-shaped cerebral commissure goes around the 

 esophagus; tlie circumpallial nerve travels along 

 the mantle's edge; and a number of nerves 

 originate from the ganglia and extend to different 

 parts of the body. The pedal ganglia, well 

 developed in many other bivalves, and tlie cerebro- 

 pedal connective are absent. These retrogressive 

 features are associated with the sedentary mode 

 of Ufe of the oyster and the loss of the organ of 

 locomotion (foot). In the evolution of bivalves 

 this simplification of the anatomy represents an 

 adaptive change and cannot be regarded as a 

 primitive trait (Jhering, 1877). 



The only organs of sense in the oyster are the 

 tentacles, along the edge of the mantle, and the 

 pallial organ inside the cloaca. The tentacles 

 are highly sensitive to changes in illumination; 

 they contract if a shadow passes in front of a 

 feeding oyster, or a beam of light is focused on 

 them. They also detect the presence of minute 

 quantities of various drugs, chemicals, excessive 

 amounts of suspended particles, and changes in 

 temperature and composition of sea water. The 

 function of the palUal organ is not well understood; 

 the organ is probably concerned with the detec- 

 tion of mechanical disturbances in the surrounding 

 water. 



The eyes found in freely moving bivalves, such 

 as scallops, are absent in adult oysters but are 

 present in fully grown larvae. 



There is no major control center (brain), and 

 in this sense the nervous system is decentralized. 

 The integi-ation of its various parts is accomplished 

 by the interconnections of the ganglia through 



FISHERY bulletin: VOLUME 64, CHAPTER XIII 



the cerebro-visceral connectives, cerebral com- 

 missm-e, and the larger nerve trunks. All these 

 parts contain groups or nuclei of nerve cells and 

 have a structure similar to that of the ganglia. 



In oysters the two visceral ganglia are fused 

 into a single organ. Its double origin is clearly 

 visible on tangential sections. 



The distribution of nerves in bivalves was 

 studied by many zoologists of the 19th century 

 (Garner, 1837; Duvernoy, 1854; Jhering, 1877; 

 Rawitz, 1887, Babor, 1896; Pelseneer, 1891; 

 Freidenfelt, 1896; Gilchrist, 1898) who described 

 the principal topographic features of the nervous 

 system of various species. The nerves of the 

 European oysters were quite accurately depicted 

 in the old paper of Duvernoy; a general de- 

 scription of the nerve system of 0. chilensis was 

 given by Dahmen (192.3); of C. angulata by 

 Leenhardt (1926) , and of 0. cucullata by Awati 

 and Rai (1931). 



METHODS 



Anatomical dissection of the nervous system of 

 the oyster is rather difficult. The nerves are 

 small, unpigmented, and are embedded in con- 

 nective tissue. In fully ripe or in so-called "fat" 

 oysters even the principal nerves of the visceral 

 mass are hidden under a thick layer of gonad or 

 are covered by large quantities of glycogen. The 

 lean and watery specimens usually found shortly 

 after spawning are most suitable for dissection. Im- 

 mersion in 10 percent nitric acid, followed by 

 washing and clarification in glycerol, may be useful 

 because the nerve tissue is stained by the acid a 

 light brown color. The entire nervous system 

 may be stained in toto by using the following 

 procedure: oyster tissue preserved in 95 percent 

 ethyl alcohol or in 5 percent formalin is transferred 

 into 1 percent aciueous solution of potassium hy- 

 droxide (solution No. 1) for 1 to 3 days. The 

 specimen is then placed in solution No. 2 made 

 of 1 part glacial acetic acid, 1 part glycerol and 6 



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