of the epibranchial chamber, then placing the 

 oyster on its posterior edge, lifting its ventral 

 side slightly toward tlie observer, and pulling the 

 dissected portions of tiie wall apart. The ganglion 

 and its nerves are then \'isible against, the back- 

 ground of the surrounding tissues. Some of the 

 individual nerves, namely, the posterior pallial 

 nerve (fig. 253, in ch. XIII, p.p.n.) and the lateral 

 pallial nerves (l.p.n.), emerge from the posterior 

 end of the ganglion and can be followed without 

 much difficulty until they begin to ramify. The 

 posterior pallial nerve follows the right side of the 

 adductor and sends a short branch to a sense 

 organ — a small unpigmented protulterance called 

 the pallial or abdomiiuil organ (p.o.). On the left 

 side of the oyster the pallial organ is much smaller 

 and is located much closer to the ganglion; in fact, 

 in many oysters only the right pallial organ is 

 present. 



The anterior pallial nerve (fig. 253, in ch. XIII, 

 a.p.n.) and branchial nerve (br.n.) leave the 

 dorsal end of the ganglion and for some distance 

 follow the nerve trunk which leads to the cerebral 

 ganglia and is known as the cerebrovisceral con- 

 nective (c.v.con.). The cerebral ganglia (fig. 71, 

 e.g.) are embedded in connective tissue at the 

 bases of the labial palps. A very thin cerebral 

 commissure passes dorsally over the esophagus 

 and connects the cerebral ganglia in a typical loop 

 or ring. The circumpallial nerve (fig. 72, cp.n.) 

 follows the circumpallial artery and can easily be 

 seen at the edge of the e.xpanded mantle. The 

 other nerves emerging from the visceral and 

 cerebral ganglia can be more conveniently studied 

 on sectioned preparations and are described in 

 chapter XIII. 



ANATOMICAL PECULIARITIES 



In several respects the anatomy of the oyster is 

 simpler than that of other bivalves. The absence 

 of a foot results in the lack of pedal ganglia; only 

 the posterior adductor muscle is present, and 

 there are no specialized organs of siglit, although 

 the animal is sensitive to change of illumination. 

 On the other hand, the edge of the mantle is 

 fringed with highly sensitive tentacles abundantly 

 supplied with nerves leading to the ganglia. As 

 in other bivalves, the nervous S3^stem is not 

 centralized but is represented by widely separated 

 ganglia. The structure of the cerebrovisceral 

 connectives, of the circumpallial nerve, and other 

 large nerves resembles more the structure of 



ganglia than that of the nerve, a condition which 

 undoubtedly results in a high degree of coordina- 

 tion among the various parts of the organism. 



In addition to performing their principal func- 

 tions, several organs of the oyster also participate 

 in other activities. The gills, for instance, are 

 not ou\y the organ of respiration but collect and 

 sort food as well. The mantle is used extensively 

 in the control of the flow of water through the 

 body; the coordinated action of the adductor 

 muscle, gills, and gonad is necessary for the 

 effective discharge of eggs by the female oyster. 

 In other animals such functions are performed by 

 special organs, but in the evolution of the oysters 

 the high degree of coordination developed among 

 different parts of the body eliminates the need for 

 specialized structures, and new and complex 

 functions are successfully performed by synchro- 

 nizing the work of the existing parts. 



BIBLIOGRAPHY 



AwATi, P. R., and H. S. Rai. 



1931. Oslrea cucidlala (the Bombay oyster). The 

 Indian Zoological Memoirs on Indian Animal 

 Types, III. Methodist Publishing House, Luck- 

 now, India, 107 pp. 

 Brooks, William K. 



1905. The oyster. A popular summary of a scien- 

 tific study. 2d ed. The Johns Hopkins Press, 

 Baltimore, Md., 225 pp. 

 Churchill, E. P., Jr. 



1920. The oyster and the oyster industry of the 

 Atlantic and Gulf Coasts. [U.S.] Bureau of Fish- 

 eries, Report of the Commissioner of Fisheries 

 for the fiscal year 1919, appendix 8 (Document 

 890), pp. 1-51. 

 Dahmen, Peter. 



1923. Anatomie von Oslrea chilensis Philippi. Jen- 

 aische Zeitschrift fiir Xaturwissenschaft, Band 52, 

 pp. 575-626. 

 Galtsoff, Paul S. 



1958. The oyster. Encyclopaedia Britannica, 1958 

 ed. vol. 16, pp. 1001-1004. Encyclopaedia 

 Britannica, Inc., Chicago, 111. 

 Leenhardt, Henry. 



1926. Quelques etudes sur Grayphea angulata (Huitre 

 du Portugal). Annalos dc I'lnstitut Oceano- 

 graphique, nouvelle serie, tome 3, fascicule 1, 

 pp. 1-90. 

 MooRE, H. F. 



1898. Oysters and methods of oyster-culture. U.S. 

 Commission of Fish and Fisheries, Part 23, Report 

 of the Commissioner for the year ending June 30, 

 1897, pp. 263-338. 

 Nelson, Thurlow C. 



1938. The feeding mechanism of the oyster. 1. 

 On the pallium and the branchial chambers of 

 Ostrea virginica, 0. edulis and O. angidaia, with 



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