wmmm^m. 



'■'^: 



.9 



& 



..■■«? 



■!^-. 



Microns 



50 



Figure 266. — Section of a small part of the ciliated 

 covering of the pallial organ of C. virginica showing 

 sensory cells between the ciliated epithelial cells. 

 Kahle, iron hematoxylin. 



for receiving and reacting to vibration. He 

 found that this organ is fully developed in 4- to 

 S-day-old spat. It is not known whether the 

 organ is present in adult 0. edulis or in C. virginica. 



SENSORY STIMULATION 



Very little experimental work has been con- 

 ducted on the physiology of the nervous system of 

 the oyster. Most of the research on neuro- 

 physiology of otlier l)i valves (Pecten, Ali/tilus, Mya, 

 and Anodonta) dealt with the action potentials, 

 tonus of the adductor muscle (see: ch. VIII), and 

 stimulation of the siphons of Mya (Hecht, 1919a, 

 1919b, 1920a, 1920b; Pieron, 1941) and Pholas 



(Hecht, 1928). From a study of the action 

 potentials along the nerve trunks of the siphon of 

 the soft shell clam, Pieron calculated that the 

 velocity of the transmission of stimuli along the 

 nerve of this mollusk is of a magnitude of several 

 meters per second. The value is probably common 

 to other bivalves. 



Neuro-secretory cells are present in a number of 

 marine bivalves (Nucula, Anomia, Mytilus, Modio- 

 lus, Chlamys, Lima, Donax, Arcopagia, Macfra, 

 Cardium, Venerupis, Venus, and Paulora) and 

 probably may be found in other genera including 

 the oyster (Gabe, 1955). These cells are typical 

 small neurones of the cerebral and visceral ganglia 

 but are absent in the pedal ganglia (Lubet, 1955). 

 The amount of secretion they contain varies with 

 the season and is apparently related to or parallels 

 the sexual cycle for it increases with the matura- 

 tion of sexual products (see: ch. XIV, page 312). 



Sensory stimulation of the tentacles of the 

 oyster by chemicals was studied by Hopkins 

 (1932a, 1932b). He measured their sensiti^^ty by 

 determining with a stopwatch the latent period, 

 i.e., the time elapsed between the application of a 

 chemical and retraction of a tentacle or a group of 

 them. The method is very simple. The mantle 

 is exposed by cutting off a portion of shell, and the 

 oyster is placed in sea water running at a constant 

 speed through a rectangular tank with two com- 

 municating parts, one of them shallow and the 

 other several inches deeper. Water in the taller 

 part is kept at a constant level. The oyster is 

 placed in the shallow portion of the tank, and a 

 vessel containing the solution to be tested floats in 

 the taller part. A three-way stopcock is mounted 

 on the wall separating the two parts of the tank, 

 one branch of the stopcock is bent horizontally and 

 ends in a capillary nozzle placed a short distance in 

 front of the tentacles. The two other branches are 

 fitted with flexible rubber tubing at the end; one is 

 lowered into sea water, the other into the floating 

 vessel with the test solution. At the beginning of 

 the test the stopcock is turned to deliver a con- 

 stant, gentle stream of sea water to the tentacles, 

 which remain in a relaxed state as long as the 

 ciuTent and temperature of the water are constant. 

 The stopcock is turned abruptly, and the sea water 

 is suddenly replaced by the solution to be tested. 

 The total time from the turning of the stopcock to 

 the observed retraction of the tentacles is measured 

 with the accuracy of one-tenth of a second. Before 

 making a test the time required for a test solution 



THE NERVOUS SYSTEM 



293 



