Centimeter 



Figure 254. — Visceral ganglion of a large C. vhginica seen through the epibranchial chamber, ad.m. — ventral side of the 

 adductor muscle; br.n. — branchial nerve; c.v.c. — cerebro-visceral connective; ki. — kidney; l.p.n. — labial palp 

 nerve; p.o. — pallial organ; py.p. — pyloric process; v.g. — visceral ganglion. The oyster was fully narcotized and 

 the adductor muscle completely relaxed. Drawn from life. 



ANATOMY 



The visceral ganglion, the largest unit of the 

 nervous system, is a wedge-shaped structure 

 embedded on the anteroventral side of the adduc- 

 tor muscle in a depression formed by the junction 

 of the translucent and opaque parts of the muscle. 

 To see the ganglion in its natural position one 

 must cut off the wall of the epibranchial chamber 

 and lift the tip of the pyloric process. The 

 location of the ganghon in such a preparation, 

 examined from the right side of the oyster, is 

 showTi in figure 253. The entire ganglion can be 

 examined in situ in an intact oyster. For this 

 purpose the oyster is narcotized until the valves 



gape, and a beam of light is directed into its cloaca 

 with the oyster's posteroventral end held toward 

 the observer; when the pyloric process is raised 

 slightly with a probe, the ganglion becomes 

 visible as a white or sometimes slightly yellowish 

 flat organ (fig. 254). In the preparation from 

 which the drawing was made the muscle was 

 completely relaxed and the gaping distance 

 between the valves measured about one-half inch. 

 Tlie visceral ganglion is higlily developed in all 

 Ostreaceae (Rawitz, 1887). The right and left 

 components, fused into a single organ, are dis- 

 tinguishable, and the ganglion appears to consist 

 of three parts, one central section and the two 



THE NERVOUS SYSTEM 



283 



