sectioned material of the oyster ganglia. The 

 crossing of some fibers from the right to the left 

 side and then- extension into the connective can be 

 seen, but there is no way to estimate the relative 

 abundance of these fibers. There is not enough 

 evidence to establish with certainty which fibers 

 run from the nerve cells of the connective into the 

 ganglion and which follow the opposite direction. 

 The tissues of the cerebral ganglion are less 

 compact than those of the visceral ganglion, and 

 the entire organ is rather indistinctly separated 

 from the underlying connective tissue (fig. 259). 

 The cerebral ganglia are located on each side of 

 the oyster directly under the surface epithelium 

 but separated from it by a thin layer of connective 

 tissue fibers. The inner part of the cerebral gan- 

 glion (fig. 260) consists of bipolar cells of medium 

 size. The outer layer, corresponding to cortex, is 

 made up of large unipolar and multipolar nerve 

 cells with dense protoplasm, stained dark with 

 hematoxylin. Loose connective tissue covers the 





cv.c 



Millimeters 



Figure 259. — Longitudinal section of a portion of thu 

 cerebral ganglion, (e.g.) at the base of the labial palps 

 of C. virginica; cv.c. the beginning of the cerebro-visceral 

 connective. Kahle, hemato.wlin-eosin. 



TiST 



Microns 



50 



Figure 260. — Small portion of a longitudinal section of 

 the cerebral ganglion of C. virginica under the epithelium 

 of the base of the labial palps. Large unipolar and 

 multipolar nerve cells form a cortex layer. Medium 

 sized bipolar nerve cells and nerve bundles occupy the 

 central part of the ganglion. Loose connective tissue 

 covers the structure (right side of the drawing). Kahle, 

 hematoxylin-eosin. 



ganglion on the outer side. The entire organ is 

 less compact than the visceral ganglion. 



The stiiicture of the cerebro-visceral connective 

 is similar to that of the ganglia (fig. 261). A 

 thick layer of large nerve cells surrounds the nem-o- 

 pile, which is divided into several bundles. There 

 is no well formed sheathing, but small connective 

 tissue cells are found along the periphery of the 

 ganglion and are scattered throughout its entire 

 structure. 



A similar pattern of ganglionic structure repeats 

 itself in many nerves emerging from the ganglia. 

 This plan of organization is found in the circum- 

 pallial, branchial, and many other nerves. 



The circumpallial nerve is surrounded by a 

 sheath of connective tissue fibers (fig. 2G2). The 



288 



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