Except for the pair of cerebro-visceral connec- 

 tives, only a few nerve trunks originate from the 

 cerebral ganglia. The common pallial nerve (figs. 

 253 and 255, com.p.n.) enters the mantle of the 

 corresponding side and establishes connection 

 with the dorsal portion of the circumpallial nerve. 

 The labial nerves (Ib.n.) branch to the four 

 labial palps. 



The circumpallial nerve (fig. 253, c.p.n.) is a 

 faii-ly large nerve trunk which runs parallel to the 

 edge of the mantle. Throughout its course it is 

 accompanied by the circumpallial artery and 

 supplies numerous branches to the tentacles and 

 makes connections to the radial nerves which 

 extend from the base of the mantle to its edge 

 (see: fig. 86 in chapter V). The circumpallial 

 nerve is connected anteriorly with the cerebral 

 ganglia and posteriorly ends in the visceral 

 ganglion. Stimuli received anywhere on the 

 mantle may be transmitted to the entire nervous 

 system of the oyster by this circular nerve. 

 Because of this arrangement the oyster may 

 respond to stimuli as a whole in spite of separation 

 of its nerve centers. 



The cerebral and visceral ganglia also are joined 

 by a pair of relatively broad and long cerebro- 

 visceral connectives (fig. 253, 255, c.v.c), which 

 constitute the main nerve trunks through which 

 communication is maintained with all the parts 

 of the body. 



MICROSCOPIC STRUCTURE 



The ganglia are formed of a central core or the 

 neuropile, tightly packed bundles of nerve fibers, 

 and the cortex made of several layers of nerve 

 cells. This arrangement gives the ganglia a 

 resemblance to the white and gi'ay matter of the 

 central nervous system of vertebrates. A layer 

 of loose connective tissue forms the outer sheath 

 of the molluscan ganglia. 



The visceral ganglion is a relatively large, 

 wedge-shaped structure embedded on the ventral 

 side of the adductor muscle in the depression 

 between its two parts (fig. 256). In the oyster 

 the ganglion is completely fused but its paired 

 origin is clearly seen on a tangential section 

 (fig. 257). 



The cortex is made of a continuous layer of 

 nerve cells. Scattered nerve cells also occur in 

 the neuropile. The ventral side of the ganglion 

 facing the epibranchial chamber is covered with 

 a unicellular layer of epithelium. 



THE NERVOUS SYSTEM 

 733-851 O— 64 19 



The nerve cells which in the oyster and other 

 bivalves form the cortex were described by 

 Rawitz (1887), whose paper remains the major 

 contribution to the histology of the molluscan 

 nervous system. To obtain the entire nerve 

 cells with their axons Rawitz macerated small 

 pieces of ganglia in 25 percent ethyl alcohol for 

 4 to 5 hours or in an aqueous solution of potassium 

 bichromate (from 0.025 to 0.1 percent) for 8 to 24 

 hours. 



Individual nerve cells of the oyster are either 

 pear- or club-shaped with one, two, or several 

 processes extending from their bodies. These 

 processes give rise to fine fibrillae which enter the 

 neuropile. Depending on the number of the 

 processes, the cells are called unipolar (fig. 258, 

 a, b), bipolar (e), and multipolar (c, d). The 

 unipolar cells are more abundant than the other 

 two types. The apolar cells, i.e., those without 

 the processes, have not been found in bivalves, 

 according to Rawitz. 



The size of nerve cells varies. The multipolar 

 cells are usually the largest; their dimensions, with- 

 out the processes, range from 14.5 ^ by 5 m to 20 /i 

 by 8 M- The unipolar and bipolar cells are smaller, 

 varying in size from 12.5 m by 4 m to 14 m by 6 m- 

 The tapered ends of the cells give rise to the nerve 

 fibers, which enter the neuropile where they 

 combine with other nerve fibers to form several 

 compact bundles. Single bipolar and unipolar cells 

 are scattered throughout the neuropile, but do not 

 aggregate into distinct nuclei or groups. The 

 protoplasm of the nerve cell is dense and is deeply 

 stained with Ehrlich's and Delafield hematoxylin. 



The nerve cells and their axons are supported by 

 a framework of connective tissue cells which 

 descend from an outer sheath of the ganglion. 

 These cells were observed first by Freidenfelt 

 (1897). Bochenek (1906) and Jakubski (1912, 

 1913) described tlie supporting elements in the 

 ganglia of Anodonta, Pinna, and several gastropods, 

 tunicates, and echinoderms, and regarded them 

 as typical glia cells. Jakubski distinguished three 

 groups of glia cells: (1) Star-shaped flat cells with 

 oval nuclei and thin processes which continue as 

 neuroglia fibers were found in the outer sheath of 

 the ganglion, (2) spindle-shaped cells with a 

 pointed nucleus usually have two outgrowths and 

 are most common in the inner portion of the 

 ganglion for which they form a supporting frame- 

 work; and (3) "neuropile glia cells", which occur 

 singly, scattered through the neuropile. Jakubski 



285 



