174 BULLETIN OF THE BUREAU OF FISHERIES. 



relation of the syphonal nerve to the posterior ventral pallial nerve is shown in figure 



161, A short branch springs from the upper end of the syphonal nerve and connects 

 above with the posterior dorsal pallial nerve in such a way as to form a triangle in the 

 mantle edge immediately behind the posterior adductor muscle (fig. 162). 



The dorsal pallial nerve arises as a fine fiber from the anterior outer side of the 

 visceral ganglion and runs forward and upward between the posterior retractor muscles 

 into the mantle, where it divides into an anterior and a posterior branch. The branch 

 which takes the anterior course sends out small nerves into the mantle edge, while the 

 posterior one makes connection with the posterior dorsal pallial nerve (figs. 159 and 



162, DPN). 



The posterior renal -nerve goes out from the visceral ganglion in close connection 

 with the dorsal pallial nerve and supplies the kidney (fig. 163, PRN). 



The branchial nerve arises from the outer posterior side of the visceral ganglion and 

 runs obliquely downward and backward to the base of the gills, which it follows to the 

 posterior extremity. Throughout its course, but more so at its beginning, it gives off a 

 great number of very fine fibrils that run in a mass anteriorly along the axis of the gills 

 (fig. 159, BrN, p. 171; fig. 163, BrN). 



The posterior adductor nerve is a small branch given off from the median side of the 

 visceral ganglion. It supplies the posterior adductor muscle and the pallial sense 

 organs in that region (fig. 163, PAN). Closely associated with the nerve endings in the 

 posterior adductor muscle Galeazzi (1888) found great numbers of ganglion cells of 

 both the unipolar and bipolar type. They form in the muscle a very fine reticulum, so 

 that a nerve fiber is able by its ramifications to innervate many muscle fibers. This 

 considerable number of ganglion cells between the bundles of muscles led Galeazzi to 

 conclude that an automatic nerve center was present in the muscle itself which explained 

 the great power of the adductor muscle in bivalves. 



The posterior pedal nerve is an unpaired fiber which springs from the middle of 

 the visceral commissure on its ventral surface. It runs posteriorly a short distance, 

 and then, turning sharply downward and forward, it passes under the commissure and 

 runs forward on the ventral wall of the body to the foot (fig. 159, PPdN, p. 171 ; fig. 



163, PPdN). 



The relation of the various nerve centers and their communications is diagrammat- 

 ically represented in figure 163. 



SENSE ORGANS. 



ANATOMY. 



The sense organs of the sea mussel fall naturally into five groups: (1) sensory cells 

 in the epidermal layer, (2) a pair of osphradia, (3) a pair of abdominal sense organs, (4) 

 a pair of otocysts, (5) a pair of eyes and extensive areas of light receptive epithelium. 



The sensory cells of the epidermal layer are present in scattered groups or as single 

 elements in the wall of the mantle cavity. The groups of cells which are sometimes 

 referred to as the pallial sense organs (fig. 164, p. 178) are particularly abundant on the 

 ventral epithelium of the posterior adductor muscle. The single sense cells, first de- 

 scribed by Flemming (1870) as pinsehellen (fig. 167, p. 178), are scattered all over the 

 inner walls of the mantle. 



