HORNELL ANATOMY OP PLACUNA 85 



Tfie parieto-splanchnic ganglion is the largest of the gauglionic centres and 

 attains relatively enormous proportions. It is not distinguishable into separate parts 

 further than may be suggested by the two small anterior lobes where the ecrebro- 

 visceral connectives enter. It rests in a slight depression on the anterior ventral 

 curvature of the adductor close to the ventral extremities of the paired region of the 

 nephridia. 



In addition to the connectives to the cerebrals, two distributory trunks, the 

 branchial nerves (N.br.), leave it anteriorly, while two other large nerves, the right 

 and left posterior common pallial, emerge from its hinder border, and a second pair, 

 the adductor nerves, pass from the posterior aspect direct into the tissues of the 

 adductor muscle. 



The two branchial nerves appear symmetric and of equal size. One leaves each 

 anterior lateral corner of the ganglion, passes forwards, and at once enters the 

 branchial mesentery ; then, abruptly and at an acute angle, it curves posteriorly, to 

 run the remainder of its course approximately parallel with the ctenidial base. At 

 first the branchial nerve runs at some distance from this axis, but gradually draws 

 nearer as it approaches the posterior extremity of the gills, where it terminates. Each 

 nerve gives off numerous branches in its course. 



The adductor nerves, as mentioned, arise right and left from the posterior surface 

 of the ganglion. Their sole function is to innervate the adductor muscle ; in saggital 

 serial sections through young individuals their dcntritic branching is beautifully 

 shown (fig. 29). 



The remaining nerve trunks, the great pair given off from the posterior border, 

 each breaks up immediately on leaving the ganglion into an inner or internal nerve 

 and several lateral ones. The first two of the latter, the pallial nerves (N.pall.), 

 radiate outwards to innervate portions of the mantle and, with the nerves from the 

 anterior common pallial trunks given off from the cerebral ganglia, form that complex 

 network of nerves within the mantle lobes known as the pallial plexus, while the third 

 passes posteriorly without turning outwards till, when close to the distal end of the 

 pyloric caecum, it bends ventrally, to pass direct to the elongated tumid pallial sense 

 organ wherein it bifurcates. 



The inner nerves (N.v.v.} pass posteriorly within the sheath of the adductor 

 muscle in the direction of the anus. They appear to innervate the tissues and organs 

 of the ventral rectal region. 



The only sense organs present are of an extremely low type, showing a minimum 

 of specialisation. Neither otocyst nor osphraclia appear to be present, A pair of the 

 pallial or abdominal organs of Thiele are, however, present. With this exception, all 

 we have is the development of sensory epithelial cells singly and in small groups upon 

 the papillate processes of the mantle edge. In general they have the structure and 



arrangement of those seen upon the pallial edge of the pearl oyster. The general 



H 2 



