bloomer: anatomy of pharella and tagelus. 117 



braiiclis and run approximately 26 in Solen vagina, 17 in Solen ensis, 12 

 in Solen fonesi, 22 in Solen [Solena) rudis and IG in Solen (Pharella) 



orientalis The two or three fihiments at the apex of the pUca 



are enlarged in Solen fonesi and Solen orientalis In Solen 



orientalis there is every gradation from a shallow frontal groove to a 

 shallow frontal ridge, even in filaments cut at the same horizontal level. 

 In all five cases the frontal groove disappears at the ventral edge of the 

 demibranch, where the principal filament presents a distinct frontal 

 ridge." 



Nervous System. 



The cerebro-pleural ganglia lie lateral to the mouth, under the 

 retractor pedis anterior muscles, and between them and the ventral 

 integument. The commissure connecting the two ganglia goes in front 

 of the mouth. Each ganglion apparently gives rise to only one anterior 

 nerve, the anterior pallial nerve, which passes gradually outward to the 

 posterio-ventral edge of the anterior adductor muscle, but divides shortly 

 before reaching it, the inner branch going underneath and innervating the 

 muscle, the outer one passing across the mantle lobe and again dividing, 

 the anterior part once more proceeds and divides; both branches join 

 the outer circumpallial nerve, while the posterior part goes posteriorly 

 as the inner circumpallial nerve. 



Posteriorly each cerebro-pleural ganglion communicates with the 

 viscero-parietal ganglion by a connective. The connective first proceeds 

 between the retractor pedis anterior muscle, and the pedal wall, then 

 emerging, runs between the latter and the viscera, and, reaching the 

 anterior portion of the retractor pedis posterior muscle, passes through 

 the pedal integument to the lateral surface of the muscle and then under- 

 neath to the viscero-parietal ganglion. 



The cerebro-pedal connective leaves the cerebro-pleural ganglion on 

 the inner side of the cercbro-visccral connective, and passing partly 

 through and then along the pedal muscles, gives off a nerve to the viscera 

 and afterwards joins the pedal ganglion. 



The pedal ganglia are situated in the centre of the foot, midway 

 between the dorsal and ventral surfaces, just over the anterior end of the 

 anterior folds of the intestine. The ganglia give off on each side a number 

 of nerves which innervate the foot. 



The viscero-parietal ganglia arc situated under the posterior adductor 

 muscle. From each ganglion arises a branchial nerve which, first goes 

 some distance anteriorly and curving outwards passes to the gills. Pos- 

 teriorly each viscera-parietal ganglion gives off a nerve, the posterior 

 pallial nerve. It passes latero-posteriorly across and under the posterior 

 adductor muscles, then there arises from it a nerve which crosses the 

 mantle lobe and joins the inner circumpalUal nerve, and afterwards 

 the outer circumpallial nerve. The main nerve goes along the proximal 



