Nervous Systetn of the Razor-shell Clam 317 



and by a circum-pallial nerve, cpn, that follows the margin of the 

 mantle lobe. There is no indication that sensory impulses ever 

 travel from one ganglion to the other through the circum-pallial 

 nerve. All of the cerebro-visceral association fibers seen to be 

 contained in the cerebro-visceral connectives. Soon after leaving 

 the visceral ganglion each paUial nerve gives rise to a branch that 

 supplies the posterior adductor muscle, Fig. i, pan. It is then 

 continued posteriorly and ventrally, sends many branches to the 

 siphonal region and then turns anteriorly along the border of the 

 mantle as the circum-pallial nerve, cpn, which joins the cerebral 

 ganglion. What service is performed by this connection is not 

 clear unless it is to afford overlap for the distribution of the motor 

 fibers from the two ganglia to the margin of the mantle. Sensory 

 fibers from the siphons all seem to go to the visceral ganglia, and 

 from the tentacles around the ventral opening in the mantle, to 

 the cerebral ganglia. For the siphons this is easily determined 

 by cutting the pallial nerves between the nerves that supply the 

 siphons and the visceral ganglia, when stimulation of the siphons 

 causes no response, and for the tentacles around the ventral open- 

 ing by cutting the circum-palUal nerves between the tentacles and 

 the cerebral ganglia, after which stimulation of the tentacles 

 causes no response. If the cut is made between the siphons and 

 the ventral tentacles, the effect of stimulating either portion seems 

 entirely normal. The motor fibers of these nerves are hard to 

 experiment with but it is evident that most of the mantle muscles 

 posterior to the ventral opening are supplied by fibers from the 

 visceral ganglia. Anterior to this opening, along the path of the 

 circum-pallial nerves, the muscles are not very well developed. 

 Abranchial nerve. Fig. 2, bn, leaves each visceral ganglion to pass 

 anteriorly and laterally to the united lamellae of the corresponding 

 pair of gills. The physiology of these nerves has not been studied. 

 Upon stimulation of the isolated visceral ganglia, slight contrac- 

 tions of the posterior foot muscles have been observed, but this 

 may have been caused by escaped current from the electrodes. 

 Nerves from the visceral ganglia to these muscles have not been 

 found. The supply of nerves to the heart and the cardioinhibi- 

 tory action have not been studied. 



