CIRCULATORY SYSTEM OF THE GIANT SCALLOP. 237 



connective, just before it joins the ganglion proper, there is a 

 ganglionic swelling (x) that supplies one of two roots of a 

 nerve (Figs. 6, 8, and 10, b)i) that leaves in an antero-dorsal direc- 

 tion along the border of the excretory organ, to bend ventrally 

 and posteriorly in the suspensory membrane of the gills, and sup- 

 ply the gills of the corresponding side. Between the points 

 where the cerebro-visceral connectives join the visceral ganglia 

 on the ventral side, there are four rather distinct swellings, with 

 three less distinct swellings posterior to them. Extending later- 

 ally from the outer side of each ganglion is a somewhat flattened 

 ridge (Fig. 10, j') from which all of the pallial nerves from this 

 ganglion originate. These nerves (Figs. 6 and 8, //w)pass laterally, 

 posteriorly and anteriorly along the surface of the adductor muscle, 

 to meet the mantle lobes and to be continued to the margins, 

 where they unite with the circumpallial nerves. It will be noticed 

 that they unite with the circumpallial nerve at intervals through- 

 out the greater length of this nerve. As the pallial nerves 

 that leave the visceral ganglia in most forms pass directly to 

 the posterior portion of the mantle, the distribution in this 

 form may be looked upon as evidence that all of this portion of 

 the mantle belongs morphologically to the posterior portion of 

 the animal. 



Other nerves leave the dorsal surface of the visceral ganglia 

 near their posterior ends, and enter the adductor muscle directly. 

 The nerves that supply the posterior division of the muscle are 

 continued along the ventral surface of the anterior portion of 

 the adductor muscle until this posterior portion is reached. 

 Small nerves also leave the ventral side of the ganglia and pene- 

 trate the visceral mass. 



All of the ganglia are well supplied with nerve cells, there 

 being very many large polar cells present, but the number of the 

 cells is far greater and their arrangement more complicated in the 

 visceral than in any of the other ganglia. 



Nerve cells are also to be found in the circumpallial nerves 

 and in the branchial nerves. So abundant are the nerve cells 

 in the circumpallial nerves that they assume the structure of 

 ganglia. The nerves by which they are connected with the 

 visceral and cerebral ganglia contain no ganglionic cells. From 



