THE MORPHOLOGY OF THE MOLLUSC A. 51 



liferation of the floor of the mantle-cavity, i.e., that the 

 ganglion is essentially a dorsal ganglion. Its final situation 

 on the course of the sub-intestinal nerve-loop is rendered 

 possible by the fact that its connectives with the visceral 

 eanolia are not delaminated from the ectoderm, as are the 

 ganglionic pleuro-visceral cords of Chiton, but are mere 

 fibrous outgrowths from the ganglia themselves. Embry- 

 ology is thus in complete accord with the views which have 

 been maintained in the earlier part of this paper as to the 

 homologies and origin of the visceral nervous system in 

 Mollusca. 



The pallial and visceral commissures in Cephalopoda. 

 — It has long been known (Hancock) that in many Cepha- 

 lopoda the stellate ganglia on the pallial nerve-cords are 

 connected with one another above the gut by a transverse 

 commissure. Is this commissure a relic of the pallio- visceral 

 nerve-ring of the Amphineura and homologous with the 

 pallial ring of Lottia, or is it merely a secondary connection ? 



In Spirilla a remarkable arrangement of the pallial 

 commissure has been recognised by Huxley and Pelseneer 

 in their recent memoir (12). The commissure is not in 

 this case a straight transverse band, but consists of two 

 curved cords which arise from the right and left stellate 

 ganglia respectively, and at their junction in the median 

 line of the body give off a median pallial nerve which runs 

 for a short distance forwards, and then passing over the 

 anterior margin of the shell — which is, of course, internal — 

 becomes recurrent and runs along the part of the mantle 

 contained within the last chamber of the shell. Pelseneer is 

 thus led to regard the commissure with its median nerve as 

 formed by the two original pallial nerves fused together. 

 The connection between the stellate ganglia having thus 

 arisen in the primitive Dibranchiates (apparently in con- 

 nection with the reduction in size and enclosure of the 

 chambered shell), higher forms show a series of stages in 

 its subsequent degradation, until it is finally lost in the 

 Octopoda. The absence of a pallial commissure in Nautilus 

 also supports Pelseneer's view that in Cephalopoda this 

 structure is not of any primary importance. 



