470 J. F. ILLINGWORTH, 



closely joined to the sides of the pedal chords, but from a series of 

 cross sections one can see by the arrangement of the ganglionic cells, 

 that these are nearly distinct chords. At the point where the pleural 

 and pedal ganglia are united, the ganglionic cells almost come together. 

 These large nerve cells are usually found only in the periphery of the 

 ganglion surrounding the central fibrous tissue, so it seems to me that 

 this arrangement is a strong indication of the fusion of two distinct 

 cords, as is also the distribution of the nerves from each. The pedal 

 chords give nerves only to the foot, while the pleural supplies the 

 mantle and epipodium. The paired pedal nerves from the anterior 

 part of the pedal cords enter the tissue of the foot almost immediately 

 upon leaving the ganglia. As we go further back we find them ex- 

 tending more and more along the lower surface of the visceral cavity, 

 lying parallel to the nerves from the pleural cords. This is eminently 

 true of the nerves which arise beyond the posterior termination of 

 the pleural ganglia. They run for a long distance posteriorly on the 

 floor of the visceral cavity but finally plunge into the tissue of the 

 foot and develope a system of branches which may be followed close 

 to the external surface. 



The anterior ends of the pleural chords are united by a heavy 

 ganglionic cross fastening or commissure; and the posterior ends are 

 joined together in a similar way, though the posterior cross fastening 

 is smaller. These two heavy commissures belong distinctly to the 

 pleural chords, while the pedal are connected by ten or twelve small 

 commissures already mentioned. The heavy, anterior, cross fastening 

 of the pleural chords, gives off two important pairs of nerves, first, 

 those forming the visceral loop, and second, the two usually called the 

 mantle nerves. The subintestinal nerve (PI. 33 Sub.g), a part of the 

 visceral loop comes from the left side of the ganglion, and goes to 

 the right, passing under the oesophagus. The supraintestinal nerve 

 (Sup) springs from the right side of the ganglion, and passes up 

 around the right side of the oesophagus uniting with the supraintestinal 

 ganglion (Sup.g) on the left side of the visceral cavity. In this way 

 these nerves cross each other forming the visceral loop. The anterior 

 mantle nerves {Mn) arise from the cross fastening near its union with 

 the pleural chords. The left one comes from the under side of the 

 ganglion, while the right is attached above. There is no crossing of 

 these nerves, as they run directly into the mantle on their respective 

 sides, and both are joined by a nerve from the gill ganglion {!). From 

 this point forward a number of inosculating nerves proceed to the 



