THE ANATOMY OF PLBtJROTOiAJ ARIA BEYRICHII. 243 



This separation is probably only au external one, since 

 sections through the cords fail to reveal any continuous 

 layer of connective tissue separating the two, and bundles of 

 nerve-fibres apparently pass from the ventral to the dorsal 

 moieties of the cord, and vice versa. 



The right and left pleuro-pedal cords now diverge slightly 

 from one another and run back within the substance of the 

 foot, each cord lying in a slight blood-sinus situated below 

 and on either side of the main pedal sinus. 



The cords extend to the posterior extremity of the foot, 

 and are furthest apart near the middle of the foot ; toward the 

 posterior end they become somewhat approximated (fig. 27). 

 These cords, along which ganglionic cells are fairly evenly 

 distributed, are, as we have seen, equally derived from the 

 pedal and pleural systems : in width each cord at its anterior 

 extremity is but very slightly if at all larger than the 

 cerebro-pleural and cerebro-pedal connectives when closely 

 approximated, i.e. there is no marked swelling indicative of 

 a great accumulation of ganglionic cells (fig. 22). In fact, 

 there is but a slight increase in number of these cells in 

 this region, and that mainly in the ventral or pedal portion. 

 It becomes then very difficult if not impossible to speak of a 

 pedal, and, as I shall endeavour to show later, inadvisable to 

 attempt to identify a pleural ganglion in this pleuro-pedal cord. 



These ganglionic pleuro-pedal cords are connected at inter- 

 vals by transverse commissures : the first of these, as already 

 mentioned, is derived from both the pleural and pedal 

 moieties ; but the posterior ones, of which there are at least 

 twelve, at first sight would be considered as derived ex- 

 clusively from the pedal portion of the cord. An examina- 

 tion of sections, however, reveals the fact that a bundle of 

 nerve-fibres comes down from the dorsal portion of the 

 pleuro-pedal cords and enters the commissure, which is there- 

 fore derived equally from both portions of the cord. 



The laterally placed pedal nerves arise like the above from 

 both portions of the cord. The double root of these nerves 

 is often very conspicuous (fig. 21, p^.), somewhat resembling 



