INTERNAL ANATOMY. 87 



of them runs beneath the outer surface of the oral hood 

 and, by the occurrence of frequent anastomoses, forms a 

 coarse network known as the outer plexus, while the other 

 set lies beneath the inner surface of the oral hood and 

 gives rise to the inner plexus. The latter was discovered 

 by FUSARI in 1889. The two plexuses are distinct from 



rn.fi 



Fig. 43. Transverse section through the spinal cord in the middle region of 

 the body. (After ROHDE. ) 



a. Giant fibre proceeding from the giant ganglion-cell A (see below), c.c. Cen- 

 tral canal, g.f 1 . Giant nerve-fibres, which traverse the spinal cord from before 

 backwards, g-f' 1 . Giant fibres, which traverse the spinal cord from behind for- 

 wards, m.p. Muscle-plates, m.r. Motor nerve-fibres, n.f. Longitudinal nerve- 

 fibres cut across, s.f. Supporting fibres, sh. Sheath of nerve-cord ( = dura mater ; 

 FUSARI). 



one another, except in so far as their component nerves 

 have a common origin from the dorsal roots (Fig. 42). 

 The outer plexus is continued up into the individual cirri, 

 while the inner plexus appears to stop short at the base 

 of the cirri. It has recently been discovered by VAN 



