858 PROF. G. C. BOURNE ON THE [NoV. 17^ 



fibres and dendrites surrounded by a cortex of nerve ganglion- 

 cells. There are three kinds of ganglion-cells : (1) Small bipolar 

 dells, very numerous, closely crowded together, and with deeply 

 staining nuclei. They occur in great numbers in all the nerve- 

 centres and are the only cells extending from the centres along^ 

 the nerve-trunks. (2) Large apparently unipolar cells, with 

 larger and less deeply staining nuclei than in 1. These are- 

 scarce and locally aggregated in the nerve-centres. (3) Large- 

 multipolar cells more numerous than the second kind and with 

 similar nuclei. They are localized chiefly at the sides of the 

 centres, and in the pedal cords their axial processes can often be 

 distinctly traced into nerve-fibres. 



The presence of the larger cells of the second and third kind is 

 characteristic of a separate nerve-centre oi- ganglion. These facts 

 are illustrated, but on a very small scale, in figs. 37-40. 



Fig. 37 is a section, taken somewhat obliquely, through the 

 pleural and pedal centres, just behind the union of the former 

 with the latter, and including the connection between the two 

 pleural ganglia. The details of the pedal centres need not con- 

 cern us ; it is sufficient to allude to the fact that they give 

 evidence of a considerable degi'ee of complication. In the pleural 

 centres it is evident that there are two principal groups of nerve- 

 cells in each ganglion : a dorso-lateral and a ventral. A bundle 

 of nerve-fibres, originating from the dorso-lateral group of the 

 left ganglion, runs across to the right and turning downwards 

 traverses, but does not enter into connection with, the ventral 

 group of cells of the right ganglion and is joined })y another band 

 of fibres originating from a distinct group in the lateral region of 

 the right ganglion. These two bundles unite to form the root of 

 the subintestinal nerve. Fig. 38, repi^esenting a section some- 

 what posterior, shows the subintestinal root still small and 

 traversing the ventral cell-grou]3 of the right ganglion. Fig. 39 

 passes through the base of the conical subintestinal ganglion and 

 shows the same bundle of fibres as in the last section, now 

 considerably larger ; and above it a new bundle surrounded by a 

 very thick mass of ganglion-cells, among M^hich is a relatively 

 large number of the larger kinds. The intervening sections 

 would show that the fibres of the upper bundle pass into or take 

 their origin from the thick dorsal and lateral mass of nerve-cells. 

 Fig. 40 is a section taken some way further back through the 

 subintestinal nerve and shows the two bundles of fibres still 

 distinct and surrounded by a thin layer of small nerve-cells. 

 Eventually when the layer of nerve-cells dies out the two bundles 

 of fibres can no longer be distinguished from one another. There 

 can be no doubt that the bundle li in fig. 37 is the origin of the 

 subintestinal nerve from the left pleural ganglion, and that the 

 bundle z represents the much-abbreviated zygoneurous connection 

 of the subintestinal with the right pleural ganglion. The mass 

 of cells sb in fig. 39 is the subintestinal ganglionic centre, and 

 the uppermost of the two bundles of fibres marked h is formed by 



