485 



In the divisions also of the central stem there are very great 

 variations, which it would be endless to describe in detail. Perhaps 

 the arrangement most generally met with, and that which I am 

 inclined to regard as the type, on account of its presenting the 

 nearest approach to symmetry, is that the main central chord sepa- 

 rates, at about '014 from its origin, into two equal branches, and 

 these again, after a course of about *01, divide dichotomously 

 (fig. 2). In such a case the division of F from B generally takes 

 place at a considerable distance from the ganglion. 



I have, however, not met with many specimens presenting even 

 this very limited amount of symmetry and regularity. 



In fig. 4 we see the two divisions (G, G) of the central chord C 

 divide almost immediately and yet not symmetrically. In fig. 5 the 

 chord C divides into two unequal divisions, the smaller of which 

 passes along for more than -014 before it redivides, while the larger 

 branch divides into three at a point only "006 from its origin. 

 In fig. 6 the central chord, just before its division into two 

 branches, throws off on each side a small branchlet ; in fig. 7 this 

 happens only on one side. Finally, figs. 4 and 7 present us with 

 some instances in which more than four branches are given off close 

 to the first division of the great chord C. 



But even in the case which I have above described as most 

 typical, the symmetry is not in fact so great as it would at first 

 sight appear to be, because the nerves on the two sides are fre- 

 quently not of the same size. Thus, in fig. 2 each of the two 

 branches of the main central stem divides, it is true, into two 

 secondary branches, one of which is smaller than the other, but the 

 two lesser branches are both upon the right side. If then, as is 

 probable, we are justified in concluding that in each animal the 

 ultimate nervous fibrils are of somewhat equal size, that they com- 

 pose the greater part of the nerve, and that the corresponding 

 organs of the two sides of the body receive an equal amount of 

 nerves, it is evident that some of the parts which on the left side 

 are supplied by the large outer branch must on the right side be 

 connected with the median branch. 



We see, therefore, that not only is the branching of the nerves 

 absolutely irregular, and that of the two sides entirely unsym- 

 metrical, but even the number of main stems proceeding from the 



