302 . ME. A. D. MICHAEL ON THE 



the different innervation prevents these organs in tlie Insecta and the Arachnoidea 

 being considered homologons ; thirdly, that although nsually in adult Arachnida 

 the mandibular (clieliceral) nerves, and possibly sometimes the palpal nerves, arise 

 from the suj)ra-a3sophageal ganglion, in the young embryo the ganglia whence 

 these nerves spring are not supra-oesopbageal, but work upward in the com-se of 

 development. 



In Bdella a very thin branch, the destination of which I was not able to trace, arises 

 on the outer side of the nerve almost immediately it leaves the brain. About as far from 

 the brain as two-thirds of the brain itself the palpal nerve gives off a second and 

 considerably larger branch, this time from its inner side ; this branch {up 2) innervates 

 the maxillary lip ; at its distal end, at the lip, it branches and forms two cone-like 

 structures (iig. 15), evidently of a sensory nature. 



The next four pairs of large nerves from the sub-oesophageal ganglion are the leg-nerves 

 (N 1, N 2, N 3, N 4) ; these are of the iisual type, and are the largest nerves in the body ; 

 each is accompanied on its anterior side by the so-called accessory nerve [iia) of Crone- 

 berg, Schaub, and Nalepa. I think I showed in my treatise on Thijas petrophilns that 

 these accessory nerves are really the first branches of the respective nerve-trunks, 

 although in many instances they have come to arise from the brain itself just at the 

 base of the leg-nerve : this, as will be seen by the drawings, is the case in Bdella 

 Basteri in some of the legs ; but, as before stated, the leg nerve-trunks run far into the 

 substance of the brain. The accessory nerve of the first leg {na 1), soon after leaving 

 the brain, expands into an almost triangular lamellar process, similar in nature to 

 that of the mandibular nerve, and two very fine nerve-branches arise from the anterior 

 angles of this expansion. The accessory nerve of the second leg {na 2) gives off two thin 

 opposite branches a short distance from the brain ; a little further on its course it gives 

 off a rather thicker branch, which extends into a process nearly similar to that from the 

 accessory nerve of the first leg, and like that gives off two fine branches from its anterior 

 corners. The third and fourth legs also are accompanied by accessory nerves {na 3, na 4) ; 

 I was able to trace two extremely fine branches from the latter, but not any processes 

 on either similar to those on the first and second accessory nerves. The nerve-trunk to 

 the third leg, about as far from the brain as half the length of the brain itself, gives off 

 a branch on its inner side, and shortly afterwards a thicker branch on its outer side ; this 

 branch soon divides dicliotomously : the trunk also gives off another branch on its outer 

 side further on. The nerve-trunk of the fourth leg is accompanied by a branch springing 

 from the brain on the inner side as well as the outer, in fact by a second accessory 

 nerve {nas). The trunk, nearly as far from the brain as the whole length of the brain 

 itself, gives off a substantial branch. I was able to trace this branch ; it goes to the 

 nmscles in the body which move the coxa of the fourth leg. 



The remaining pair of large nerves are the genital nerves {ng) ; these arise close 

 together in the centre of the posterior edge of the ganglion ; indeed, they are so close 

 that just at first their inner edges fuse, making a square, central, posterior projection 

 from the ganglion, I was able to trace five branches on the underside of each of these 

 nerve-trunks ; that on the left is shown in. the drawing slightly turned over to expose 



I 



