304 FRITZ MULLERj ON TIlE 



The hasal ganglia and the nerve-trunks_, -with favorable 

 illumination^ may often be readily seen, even with a pocket 

 lens. 



On the upper side of the trunk, sometimes closely cover- 

 ing it, sometimes spread over it in wide reticulations, rests a 

 plexus of more delicate nerves (figs. 3 — 5 p), which spreads 

 out laterally towards the line of origin of the polypide cells, 

 and is richly developed, especially at the extremity of the 

 branch between the basal ganglia of the succeeding inter- 

 nodes. In this terminal plexus, however, besides the branches 

 going to the ganglia above mentioned, at least one arch 

 appears to be formed between each two of the branches 

 springing from tlie smooth main nerve-trunk (fig. 4 h) . The 

 nerves composing the plexus are distinguished from the main 

 trunk principally by the circumstance that their surface is 

 rendered uneven, and more or less nodulated or tuberculated, 

 by the presence of nucleated cells. Chromic acid causes the 

 disappearance of these cells ; and the nerves, in consequence, 

 acquire sharper and even outlines, upon which, however, the 

 nuclei of the dissolved cells may still he perceived in the form 

 of minute, strongly refracting granules. This plexus is par- 

 ticularly well developed on those parts of the branches upon 

 which the cells are placed ; and it is especially complicated in 

 the older branches, in which a series of successive generations 

 has taken place. Towards the origin of the branch, the 

 plexus does not usually spread laterally beyond the nerve- 

 trunk, from which it can then hardly be distinguished. In this 

 case, on viewing the nerve from above, it will be found to 

 present an uneven border on either side ; whilst on a side 

 view, the uneven contour of the plexus will be seen above, 

 and the smooth border of the nerve-trunk beneath. In this 

 sterile portion of a branch, sometimes no peripheral nerves 

 at all can be perceived, sometimes only a few isolated fila- 

 ments, usually passing in a backward direction ; and occa- 

 sionallj^ even a tolerably well developed plexus may be 

 noticed, which, however, in this case spreads vertically up- 

 Avards from the trunk, whilst the expansion of the plexus in 

 the neighbourhood of the cells is more or less horizontal. 

 AVitli respect to the latter plexus, it may also be remarked, 

 that occasionally, though by no means constantly, its fila- 

 ments maj^ be seen to coalesce into a somewhat stronger 

 cord running beneath the line of origin of the cells. 



It remains to notice the connexion of the above described 

 common nervous system with the individual polypides. 

 This connexion is not always readily made out, which arises 

 from the circumstance that in order that tlie region under 



