OF THE NEMATOIDS, PAEASITIC AM) FEEE. 601 



or at their terminations ; their breadth varies, however, from yoov" to y^" in A. mega- 

 locephala. When examined with a power of about 200 diameters, it can be seen that 

 the granules between which the vessel lies are displaced by an intervening body, and 

 that there is something more than a mere lacunar canal. It may frequently be seen 

 that one of the vessels composing the pair, in the neighbourhood sometimes of the dorsal, 

 and sometimes of the ventral median line, is more irregular and undulating in its course 

 than usual, and from this portion short branches appear occasionally to arise which extend 

 into the neighbouring granular material. This was also observed and figured by Scuj^eideb. 

 Sometimes, moreover, a slender transverse branch may be seen uniting the two vessels in 

 the same situation ; but with these exceptions the vessels appear to be quite unbranched. 

 All attempts to follow them into the substance of the median lines have utterly failed, and 

 after the most careful examination I have also equally failed to discover any channels 

 through the integument opposite theii* terminations ; and I have so repeatedly searched 

 for these in vain, that I can almost confidently assert they do not exist. When portions 

 of the vessels are isolated from the layer in which they are imbedded they appear as flat 

 bands, whitish in colour, and as if composed of longitudinal fibres with minute inter- 

 spersed molecules, though it is extremely difficult to convince oneself in this way that 

 they are in reality hollow tubes. They have much the appearance of flat solid bands, 

 such as Schneider believes them to be. But I am almost convinced this cannot be the 

 case, since several times, on examining a portion of the animal in which I had been en- 

 deavouring to isolate these bodies, I have found the vessels torn across in some part of 

 their course, whilst the remaining portion still lying in an undisturbed part of the cel- 

 lulo-granular layer, now no longer occupied the whole breadth of the lacimar channel in 

 this structure — as they all invariably do before they have been disturbed — ^but was so 

 diminished in size as to occupy no more than one-third of this space. Is it likely that 

 the traction exercised in the act of tearing this structure across would, if it were a solid 

 band, cause it to shrink to this extent 1 I think not ; though this might very easily occur 

 with a thin-walled vessel, such as I believe this structure to be. If they really are solid 

 fibrous bands, what can they be except either muscles or nerves 1 But putting aside the 

 prima facie improbability of such an extraordinaiy and unsymmetrical arrangement of 

 muscular bands, such a view as to their nature seems absolutely negatived by the fact of 

 their undulating course, and their situation in the midst of a structure which would be dis- 

 turbed and destroyed by their action. Then, again, with regard to the other view, which 

 seems to find most favour with Schneidek*, that these bands belong to the nervous system, 

 we are met by the fact that, according to my measurements, they are often three times as 

 broad as the largest nerve-fibre I could find issuing from a ganglion-cell in connexion 

 with the oesophageal ring. In addition to this there is the difficulty of their asymme- 

 trical distribution, which seems quite inexplicable on the supposition of their being 

 nerves, though it can be smoothed over to a certain extent if we regard these structures 

 as transverse vessels simply destined to bring the dorsal and ventral median luies into 



* MtJiXEK's Archiv, 1863, S. 10. 

 MDCCCLXVI. 4 



