or THE NEMATOIDS, PARASITIC AND FREE. 589 



than in the other. In A. lumhricoddes a distinct walled vessel does exist, though its 

 isolation from the surrounding tissue is by no means easy ; the arch also is less pro- 

 nounced. Neither in this species nor in A. megalocephala could I see, when viewed from 

 within, the straight exit-tube of the arch ; it does not seem to lie along in the median 

 line, but to proceed almost directly outwards through the muscular layer and integument 

 to the median aperture, which is I think only very slightly anterior in position to the 

 arch itself It is not easy to follow these vessels in the lateral bands towards the posterior 

 extremity of the body, but after the most careful examination of individuals of these 

 species and of A. marginata, I feel quite confident that no posterior union between the 

 vessels exists, but that in all they appear gradually to diminish in size and in the thick- 

 ness of their walls, and either disappear or dwindle into mere lacunar channels at about 

 the level of the anus. Thus in a specimen of A. marginata one of the thick-walled 

 vessels, just behind the arch, measured 43^" in diameter, having a central lumen 2^0 a " 

 in diameter, whilst close to the termination of the vessel, where I was fortunate enough 

 to be able to isolate it from the band in which it was imbedded, at a distance of f " from 

 the posterior extremity of the animal, the diameter of the entire vessel was only -g-jnjo"- 

 I have never been able to detect branches of any kind given off from these lateral canals 

 or vessels. Their walls are composed of a dense tissue ; and though I have not been 

 able to make out its exact structure, I have no doubt that it is a kind of muscular 

 tissue, capable of contracting and dilating. Contiguous portions of the canals may 

 frequently be seen with a vaiying calibre; and on the left side of the arch I have 

 occasionally seen the canal very narrow opposite the peculiar, large, more or less round 

 or ovoid cell, enclosed in its thickened outer wall in this situation (Plate XXV. fig. 9). 

 This body is altogether remarkable ; imbedded in the very substance of the wall of the 

 canal it has a distinct bounding membrane and is densely filled with small granules, 

 which generally effectually conceal a clear nuclear body existing in the centre. Curi- 

 ously enough, too, I have detected three bodies presenting a precisely similar appearance 

 in what I believe to be muscular tissue, surrounding the rectal termination of the intes- 

 tine in A. megalocephala (Plate XXV. fig. 5). What their nature can be I have not the 

 slightest idea, neither have I met anything similar to them elsewhere among the Nema- 

 toids. In those Ascarides having the above-mentioned arrangement of their excretory 

 tubes, a body of this kind seems always to exist on the left side of the arch, and at the 

 same time the walls of the vessel on this side are generally much thicker than on the 

 other. What relationship there is, however, between these two facts I am unable to 

 say, and it seems very difficult to explain why one side of the arch should be larger than 

 the other. The most extreme divergence between the two sides exists in A. marginata, 

 in which animal these canals are remarkable, not only for the thickness of their walls, 

 but also for their great proportionate size as compared with that of their containing 

 bands (Plate XXVI. fig. 2). In A. mystax, on the contrary (Plate XXVI. fig. 4), the 

 vessels have about the same proportionate development as in A. lumbricoides. 



An arrangement of the excretory tubes similar to that just described also exists, in all 



