OF THE NEMATOIDS, PARASITIC AND FEEE, 599 



apparently have an independent origin, and no connexion with the system of tubes com- 

 municating with the ventral pore. The first variety is met with amongst the various 

 species of four land and freshwater genera, all of which possess certain common charac- 

 ters, and amongst others an extraordinary tenacity of life and power of resuming their 

 vital manifestations after the most prolonged periods of desiccation. These four genera 

 are Tylenchm, Plectus, Aphelenchus, and Cephalohus. I have examined the vessels them- 

 selves most fully in adult specimens of Tylenchus tritici*. In this species they are single, 

 unbranched vessels, about ^-gVo" ^^ diameter, with thin transparent parietes and colourless 

 contents, each pursuing an undulating and in some places an almost convoluted course 

 along either side of the body. They are, certainly, throughout the greater part of their 

 extent, unattached to the parietes of the body, but how they end either anteriorly or 

 posteriorly 1 have been quite unable to ascertain. I have succeeded in tracing them to 

 within a very short distance of each extremity of the animal, but never could detect 

 either any external aperture with which they communicated, or union of the vessels of 

 opposite sides. From the freedom, howevei', with which I have seen these vessels 

 extruded with other contents of the general cavity, through ruptures of the posterior 

 part of the animal by pressure of the covering glass, I am inclined to think that, poste- 

 riorly at least, they are unattached to the parietes of the body. When whole coils of 

 these vessels were thus brought fairly into view, I never saw the slightest evidence of 

 contraction in any part of their extent, neither could I detect any cilia in their interior ; 

 they seemed to contain a clear fluid devoid even of suspended molecules. In the other 

 genera named I have never seen any approach to a convoluted arrangement of these 

 lateral vessels, and they seem to pursue nearly a straight course. In the genus Plectus, 

 however, each seems to start from, and to be connected anteriorly by a short narrower 

 portion, with a circular marking (perhaps having a minute orifice in its centre) of the 

 integument (Plate XXVIII. fig. 14). This is all I have been able to ascertain concerning 

 these structures, and it is from what I have seen in Tylenchus tritici only, that I have 

 been led to infer as a probability that the similar vessels met with in the other genera, 

 occupy the same position in the general cavity of the body as I have proved them to do 

 in this species. These four genera are further allied to one another by the fact that 

 they alone of the land and freshwater Nematoids yet discovered possess any modification 

 of the ventral gland before described, and that it exhibits essentially the same formation 

 in all the species of these four genera, whilst this formation differs somewhat from the 

 common type. 



The other vessels in the Nematoids were, I believe, structures first noticed by 

 ScHNEiDERf in Ascaris lumbricoides, though neither at that time nor at the present^ 

 does he take this view of their nature. He is somewhat perplexed by them, but upon 

 the whole regards them rather as nerves than vessels. Leydig has also examined these 

 structures, and believes them to be vessels — an opinion which I had, moreover, formed 

 of them at a time when I was quite in ignorance of Leydig's views upon the subject. 



* Olim, Vibrio tritici. f MCller's Archiv, 1861), S. 240. Taf. \-i. 10 u. 11. + Hid. 1863. 



