OF THE NEMATOIDS, PARASITIC AND FEEE. 597 



some species of free Nematoids, although their minute size has hitherto prevented their 

 detection. 



Eberth still believes that the median lines in some Nematoids constitute a part of the 

 water-vascular system, whilst Walter goes much further, and states that they anastomose 

 with the lateral vessels, and also send off a number of transverse branches on either side. 

 These transverse branches are what Walter formerly described as peripheral nerves, 

 but which have now been most conclusively shown by Schneider and myself to be 

 merely transverse prolongations from the muscle-cells to the median lines. Schneider 

 does not look upon the median lines as belonging in any way to the vascular system. 

 I have no doubt concerning the general structure of the median lines ; and the descrip- 

 tion given of them in several species, both by Walter and Eberth, to the effect that 

 they are composed of a delicate membrane enclosing a number of highly refracting par- 

 ticles, is not only in accordance with my own observations, but also in harmony with 

 the belief that they are developments of the deep integumental layer. As I stated, 

 however, in my paper upon the Guineaworm, these structures in this animal have a 

 somewhat different formation, not presenting the granules and being of a more fibrous 

 nature ; and after the most careful investigation of them, I came to the conclusion that 

 they were hollow, and in fact constituted vessels of some kind. At the same time I 

 stated that I had frequently been almost led to believe that a lacunar channel did exist 

 in the median lines of Ascaris lumbricoides. The difficulties besetting the investigation 

 of these structures is extreme, and I am still somewhat undetermined upon this point. 

 I have fancied from time to time that a lacunar channel existed in these bodies both in 

 A. lumbricoides and A. megalocephala, but can say nothing positive upon the subject. 

 The existence of peculiar transverse vessels to be presently described in the deep inte- 

 gumental layer of both these animals, also seems to lend support to this idea. Dr. 

 Schneider's views concerning the structure of the median lines in the Ascarides have 

 already been alluded to when speaking of the nervous system. 



A statement has now been made of almost all that is at present known, not only con- 

 cerning the nature of the structures in connexion with the anterior ventral aperture of 

 the Nematoids, but also concerning the formation of the lateral and median bands. And 

 at present the balance of evidence is decidedly in favour of the view I have adopted, that 

 the so-called ventral glands met with in some Nematoids, and the axial tubes seen in 

 the Ascarides, Cucullanus, and other animals — organs communicating with the exterior 

 by a median pore — are all only modifications of one and the same structure. From the 

 abortive saccule of Oxytms amhigua, we see the organ progressively developing into a 

 simple tube in some of the free Nematoids, a tube with a dilated nucleated extremity in 

 others, and in the Strongyli, Sclerostomata, and others, we get a bifurcation of the 

 organ, which still opens, however, by the same median pore. In Ascaris megalocephalay 

 A. lumbricoides, &c., the outgoing tube and the arch are still situated, like the ventral 

 glands, in the general cavity of the body, so that up to a certain stage of development 

 we may imagine the structure of one of these animals, so far as this organ is concerned. 



