OF THE NEMATOIDS, PAEASITIC AJUiD FREE. 679 



contained within its sheath, and the isolated condition of what should have been its 

 axial lumen or canal. In this animal, lining the inner side of the sheath of the oeso- 

 phagus, was a longitudinal layer of fibres apparently muscular* in their nature. This 

 is the only Nematoid in which such tibi-es have yet been recognized. 



The remaining or intestinal portion of the alimentary canal is subject to much less 

 variation, and its essential structure is the same in all Nematoids. It is separated by 

 a well-marked constriction, as well as by a difference of tissue, from the oesophagus, being 

 composed of an external structureless membrane, bounded internally by a more or less 

 developed layer of cells, each of which contains a clear nucleus-like body and a number 

 of dark spherical jjarticlesf . The nucleus very frequently appears to be absent, though 

 this may be owing to its being obscured by the pigmentary particles ; and where present 

 it often appears only as a clear space free from granules (Plate XXVIII. figs. 6, 10, & 29). 

 Internally the cells are bounded by a kind of structureless cuticle, which perhaps is 

 not an independent formation, but is produced by a thickening of the walls of the cells 

 next the central cavity. In Ascaris lumbricoides and many other species of the same 

 genus, these intestinal cells are long and columnar, and when an isolated portion of 

 intestine is spread out and examined through its outer membrane, the hexagonal bases 

 of these cells may be seen, presenting a regular tessellated appearance (Plate XXIII. 

 figs. 10 & 11). These cells are very long in A. megalocephala and A. mystax, and in 

 this last animal the internal cuticle is very thick and well developed (Plate XXVI. fig. 5). 

 The cuticle is distinct also in A. osculata, whilst in many of the free Nematoids it forms 

 an inner tube quite distinct, and capable of being isolated from the outer and the inter- 

 vening cellular layer ; this may be well seen in the members of the genera Tt/lenchus, 

 Aphelenchus, and Monhystera (Plate XXVIII. fig. 28). In A. osculata, as well as in 

 A. spiculigera and Filaria piscium from the Haddock, I have found the cavity of the 

 intestine almost obliterated by the great development of the cellular layer, and its pro- 

 jection inwards in the form of large rounded masses (Plate XXVI. fig. 10) ; some of the 

 cells composing them are elongated, though for the most part they have a rounded form. 

 I have seen similar nodosities of the cellular layer at the posterior part of the intestine 

 in A. viegalocephala. In several Strongyli that I have examined I have found the 

 intestinal granules very large and somewhat light coloured, though I have not been 

 able to detect the separate cells in which the granules are contained, and the same diffi- 

 culty occurs with many other Nematoids. This cellular or glandular layer is described 

 by DujARDiN as being " d'un rouge vif " colour in Filaria labiata, though in other animals 

 it seems to vary from a whitish to an almost black colour, the predominant tint being 

 olive-bro^vn. In some species the cells are of a rounded shape, and this, allowing for 



• Trans, of Linn. Soc. vol. xxiv. p. 116. 



t These particles, when liberated by rupture of the cells, generally exhibit an active molecular movement ; 

 I have often ■watched for evidences of this movement in the unruptured cells, but have always failed except on 

 one occasion, and then I witnessed a most active molecular movement within the cells of a portion of intestine 

 which had been expressed from the cavity of the body of one of the freshwater species of the free Nematoids. 



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