0)1 an Australian Land Nemerline. 101 



We will first describe the median or dorsal vessel. In the 

 first place it does not keep by any means in the middle line, 

 but is generally found to one side or the other and also 

 curves about considerabl}'. Generally, at any rate in the 

 anterior portion of the body, it appears to keep constantly 

 on one side of the middle line (Figs. 7, 8, m. v.) 



The diameter of the vessel is by no means uniform. For 

 perhaps the greater part of its length it is a nan-ow cylindrical 

 tube, but occasionally, and more esj)ecially towards the 

 anterior end, it swells out somewhat sudden!}' into a wide, 

 irregular, lacunar cavity (Fig. 21). The structure of the 

 wall of the vessel is decidedly complex. On the inside, in 

 transverse sections, we see irregularly disposed, deepl}' 

 staining nuclei (Fig. 22, nu. v.), usually projecting more or 

 less into the lumen of the tube. From considerations to be 

 adduced hereafter I doubt whether these are the nuclei of a 

 properly defined epithelium. Outside this nucleated layer 

 there comes a thin layer of very delicate fibres, doubtles<i 

 muscular, arranged in a circular direction around the vessel 

 (Figs. 22, 23, c. m. v.) Outside the muscular layer comes a 

 single layer of large, vesicular-looking, irregularly ovoid, 

 faintly staining cells with small nuclei and slightly granular 

 contents (Figs. 22, 23, ves. c.) The wall of the vessel then, 

 in its narrow portions, is made up of three distinct layers. 

 In the swollen, lacunar portions of the vessel (Fig. 21, 1, m. v.) 

 only the two inner layers can be made out, the outer layer 

 seems to be entirely wanting. 



At its extreme anterior end the median vessel becomes 

 narrow again, after swelling out into a series of irregular 

 lacunse as above described, and passes forward between the 

 oesophagus and proboscis sheath to the level of the ventral 

 comn)issure (Figs. 6, 18). Here it terminates in a very 

 remarkable manner. A transverse section taken through 

 the region of the ventral commissure will, if taken at exactly 

 the right level, show two curious bodies imbedded in the 

 proboscis sheath, one on each side of the mid-ventral line. 

 The.se bodies have the form of cellular plugs, containing small,, 

 very deeply staining nuclei and frequently projecting very 

 markedly into the cavity of the proboscis sheath. The exact 

 form and position of one of these curious structures will be 

 best understood by reference to Figure 18, representing a 

 small portion of a longitudinal vertical section taken at one 

 side of the median line. It will be seen that in its deeper 

 part, as it passes through the muscular proboscis sheath, the 



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