MUSEUM OF COMPAEATIVE ZOOLOGY. 147 



to the median lines of other Nematodes. The curious torsion, by which 

 in the normal position of the body they come to lie laterally for the 

 larger portion of its length, has already been described. 



The two lines, dorsal and ventral, are very similar in form and struc- 

 ture (Plate VIII. Figs. 101, 102), except that the ventral line contains 

 the prominent ventral nerve cord, which will be described in connection 

 with the other portions of the nervous system. 



The dorsal line can first be seen distinctly immediately behind the 

 partition which cuts off the anterior chamber. In front of this I find no 

 dorsal differentiation of the hypodermis, and consequently no dorsal line. 

 At its anterior end the dorsal line has a thickness of 20 /x ; passing poste- 

 riad, this gradually increases to 40 /*, and this thickness remains nearly 

 constant until within a short distance from the end of the body, where 

 it becomes gradually reduced and finally disappears. The line is sepa- 

 rated from the body cavity by a prominent basement membrane, the 

 direct continuation of that which separates hypoderm and muscular 

 layer. The elements which make up the dorsal line (Fig. 102) appear 

 both in longitudinal and in transverse sections as a row of elongated 

 cells, the walls of which are usually first visible a short distance below 

 the cuticula, although in one specimen preserved in Flemming's mix- 

 ture they could be traced even up to the lower surface of the cuticula 

 itself. The nuclei are oval, poor in chromatic substance, hence pale and 

 not at all prominent. They lie with the long axis perpendicular to the 

 surface of the body, nearly or quite filling the entire diameter of the cell. 

 In most cases they are found at about the same level in the different 

 cells, which thus form a regular epithelial layer. The deep ends of 

 the cells are prolonged into processes which extend down to the base- 

 ment membrane through a mass of fibres which cross in every direction. 

 Among this net- work of fibres in the lower portion of the line one finds 

 occasional cells with branching processes (*, Fig. 102), which may be 

 nervous. There is however no definite nerve cord extending through 

 the line, and no evidence was found of the connection of these cells with 

 other parts of the nervous system. 



The ventral line is similar in structure to the dorsal line, in that it 

 consists likewise of a layer of high cells of an epithelial character im- 

 mediately imderlying the cuticula (Fig. 101). Their deep ends are also 

 prolonged into processes, which are here bent around the ventral nerve 

 cord, which lies in the centre of the line. Between the cord and the 

 basement membrane below is seen again a confused mass of fibres, 

 into which, as in the case of the dorsal line, the deep ends of the cells 



