MUSEUM OF COMPARATIVE 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 p ; passing poste- 
riad, this gradually increases to 40 w, 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 underlying 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 
