15 
THE MUSCULAR SYSTEM. 
Together with the integument, muscular layers of different thick- 
ness participate in the formation of the bodywall. Superfieial in- 
spection shows that, with the exception of a short distance right and 
left of the ventral furrow, the thiekness of this muscular investment 
is far inferior to that of the integument: generally it only measu- 
res one third. The muscular layers firmly adhere to the integu- 
ment; separation of these two is nevertheless not diffieult and in 
sections through a portion of the bodywall this separation has gen- 
erally commenced at the free border of the section. 
Immediately beneath the integument is a circular muscular layer 
which in both the specimens examined had an average thickness of 
0.025 mm., whereas the body-cavity is lined by a somewhat thicker 
layer (0.04 mm. on the average) of longitudinal muscular fibres. 
Apparently these two layers fit together, but tangential sections 
furnish conclusive evidence that between them there is a thin layer 
of fibres erossing both the longitudinal and circular one at angles of 
about 45°. This intervening layer is built up of two sets of 
fibres, each of them parallel to the two sets of spicules in the in- 
tegument, which as described above, are in their turn placed at 
angles of 45° with respect to the body axis. Thus the muscular fibres 
in the bodywall are very regularly distributed running paralel to 
each other according to either of the four directions just mentioned. 
Both in the anterior and posterior extremity of the body as well, 
as on both sides of the ventral furrow this arrangement of the 
muscles in the bodywall becomes somewhat modified: at the ex- 
tremities in correspondence with the blunt termination of the body, 
and all along the ventral furrow because of a considerable thicken- 
ing of the layer of longitudinal muscular fibres (fig. 24). I need 
not enter into a more detailed description of these museles which 
were already found in Neomenia by v. Grarr and TULLBERG. v. GRAFF 
calls them the longitudinal muscle bands. It will also suflice to 
repeat that in Proneomenia the intestine is likewise suspended to the 
bodywall by radial fibres, which however nowhere penetrate into 
