127 
of Edinburgh , Session 1875 - 76 . 
arrangement formerly noticed does not appear in this form. The 
two neural canals soon increase in size, and approach each other in 
the middle line. With the exception of the great development of 
the ventral longitudinal muscles posteriorly, little further change 
takes place in the structure of the body-wall. 
The situation of the central ganglia in Scolecolejois cirrata, Sars., 
corresponds with the preceding, and the nerve-cords follow the 
progress of the oblique muscles toward the ventral surface, each 
trunk having a small neural canal. When the body wall is com- 
pletely formed (for instance about ^ in. from the snout), the great 
size of the longitudinal muscles is conspicuous. The dorsal form a 
thick superior arch, and proceed a considerable distance down the 
lateral wall; while the ventral muscles constitute two great curved 
masses in transverse section, the inner border of each being so 
carried upward that a deep ventral sulcus is formed for the nerve- 
trunks and their hypodermic external investment. For the same 
reason the strong oblique muscles are rendered nearly horizontal. 
The rounded firm nature of the alimentary canal gives little scope 
for the development of vertical fibres. 
The structure of the body-wall in Spio, and the position of the 
ganglia and nerve-trunks correspond with the foregoing in general 
features. The same may be said of the rarer Prionospio, which 
has two neural canals interiorly. 
In Polydora ciliata, Johnst., the body-wall anteriorly is charac- 
terised by the great development and bifid nature of the median 
ridge, which is flanked on each side by a prominent process of the 
hypoderm. In transverse section, the snout a little behind the 
tip presents, on each side of the dorsal process, a large rounded 
lobe, which projects downward to the oral aperture. Exter- 
nally, the lobe is composed of a thick layer of hypoderm, having 
internally a series of circular fibres, which come from the transverse 
dorsal arch in the form of a loop on each side. The fibres pass 
downward within the hypoderm, curve inward ventrally, and then 
proceed upward over the oesophagus to the point of commencement. 
A well-marked series of longitudinal fibres lines the outer division 
of the loop, and afterwards merges into the ventral longitudinal 
muscle of the side. The dorsal arch of transverse fibres cuts off 
the hypodermic process, containing the nerve-ganglia, and in the 
