388 
Tetrarhynclms erinaceus 
and has been reconstructed from serial sections ; and Fig. 6 is a similar 
reconstruction projected in a single transverse plane. Fig. 7 is a pure 
diagram but inay be useful in enabling the reader to follow the de¬ 
scription. The exact situation of the central nervous system may be 
fixed by a reference to the diagrammatic transverse section (Text-fig. 3, 
p. 379). It is contained in the space bounded by the dorso-ventral 
muscles c, the horizontal muscles a, and lying between the dorsal and 
ventral pairs of proboscis sheaths. It is all contained within relatively 
few sections of the series. 
Lateral nerve coids 
Fig. 7. The central nervous system. The figure is entirely schematic, the commissures 
being rather widely separated so as to make their relations as clear as possible. 
The whole is built up round four paired masses which in order to 
avoid circumlocution I will call ganglia. These structures are arranged 
as right and left dorsal, and right and left ventral. From each of them 
a stout nerve, hardly less in diameter than the ganglion itself, proceeds 
and runs transversely and anteriorly into the antero-lateral parts of the 
bothridia. These are the paired anterior bothridial nerves—four in 
number. The ganglia are connected together by commissural tracts. 
The corresponding right and left centres are joined together by paired 
transverse commissures. These two commissures run transversely across 
the scolex from side to side and near to the middle line—much 
nearer than is suggested in the diagram Fig. 7. Posterior to them the 
anterior ganglia of the same side approach each other in the middle line, 
and right dorsal and right ventral masses fuse together, also left dorsal 
and ventral centres. From the points of fusion another commissure 
takes origin: this is the posterior median commissure : it crosses the 
scolex from side to side just posterior to the anterior tracts. Thus we 
