At the posterior end of the body there are likewise seven separate ganglia partially fused to form a single ganglionic mass, which innervates the segments lying behind the anus and corresponding to the posterior sucker.
Ganglia, rather than cord-like nerve- br', Parabranchia (= the tracts containing both nerve-cells or comb-like osphra ganglionic elements and nerve-fibres.
The nervous system of the embryonic head exhibits three ganglionic masses, anterior to the thoracic ganglionic masses; these three masses subsequently amalgamate and form the sub-oesophageal ganglion, which supplies the trophal segments.
The position of the chelicerae of Limulus and of the ganglionic nerve-masses from which they receive their nerve-supply, is closely similar to that of the same structures in Scorpio.
The concentration of nervous matter and ganglionic substance at the oral end of Trematodes is equivalent to the " brain " of the Planarians, but the similar thickening in the scolex of Cestodes is by no means so certainly to be called by that name.