The Mollusca possess a nervous system, which usually consists of a number of nerve centres or _ganglia_, linked together by bands (the _commissures_) and sending out thread-like nerves which ramify into the various organs. The character of the nervous system varies greatly in different groups, ranging as it does from a condition of extreme complexity, in which the ganglia are numerous and the commissures equally so, to that of considerable simplicity, in which ganglia are almost entirely absent. The most important ganglia are (1) the _cerebral_,[313] which are always placed above or on either side of the mouth, and from which proceed the nerves of the eyes and tentacles; (2) the _pedal_, which in Gasteropoda are situated below the oesophagus, in Pelecypoda at the base of the foot, and from which the nerves of the foot and sometimes the acoustic nerve arise; (3) the _pleural_,[314] whose position varies considerably, but is always below the oesophagus and slightly above the pedal ganglia; these innervate the mantle, branchiae, heart, and viscera generally. =Gasteropoda.=--The simplest form of nerve system as thus understood occurs in the Amphineura, and more particularly in the Chitons. Here we find four longitudinal nerve-cords, parallel to one another for nearly the whole length of the mollusc. The two exterior cords probably represent the pleural, the two interior the pedal nervous system. There being no head or tentacles, but simply a mouth at the anterior end, the cerebral ganglia do not exist, but they are represented by the curved ring formed by the massing together of the two nerve-cords on each side. The only distinct ganglia are a pair of buccal ganglia (which are developed on a pair of commissures which pass forward from the cerebral mass and innervate the lips and buccal region), and a much smaller group, the sublingual. The two pedal cords are united by a number of transverse parallel connectives, which recall similar modes of connexion in the Chaetopod worms and in Arthropoda. This quadruple set of nerve-cords is characteristic of all the Amphineura, but the absence of ganglia is most marked in the Chitons. In _Proneomenia_ and _Neomenia_ there is a distinct cerebral ganglion, formed by the massing of the two ganglia into one, while in _Proneomenia_ the lateral cords are joined to the pedal, as well as the pedal to one another, by connectives. In _Chaetoderma_ the cerebral ganglia, though adjacent, are distinct, and both the pedal and lateral cords connect directly with them, while there are no transverse connectives.