II. Right side of the head of _Limnaea stagnalis_. The overhanging flap of the mantle has been cut in the middle line, and the right half twisted back, so as to expose the pulmonary orifice, etc. The points =A= =A= on the mantle edge were continuous before the mantle was cut; the line =BA= is part of the free edge of the mantle. =An=, anus; =F=, female generative orifice; =J=, portion of jaw; =M=, male generative orifice under right tentacle; =Os=, osphradium; =P.O=, pulmonary orifice.] Of the streptoneurous Gasteropoda, the nervous system of _Fissurella_ and _Haliotis_ shows distinct points of similarity to that of the Amphineura. The pedal nerves are united by transverse commissures throughout their entire length, while a double commissure unites the cerebral ganglia to the mass from which the pedal nerves proceed. In the great majority of the Streptoneura the ganglia (except the visceral) are more concentrated and the commissures are consequently much shorter. The accompanying figure of _Cyclostoma_, in which the animal is represented as in that of _Physa_ just described, illustrates this grouping of the ganglia, the twist of the visceral loop, and the position of the visceral ganglia at its posterior end. [Illustration: FIG. 103.--Nervous system of _Physa acuta_ Drap., showing the massing of the ganglia at the hinder end of the pharynx: _e_, _e_, eyes; _m_, mouth; _m.l_, _m.l_, mantle lappets; _o.f_, female generative orifice; _o.m_, male generative orifice; _os_, osphradium. (After Lacaze-Duthiers.)] [Illustration: FIG. 104.--Example of a streptoneurous Gasteropod (_Cyclostoma elegans_ Drap.): _c.g_, _c.g_, cerebral ganglia; _e_, _e_, eyes; _os_, osphradium; _ot_, _ot_, otocysts; _p.g_, _p.g_, pedal ganglia; _pl.g_, _pl.g_, pleural ganglia; _sp.g_, supra-intestinal ganglion; _sb.g_, sub-intestinal ganglion; _t.n_, tentacle nerve; _v.g_, visceral ganglion. (After Lacaze-Duthiers.)] =Scaphopoda.=--In the Scaphopoda the nervous system resembles that of the Pelecypoda. The cerebral and pleural ganglia lie close together, while the pedal ganglia are placed in the anterior part of the foot, connected with the cerebral ganglia by long commissures; the visceral loop is rather long, and the two visceral ganglia are adjacent to the anus.