The remaining three great divisions of Gasteropoda, namely, the Prosobranchiata, Opisthobranchiata, and Pulmonata, may be regarded as comprising two distinct types of nervous condition, according as the loop formed by the two visceral nerve-cords is twisted over itself, forming a figure of 8, or continues straight and uncrossed. In the former case, we get the condition known as _streptoneurous_, in the latter that as _euthyneurous_.[315] The _Euthyneura_ include the whole of the Opisthobranchiata[316] and Pulmonata, the _Streptoneura_ all the Prosobranchiata. [Illustration: FIG. 101.--Nervous system of the Amphineura: =A=, _Proneomenia_; =B=, _Neomenia_; =C=, _Chaetoderma_; =D=, _Chiton_; _c_, cerebral ganglia; _l_, _l_, lateral cords; _pc_, posterior commissure; _s_, sublingual commissure or ring, with ganglia; _v_, _v_, pedal cords. (Alter Hubrecht.)] The simplest form of nervous system in the euthyneurous Gasteropoda occurs in the Opisthobranchiata. The cerebral, pleural, and pedal ganglia tend to become concentrated in a ring-like form, united by short commissures at the posterior end of the pharynx. The visceral loop is in some cases long, and the two or three visceral ganglia are then situated at its posterior extremity. The nervous system of the Pulmonata is of a similar type, the visceral loop being often much shorter, and tending to draw in towards the central group of ganglia. The tentacular and optic nerves are, as usual, derived from the cerebral ganglion, with which also the octocysts are probably connected by rather long nerves. A pair of buccal ganglia innervate the buccal mass, and are united by commissures with the right and left cerebral ganglia. The osphradial nerve springs from one of the ganglia on the visceral loop, the osphradium itself being situated (in _Limnaea_) immediately above the pulmonary orifice and adjacent to the anus (Fig. 102). This massing of the ganglia is still better illustrated by the accompanying figure of _Physa_ (Fig. 103), in which the animal is represented as if transparent, so that the ganglia and nerves are seen through the tissues. [Illustration: FIG. 102.--I. Nervous system of _Limnaea stagnalis_ L. The oesophagus has been cut and pulled forwards through the nerve-collar, so as to expose the lower surface of the buccal mass(dissected by F. B. Stead). =B.M=, buccal mass. =B.G=, buccal, =C.G=, cerebral, =Os.G=, osphradial, =Pe.G=, pedal ganglia. =Pl.G=, pleural ganglia. =Op.N=, optic, =Os.N=, osphradial, =Te.N=, tentacular nerve. =Ot=, otocyst; =V.L=, visceral loop. =R=, rectum, dotted in to show its position relative to the osphiadium.