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.
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.
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.
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.)