The ganglion cells of Helix vary from 0.4 mm. in the visceral ganglia to 0.16 mm. or 0.007 mm in the cerebral. Some of the larger cells have a connective tissue capsule. The majority are unipolar, but bi- and multi-polar cells are also found. The central region of the ganglia is occupied by Leydig's 'Punkt-substanz,' which is composed really of a network of very fine fibres derived from the ganglion cells. The nerves are composed of an outer sheath of vesicular connective tissue, and an inner membranous sheath continuous with internal septa dividing the nerve-fibrillae into bundles. The fibrillae and fibrillar bundles originate from the 'Punkt-substanz.'

The organs of special sense are sensory epidermic cells, the tentacular ganglia, the ganglia of the oral lobes, the eyes, and the otocysts. The sensory cells are most numerous on the tentacles, oral lobes, and on the sides of the foot, where they have the form of cylinders with a fine point composed of delicate hairs, which do not project above the level of the cuticle of the ordinary epidermic cells. At the actual edge and on the sole of the foot, parts covered by mucus, the hairs are distinctly separate, and project beyond the cuticle of the surrounding cells. Hence there is a closer resemblance with the 'Pinselzellen' of aquatic Pulmonata and other Mollusca in which the cell is terminated by a distinct head and a bundle of long projecting hairs.

The nerve which enters each tentacle ends at its summit in a knob composed of nerve-fibrils and ganglion-cells, the whole surrounded by a muscular sheath derived from the retractor muscle of the tentacle. There are masses of small ganglion-cells underlying the epidermis, which is modified in the region of this terminal ganglion. The ordinary epidermic cells acquire an exceedingly well-developed cuticle vertically striated. A few goblet cells occur among them, and numerous sense-cells connected each with a fibril from a ganglion-cell. The sense-cells are. flask-shaped, and end (?) either in a single point or several hairs. They do not reach as far as the cuticle of the epidermic cells. These tentacular organs appear to be olfactory in function. As long as one or both pairs are present the snail recoils from strong-smelling liquids, e.g. turpentine. If both pairs are removed it creeps into them. The ganglionic structures in the large oral lobes or lips, and the smaller series of oral lobes are similar to the tentacular ganglia. The overlying epidermic cells have also a very thick cuticula.

A tentacular ganglion is found on the outer side near the bases of the two tentacles of Basommatophora, and an oral ganglion in the oral lobes, e. g. of Limnaeus.

The eyes are situated close to the apex of the upper tentacles, a little to their inner side. The optic nerve is a branch of the tentacular nerve in the Siylom-matophora, an independent nerve in the Basommatophora. The retina is composed of a single layer of cells derived by invagination from the epidermis. The cells are of two kinds, pigmented and non-pigmented, both continuous basally with the nerve-fibrils derived from a peripheral optic ganglion. The pigmented cells are widest at their inner, the non-pigmented at their outer ends. The inner ends of the non-pigmented cells end in a flask-shaped (Stylommatophora) or a fine (Basommatophora) visual rod. The inner ends of the pigmented cells form transparent processes. Each non-pigmented cell is surrounded by a zone of pigmented, and the visual rods are encased in the transparent processes of the latter. The retina is continuous at its anterior margin with a layer of transparent cells which lines the cornea (=pellucida). The optic cavity is filled by a structureless substance, the vitreous body (Carriere), commonly called the lens. By Hilger a vitreous body is distinguished from a lens in all the Gastropoda he examined except the Stylom-matophora, which, according to him, possess a lens but no vitreous body.

It is probable that the lens, so called, is a differentiated portion of the vitreous body, and not an independently formed structure. The cornea consists of an outer layer of transparent cells with underlying connective tissue.

The otocyst is small in size, and is to be found on the pedal ganglion close to the spot where the anterior bundle of pedal nerves quits it. To the naked eye it appears like a minute white dot. It consists of a connective tissue capsule nearly, but not quite, spherical, and an epithelial lining with a central cavity. The latter contains a fluid and a number of otoliths, as is usual in Gastropoda. The otoliths are more or less oval, and consist of a small quantity of an organic substance and a large quantity of calcium carbonate. The cells of the epithelium are not clearly separated from each other in the fresh state, according to Leydig. At the spot opposite to the auditory nerve (or the auditory canal) they are of larger size. All the cells are ciliated, and the cilia are short. It is doubtful whether or not long auditory or sensory hairs are present such as are found in Natantia (=Heteropoda) and in Cyclas (a Lamellibranch). Leydig believes them to be present. The otoliths are in constant motion in the living animal. The otocyst is connected to the cerebral ganglion by a delicate filament. This, according to Leydig, as in some other Gastropoda, is a narrow canal lined by cells, and continuous with the cavity of the otocyst. Nerve-fibrillae are therefore not present.

But if this is the case, it is difficult to see in what way auditory impulses can affect the nervous system. And, judging from the analogy of other Mollusca, auditory nerve-fibrils are probably present. The connection of the otocysts with the pedal ganglia is only accidental. Both sets of structures are enveloped by a common investment of loose connective tissue.

An osphradium or olfactory organ in connection with the organ of respiration has not been detected in Helix pomatia and many of its allies. In Helix personata Sarasin found a nerve arising from the region of the right visceral ganglion which passes beneath the epithelium of the pulmonary chamber, the cells being more columnar where the nerve is in contact with them than elsewhere. It bends round the anterior edge of the pulmonary aperture, and at the spot where it bends swells into a small ganglion with large ganglion-cells. A nerve extends from the ganglion to the glandular cells of the epidermis (?) of the collar. The homologous nerve, but devoid of a ganglion, is present in Succinea amphibia, Bulimus detritus, B. decollates and Limax cinereoniger. It is distributed to the glandular epithelium as in H. personata. The corresponding nerve is apparently present in H.pomatia1. The osphradial apparatus is well developed in the aquatic Pulmonates, Limnaeus, Planorbis, and Physa. In the first of the three, which has a dextral shell, the nerve is derived from the right visceral ganglion; in the other two, which have sinistral shells, from the corresponding left ganglion.

The nerve ends in a ganglion with large ganglion cells, in connection with a ciliated canal or depression in the pulmonary chamber above and behind its orifice. In branchiate Gastropoda the nerve is also derived from the visceral ganglion, and ends in a similar ganglion lying immediately beneath the epidermis close to the ctenidium. The epidermic cells above this ganglion are large and columnar. The apparatus is paired when the ctenidia are paired. See general account of the Class.

Nervous system: of Helix pomatia and Limnaeus, Bohmig, Inaugural Dissertation, Leipzig, 1883; of terrestrial Pulmonata, Leydig, A. M. A. i. 1865; of aquatic Pulmonata, De Lacaze Duthiers, A. Z. Expt. i. 1872; of Mollusca, von Ihering, Vergleich. Anat. des Nervensystems, etc. der Mollusken, Leipzig, 1877.

Homology of the ganglia.Spengel, Z. W. Z. xxxv. 1881. Cf. Ray Lankester, 'Mollusca,' Encyclopaedia Brit. (ed. ix.) xvi. p. 636, and Fig. 1 d.

Pedal nerves: of Zonites, Nalepa, SB. Akad. Wien. lxxxvii. Abth. 1. 1883, p. 282; of Limax, Helix, Arion, Simroth, Z. W. Z. xxxii. 1879, pp. 304-318; of Vaginulus, Semper, A. M. A. xiv. 1877, p. 123.

Histology.See Bohmig, op. cit. supra; Vignal, A. Z. Expt. (2) ii. 1883. Cf. Haller, Marine Rhipidoglossa, ii. M. J. xi. 1886.

Organs of special sense.Sensory epithelium. Flemming, A. M. A. v. 1869. Tentacular ganglion, etc, Id. A. M. A. vi. 1870; Sarasin, Arb. Zool. Zoot. Inst. Wurzburg, vi. 1883. Oral ganglion (=Semper's organ), Sarasin, op. cit. Eye, Hilger, M. J. x. 1885. Carriere, Sehorgane der Thiere, Leipzig, 1885; Z. A. ix. 1886. Otocyst. Leydig, A. M. A. vii. 1871. Cf. Simroth, Z. W. Z. xxvi. 1876, pp. 278-281; De Lacaze Duthiers, A. Z. Expt. i. 1872. Osphradium (=olfactory organ): of Mollusca, Spengel, Z. W. Z. xxxv. 1881; of aquatic Pulmonata, Simroth, Z. W. Z. xxvi. 1876, p. 308; De Lacaze Duthiers, A. Z. Expt. i. 1872 (=organe nouveau d'innervation); of terrestrial Pulmonata, Sarasin, op. cit. supra.

Regeneration of eye, etc. in Pulmonata. Carriere, Studien uber die Regenerations-erscheinungen bei den Wirbellosen, i. Wurzburg, 1880.

1 Simroth has described (Z. A. v. 1882, and with figures in J. B. Mal. Gesellsch. x. 1883), in the slug Parmacella Olivieri, a groove with projecting edges extending from the pulmonary aperture forwards to the left, and lying in the furrow between the edge of the mantle-fold and body. Ganglion cells underlie the groove and its edges, which are supplied by the right pallial nerve, as well as by a branch from the left pallial nerve. The position of the organ external to the pulmonary chamber appears to render an homology with the osphradium impossible. Zonites has a sac opening close to the pulmonary aperture and lying in the roof of the pulmonary chamber. It is beset with gland-cells and supplied by the 'olfactory' nerve, which is, however, devoid of a ganglion (see Nalepa, op. cit. ante, p. 239).