EDIBLE SNAIL. 121 



not common in the course of the two main nerves. The nerves to the margins of 

 the foot originating from the pedal nerves form a similar network with nodal ganglia. 

 In Vaginulus, according to Semper, the pedal nerves run parallel to one another, 

 and are provided at stated intervals with ganglia from which spring transverse 

 commissures. The similar transverse commissures ofLimax appear from Simroth's 

 description to be rather irregular, and they give origin to a network of fibres with 

 nodal ganglia which unite them together. The nerves passing across the foot in 

 Helix anastomose, but they are very irregular in arrangement ; and in Arion they 

 break up into a fine network, and there is nothing to compare with the commissures 

 of Limax either in the direction or in the size of the nerves ; but the networks of 

 nerve-branches are furnished as usual with nodal ganglia both in Helix and Arion. 

 The regular arrangements of transverse commissures in Zonites and Vaginulus 

 recall the ladder-like structure of the pedal nervous system in Chiton, Haliotis and 

 Fissurella among marine Gastropoda. It is not certain, however, whether or no 

 they are strictly comparable with one another. 



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 con- 

 nective 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 c 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 



