Some aspects of the structure and function of the nervous system ofArchachatina (Calachatina) marginata(Swainson)

Abstract

This account is restricted to the ganglia of the central nervous system and the pallial, visceral and cervical nerves. The histology of the ganglia, nerves and their sheaths is described, attention being drawn to the vascular sheaths which enclose the system. Cell sizes within the ganglia are indicated and the giant cells and their axons are described. The possibility that a synaptic connexion exists between medium sized (40 to 60 u) nerve cells and the giant axons is discussed. The active and contractile nature of the peripheral nerves is described and some measurements of nerve lengths and diameters under extended and contracted conditions are given. Maximum extended lengths are approximately double the contracted lengths and under extended conditions, their mean diameters are reduced by a factor of 0.75. More than 60 nerve fibers are estimated to be present in the middle fine right pallial nerve and more than 1000 in the right pallial nerve. The innervation of the collar and the lateral body wall receives brief mention. Observations and measurements of active snails and of dead, expanded animals, have lead to the development of a technique for the estimation of maximum functional lengths of the nerves in terms of the shell dimensions. The method is discussed with reference to the danger of multiple pick-up of single-action potentials during investigations into the electrical activity in the nerves. A factor based upon the breadth of the shell gives reliable estimates of maximum nerve lengths. Attention is directed to the following activities: protraction and retraction of the head and cervical region; pedal locomotory waves and the dorsal body waves, their relative velocities and wavelengths; and opening and closure of the pneumostome. Activity in the collar and the cervical region is important in relation to electro-physiological inves -tigations reported elsewhere. The nerves concerned are the pallial and cervical nerves. It is possible that an integration centre for the pallial system is situated in the abdominal ganglion. The enclosure of the nervous system within a special part of the vascular system is discussed with reference to conditions in the primitive gastropod. The visceral nerve of Archachatina is compared with the nervus intestinalis. of Helix pomatia as described by Schlote (1955) and attention is drawn to the relative advantages to be gained from the use of the pallial nerves, in which cell somata are few in number. Conditions within the ganglia are discussed. Little is known of cell relationships but it is suggested that (i) the large fibers in the pallial nerves are the axons of the giant cells,(ii) the form of their collaterals in the ganglia furnishes a possible explanation for observed changes in the action potentials of the peripheral nerves, (iii) the regular patterns of nervous activity may be due to group control of discharge rates throught the mediation of the giant cells.