Structure

Abstract

The outline of a typical slug (Fig. 2.1) is elongated and streamlined, with the visceral mass incorporated completely within the cavity of the head-foot. Tillier (1984) considered this character to be a distinguishing feature of the slug and suggested that a ‘semislug’ evolved into a slug when the stomach sank into the foot cavity, lower than the posterior edge of the foot cavity. The sequence of stages in limacization of urocyclids.has been clearly illustrated by van Mol (1970). Some urocyclid slug-like forms still retain a prominent dorsal visceral hump. The mantle covers about a third to a quarter of the anterior dorsal region of the body, giving some protection to the pallial complex and enclosing the vestigial shell when this is present. The anterior border is usually free, forming a flap which may cover the head and neck of the slug when these are contacted. The pneumostome, which opens into the lung-like mantle cavity, is located on the right side of the mantle edge and its position varies with the different slug families (Chapter 1). The pneumostome is opened and closed during respiration. The lips of the pneumostome are thickened and contain additional mucous gland cells. The structure of the pneumostome of L. flavus was described by Davies (1974) while Cook and Shirbhate (1983) showed that, in section, the pneumostome is surrounded by an annulus of large cells which may function as an elastic annulus.