Anatomy of a Slug: Overview

Before we examine the slugs' behavior, let's explore their anatomy in a bit more detail. The basic structure of a slug does not vary with species, though the location or existence of certain organs does. The following website, although it discusses Limax maximus (the great slug) and other species, should suffice to delineate the common traits of slugs. Richard Fox of Lander University guides his students through a dissection (there are no photographs, only diagrams). After reading these notes, I came away in awe of the intricacy of slugs; externally they appear so simple.

This photo shows many of the external features of a slug. Here you can see the banded visceral mass, the mantle, the pneumostome, the foot, the cephalic and oral tentacles, and the head. You cannot see the gonopore as it is normally hidden. 

This is Fox's diagram of a dissected Limax maximus. Compare it to the photograph below of a dissected Lehmannia valentiana. Note that the Limax maximus is much larger than the Lehmannia valentiana, so the spatial proportions of the internal organs differ between the species. As you can see from the photo, the organs of the three-band slug are smaller and more compressed. Nevertheless, the two species share a basic organization. 

Credit: Charles Olsen, USDA APHIS PPQ, Bugwood.org