Feeding, digestion and metabolism

Abstract

The digestive tract of A. ater has been described and illustrated by Roach (1968) and that of D. reticulatum by Walker (1969) and by Runham and Hunter (1970). The mouth opens into the buccal cavity and, in D. reticulatum, it is surrounded by a number of small lips, including three anterior lips, five pairs of lateral lips and one single posterior lip (Walker, 1969). These are flanked by a pair of larger mouth lobes (oral lappets). The two most posterior pairs of lateral lips fuse inside in the mouth to form lateral folds, which extend back through the buccal cavity and are continuous with the lateral folds in the oesophagus (Runham, 1975). Runham also described how the surface of these lips, the mouth and the buccal cavity were lined by a cuticle which was thickened inside the buccal cavity to form a jaw and probably also the radula. The jaw consisted of chitin and protein together with mineral salts. The form of the jaw varies with different families (see Chapter 1) and it is lacking in the family Testacellidae. The range of jaw types was described by Taylor (1902–1907) and the jaws of most slugs may be divided into two groups. They are either crescent-shaped and markedly ribbed (odontognathous; Fig. 3.1(a)) or relatively smooth and usually with a median anterior projection or beak (oxygnathous; Fig. 3.1(b) and (c)). The former group includes the arionid slugs while the latter group includes the limacid and milacid slugs.