Structure and function of the digestive system in molluscs.

The phylum Mollusca is one of the largest and more diversified among metazoan phyla, comprising many thousand species living in ocean, freshwater and terrestrial ecosystems. Mollusc-feeding biology is highly diverse, including omnivorous grazers, herbivores, carnivorous scavengers and predators, and even some parasitic species. Consequently, their digestive system presents many adaptive variations. The digestive tract starting in the mouth consists of the buccal cavity, oesophagus, stomach and intestine ending in the anus. Several types of glands are associated, namely, oral and salivary glands, oesophageal glands, digestive gland and, in some cases, anal glands. The digestive gland is the largest and more important for digestion and nutrient absorption. The digestive system of each of the eight extant molluscan classes is reviewed, highlighting the most recent data available on histological, ultrastructural and functional aspects of tissues and cells involved in nutrient absorption, intracellular and extracellular digestion, with emphasis on glandular tissues.