Читать книгу Marine Mussels - Elizabeth Gosling - Страница 11

The phylum Mollusca is the second largest phylum of animals, with about 130 000 named extant species and 70 000 described fossil species (Haszprunar et al. 2008). While most of these are marine, many live in freshwater and terrestrial habitats. Research has indicated that molluscs had a terminal Precambrian origin, with rapid divergence occurring in the Cambrian era some 540–560 million years ago (Stöger et al. 2013). All molluscs have a soft body that, with the exception of some groups (see later), is protected by a hard calcium shell. Inside the shell is a heavy fold of tissue called the mantle that encloses the internal organs of the animal. Another feature of the phylum is a large muscular foot that is generally used for locomotion. Although most molluscs share this basic body plan, the group is characterised by a great diversity of form and habit.