BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.8. Kingdom Animalia (animals)

2.8.7. Phylum Mollusca (molluscs)

The Classification and key CHARACTERISTICS OF THE phylum Mollusca are summarized in Table 2.18 (data are presented for only three of the six classes). This phylum comprises a diverse array of organisms, ranging from slow-crawling snails and slugs to relatively sedentary bivalves, such as edible mussels, cockles, and others, as well as highly active cephalopods (Fig. 2.63).

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Fig. 2.63. Diversity of Molluscs.

Table 2.18. Classification and key characteristics of THE PHYLUM MOLLUSCA (molluscs)

Phylum Mollusca
Key characteristics
Unsegmented, triploblastic, coelomate animals Usually bilaterally symmetrical Body soft and fleshy, divided into a HEAD, a ventral muscular FOOT, and a dorsal visceral hump* Skin (mantle) covering the hump secretes a calcareous shell Main body cavity is a haemocoel Limbs absent
Class Gastropoda	Class Pelycopoda (Bivalvia)	Class Cephalopoda
Terrestrial, marine, and freshwater	Aquatic	Aquatic. The largest and most complexly organized molluscs
Asymmetrical	Bilateral Symmetry	Bilateral symmetry
Shell is univalve, typically coiled As a result of the Torsion of the visceral hump during growth	Shell consists of two hinged halves called Valves (hence the term "bivalve"). The body is enclosed within the valves and laterally compressed	Shell internal, often reduced or absent
Large, flat foot used for locomotion	Foot small; frequently used for burrowing in sand or mud	Adapted for fast swimming. Foot modified to form part of the head and tentacles
Head, eyes, and sensory tentacles present	Head very small, tentacles absent	Well-developed head bearing tentacles with suckers; well-developed eyes
Radula — a ribbon-like, Tongue-like Structure used for scraping and grinding food	Filter feeders	Radula and horny jaws
Anus located at the anterior end of the body	Anus located at the posterior end of the body	Anus located at the posterior end of the body
Examples: Helix aspersa (garden snail) Patella (limpet) Buccinum (whelk) Limax (slug)	EXAMPLES: Mytilus edulis (edible mussel) Ostrea (oyster)	EXAMPLES: Sepia officinalis (cuttlefish) Loligo (squid) Octopus vulgaris (common octopus)
* Diagnostic feature

This phylum includes over 80,000 living and about 35,000 fossil species; in terms of species richness, molluscs rank second only to Arthropods. One mollusc, the giant squid, is the largest invertebrate, with an estimated mass of several tonnes and a length of 16 m.

The Development of a protective shell and The Use of gills or Lungs for gas exchange have enabled molluscs to colonize both aquatic and terrestrial habitats and occupy A wide variety of ecological niches. However, the shell can impede movement; consequently, in the most active molluscs, the shell has either been reduced or lost entirely.

Last update: 06/08/2026

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