There are three great invertebrate phyla: Mollusca, Annelida and Arthropoda. Together they make up the majority of invertebrate species. Molluscs are true shellfish and they first appeared back in the Cambrian Period or even earlier. The phylum is quite diverse, containing snails and slugs, classic shellfish like oysters and mussels, as well as squid and octopus. These animals may look quite different but they share some common characteristics that joins them together in Phylum Mollusca.
Most molluscs are unsegmented and this differentiates them from the annelids and arthropods. One primitive group, the monoplacophora, show segmentation, so it appears that this characteristic was present in early molluscs but lost in most groups.
Molluscs all have a head that contains the sense organs, mouth and nervous system. There is a nerve ring rather than a brain and this is connected by two pairs of nerve chords that connect to the foot and to the viscera and mantle. Molluscs have a coelom but it is reduced in size. Their body wall is thick and muscular and most have a muscular foot on the ventral side of the body which is the main form of locomotion. In most molluscs, the dorsal body wall is extended and forms a mantle, from which the shell is secreted in all the shelled molluscs. Some molluscs have secondarily lost the shell, such as squid, octopus and slugs but most molluscs still have this most characteristic feature, which acts as the main defence against predators.
Molluscs have a well developed digestive tract that is regionalised. The mouth contains a proboscis-like radula for scraping and chewing. The esophagus follows with specialised regions for food storage and breakup. There is a stomach and a pair of digestive glands, an intestine and an anus that usually opens into the mantle cavity. Molluscs also have a pair of kidneys to remove wastes.
Almost all molluscs also have a well developed circulatory system that includes a heart. Some have blood vessels but in many molluscs the blood just circulates through open spaces. The blood usually contains the respiratory pigment haemocyanin rather than haemoglobin.
Development is similar in most molluscs. Eggs are small with little yolk, cleavage is spiral and determinate and the egg develops into a free-swimming, ciliated trochophore larva.
Because virtually all molluscs share the above characteristics, it is believed that they all come from a common stock and therefore deserve to be included in a single phylum.
There are seven classes of living molluscs:
1. Monoplacophora, represented by ten species and considered to be the most primitive class of the phylum. They were only known from fossils until a living species was discovered in Mexican waters in 1952.
2. Aplacophora. The solenogasters and caudofoveata. There are about 250 species of these shell-less aberrant molluscs.
3. Polyplacophora. These are the chitons, which are characterised by 8 overlapping plates instead of a single shell. There are about 600 species worldwide and they are common in rocky intertidal zones, where they can be found in hollows that they make in the rocks. At night, they move around, grazing on algae.
4. Gastropoda. These are the single-shelled molluscs such as snails, abalone, limpets and cone shells. This is the largest, most widespread and successful group of molluscs, with about 40,000 known species. Their shells are spirally curved and their heads are well developed. They are found in marine, freshwater and terrestrial environments. Most are slow-moving herbivores that crawl around on their muscular foot and use their radula to scrape plant matter off rocks. Some gastropod feet have been modified for swimming or burrowing instead of crawling. Gastropod shells are extremely variable in shapes, colors and patterns and so are favored by collectors.
5. Pelecypoda. These are the bivalve molluscs such as clams, oysters, mussels and scallops. This is the second most successful group of molluscs with about 8000 species worldwide. Their mantle secretes two shells joined at a hinge. They live in sediments and on hard substrates such as rocks. Some of the most commercially important shellfish belong to this group. Most are filter feeders and have a reduced brain compared with the gastropods.
6. Scaphopoda. Tusk shells (200 species) These are marine burrowers with a characteristic tooth-shaped shell.
7. Cephalopoda. Squid, cuttlefish and octopus. There are about 800 living species of this group, the most highly evolved of the phylum, with well developed sense organs and large brains. These animals are predators and most have secondarily lost their shells. Instead they depend on quick movements and camouflage to protect themselves from other predators.
There is a lot more that could be said about molluscs and their importance in marine and freshwater ecosystems. Their larvae form an important part of the zooplankton while the adults form a major part of benthic communities. Molluscs can be found at every trophic level as well: herbivores, carnivores, filter feeders and top carnivores in their ecosystems. They are also economically important in many human cultures from the pearl oyster industry to edible oysters, clams and mussels to the common garden snail which plagues many gardeners while being a delicacy in France. This is a most important group of animals.
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.