Mollusks: Ancient Wonders Of The Phylum Mollusca

Etymology: From the Latin Molluscus, meaning soft of body.

Characteristics of Mollusca:

  1. Bilaterally symmetrical.
  2. The body has more than two cell layers, tissues, and organs.
  3. Body without cavity.
  4. The body possesses a thorough gut with a mouth and anus.
  5. Body monomeric and highly variable in form may possess a dorsal or lateral shells of protein and calcareous spicules.
  6. Has a nervous system with a circum-oesophagal ring, ganglia, and paired nerve chords.
  7. Has an open circulatory system with a heart and an aorta.
  8. Has gaseous exchange organs called ctenidial gills.
  9. Has at least one pair of kidneys.
  10. Reproduction is normally sexual and gonochoristic.
  11. Feed on a wide range of materials.
  12. Live in most environments.

Introduction To The Phylum Mollusca

After the Arthropods, the Molluscs are the most successful of the animal phyla in terms of number of species. There are about 110,000 species known to science, most of which are marine.

They occupy a vast range of habitats, however, both aquatic and terrestrial. From the Arctic seas to small tropical streams and from valleys to mountainsides 7,000 metres high. There are a few adapted to live in deserts and some are parasitic. Most are marine.

They also exhibit an enormous range in size, from species that are almost microscopic to the largest of all invertebrates – the giant squid which can weigh 270 kg and measures up to 12 metres long (in the body, with tentacles as much as another 50 metres in length).

Architeuthis giant squid
Scientists from NOAA and the Delaware Museum of Natural History dissect a giant squid specimen (Architeuthis sp.)

Many species are common and many more are beautiful. Most species secrete a shell of some sort. These shells are long-lasting and have been collected by human beings for thousands of years. Some of these shells – and the pearls that come from oysters, which are also mollusks – may be among the earliest forms of money.

Molluscs are very ancient organisms believed to have evolved from a flatworm like ancestor, during the Precambrium about 650 million years ago. Because many species secrete a shell of some sort, the fossil record is good.

Different classes of molluscs have been predominant in the past and the Ammonites represent a group of Cephalopods that were extremely abundant for millions of years before they became extinct. Their close relatives, the Nautiloid cephalopods, were also once very successful but are now only represented in the world by one species, Nautilus.

Mankind and Molluscs

Mollusks, because of their ease of capture, edibility and beauty have long been important to mankind.

Molluscs of many sorts are eaten by humans: Abilone, Clams, Cockles, Muscles, Octopus, Oysters, Periwinkles, Scallops, Snails, Squid, Whelks, Winkles and many more are all molluscs and all make their contribution to the human diet.

Mankind has been deliberately culturing molluscs as food for a long time. The earliest known records of someone farming mollusks for food come from Rome, where one Sergius Orata bred oysters.

mollusca tusk shell
Mollusc shell is usually a calcareous exoskeleton that protects the soft parts of the animal

Mollusc shells have also had a long history of usage by mankind. Many have been used as decorations, or as a substance to carve into cameos and buttons.

In North America, Tusk shells on the west coast and Cockles on the East supplied the basis of a system of money. In many tropical countries, the shells of coweries were until recent times used extensively in trade. Pearls, which arise in oysters as a result of the oyster’s attempts to cover up a grain of sand within its mantle, have been – and still are – much sort after.

The ‘mother of pearl’ used to make pearl buttons comes from bivalve shells… and so great was the market for it, that the Mississippi and Missouri river basins have been seriously over collected and the bivalves are now quite scarce.

In ancient times, the city of Tyre was famous for its purple dye. This dye was made from a marine mollusc called Murex sp. While Sepia, a brown pigment used by artists, was (and perhaps still is) made from the ink of Cuttlefish.

Not all the interactions between man and mollusks are to man’s benefit, however.

Slugs and snails are, in some places, serious pests of crops and are often a nuisance in people’s gardens. Wooden ships and wharves can be destroyed by burrowing bivalves such as Teredo navalis, known as ship worms – which weaken the timbers until they collapse or fall apart.

Mollusc Anatomy & Visceral Mass

Although the original ancestor of the mollusks is lost in the dawn of time, scientists have theorised that the original mollusc arose from a flatworm (Platyhelminth) like organism. The similarities are listed below:

Characteristic Molluscs Turbellarian Platyhelminth
Externally ciliated. Yes Yes
Movement by cillial gliding or ventral muscular wave. Yes Yes
Possession of mucous glands. Yes No, but Rhabdites very similar
Intracellular digestion. Yes Yes
Cuticle absent. Yes Yes
Setae absent. Yes Yes

Though the modern mollusk show quite a wide degree of adaptable variability in form, there are several basic anatomical characteristics that can be found in all or most of them.

The body is divided into two functional regions, the head-foot and the visceral mass:

mollusk anatomy diagram with visceral mass

The head-foot is the part you see most easily in slugs and snails. It is mostly a muscular organ covered in cilia and rich in mucous cells, which the mollusc uses to move around. It normally tapers to a tail at one end and has a head incorporated in the front.

The head includes a mouth, eyes and tentacles – the last two may be much reduced or even absent. In those species with shells, the head-foot can be drawn into the shell.

The rest of the body is the visceral mass. This is entirely non-muscular and contains the organs of digestion and reproduction. It includes the gonads, the kidney, the heart and the digestive diverticulum.

Attached to the dorsal surface of the visceral mass is – and hanging freely down the sides of it – the mantle (often called the skirt or pallium). There is a space between the mantle and the viseral mass. This space is greatest towards the rear of the animal, where it is called the mantle cavity or the pallial cavity. The mantle cavity generally contains the gills or ctenidia (gill-like organs used for respiration).

mollusk visceral mass
African Giant Land Snail (Achatina fulica) hiding its visceral mass under its shell

A water current, generated by beating cillia, enters the mantle cavity at the sides, passes over the gills and departs centrally, i.e. the outward bound current runs out between the two inward bound currents.

Near the head, just behind the mouth is a pair or more of ganglia and a nerve ring – from which two nerve chords arise that reach out through the body. Mollusks are true coelomic animals. Though the coelom they have is small, enclosing only the gonads and the heart where it is called the gonodial cavity and the pericardial cavity respectively.

This then, is the plan of a basic unevolved mollusc. This basic plan is changed and adapted, for the requirements of different lifestyles, almost beyond recognition in some of the 7 classes of Mollusca.

Over the last few decades there has been much scientific discussion concerning the true taxonomic relationships within the Phylum Mollusca, and even suggestions that monoplacophora were not really molluscs at all.  However in January 2020 Kevin M. Kocot et. al. published an in-depth phylogenetic analysis of the phylum mollusca in the journal Nature that resolved many, if not all, of these issues for the present.

Classes of Phylum Mollusca

Squid image license: Creative commons

Gordon Ramel

Gordon is an ecologist with two degrees from Exeter University. He's also a teacher, a poet and the owner of 1,152 books. Oh - and he wrote this website.

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