The Phylum Mollusca


 

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

Characteristics of Mollusca:-
1)Bilaterally symmetrical.
2)Body has more than two cell layers, tissues and organs.
3)Body without cavity.
4)Body possesses a through gut with 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 a pair of kidneys.
10)Reproduction normally sexual and gonochoristic.
11)Feed a wide range of material.
12)Live in most environments.

Introduction

After the Arthropods the Molluscs are the most successful of the animal phyla in terms of numbers 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. They also exhibit an enormous range in size, from species which are almost microscopic to the largest of all invertebrates the giant squid which can weighs 270 kg and measures up to 12 metres long in the body, with tentacles as much as another 50 metres in length. Many species are common and many more a 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 which come from oysters, which are also molluscs may be among the earliest forms of money.

 

Most molluscs are marine. 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 which were extremely abundant for millions of years before they became extinct. There close relatives the Nautiloid cephalopods were also once very successful but are now only represented in the world by one species, Nautilus.

Molluscs, 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 there contribution to the human diet. Mankind has been deliberately culturing molluscs as food for a long time and the earliest known records of someone farming molluscs for food come from Rome where one Sergius Orata bred oysters.

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 oysters 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, perhaps still is, made from the ink of Cuttlefish. Not all the interactions between man and molluscs are to man's benefit however, slugs and snails are, in some places, serious pests of of crops, and are often a nuisance in peoples 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.

General Anatomy

Although the original ancestor of the molluscs 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 in table 1.

Table 1. Comparison of similarities of Molluscs and Turbellarian Platyhelminthes
Characteristic Molluscs Turbellarian Platyhelminth
Externally ciliated. YesYes
Movement by cillial gliding or ventral muscular wave.YesYes
Possession of mucous glands.YesNo, but Rhabdites very similar
Intracellular digestion.YesYes
Cuticle absent.YesYes
Setae absent.YesYes

Though the modern molluscs 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 lump. 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 nonmuscular 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 is 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, 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. Molluscs 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 6 classes of Mollusca. Hopefully I will get something written about each class in the not too distant future.

The Classes of Phylum Mollusca
AmphineuraNeopilina galatheae
MonoplacophoraChitons
GastropodaCowries, Limpets, Slugs and Snails
ScaphopodaTusk Shells
BivalviaBivalves = Muscles, Clams etc.
CephalopodaNautilus, Octopus and Squid

 




Book Reviews


The Biology of Terrestrial Molluscs, by G.M.Baker (Ed.)

 

 

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