The molluscs are a very large and diverse group. Mollusca is the second largest Phylum with over 100,000 species living in marine, freshwater and terrestrial environments. Molluscs probably evolved from a primitive flatworm ancestor. Today, their closest living relatives are the segmented worms.
The molluscs are a very successful group. If success is measured in terms of number of species and variety of habitats to which they have become adapted, then molluscs are one of the three most successful groups in the animal kingdom. Over 160,000 species have been described, of which around 128,000 are living, and about 35,000 are recorded as fossil species.

Molluscs have evolved a distinctive and highly successful body plan that features a mantle, shell, muscular foot, and radula. These features are evident in most contemporary molluscs, but have been highly modified in some groups.

Mantle

The soft body of molluscs is covered by a structure called a mantle that secretes the molluscan shell.

The mantle is a sheet like organ that forms the dorsal body wall. It grows during development as one or two folds that contain muscle layers and hemocoelic channels.

Earliest molluscs probably did not possess a shell. Instead, their dorsal side was protected with the mantle. In these ancient molluscs the mantle was covered by a hard cuticula of a horn-like material. Calcium carbonate taken in with the food was assembled in the cuticula, rendering it additionally resistive. The most ancient molluscs alive today are the so-called aplacophorans. Though these groups have certainly evolved further since the Cambrian, this state today is estimated nearest to the basic original. Both groups possess a hard cuticula strengthened with calcareous scales and sometimes calcareous spikes. The fuzz on the aplacophoran below are actually calcareous spikes.

Shell

The molluscan shell was an important evolutionary innovation.

The shell consists of at least two layers of calcium carbonate deposited in a protein framework, with a thicker protein layer covering the outer surface.

• 

• The shell is loose-fitting and never shed, but can increase in size as the mollusc grows. Note the three layers in the diagram below. The edge of the mantle secretes the outer layers (forming rings of shell as the animal grows), whereas the entire mantle surface secretes the inner shell layer. Thus the inner, nacreous layer can thicken throughout life.
  

  A grain of sand or some other foreign body can cause pearl formation. Your book indicates that probably only 1 in 1000 oysters is likely to have a valuable pearl.

  

Shells are used by other invertebrates.

In some forms the shell is reduced.

Cuttlebones or reduced internal shells of cuttlefish.

Foot

Molluscs have a ventral "foot" that is utilized for locomotion.

The movement is due to the dorsal ventral (your book) or longitudinal muscle (diagram) and transverse (your book) or oblique muscles (diagram). Mechanism consists of creating a partial suction with transverse muscle contraction and then the wave of contraction of dorsal ventral muscles push this area along as it does not relax until the wave passes over the area.

 

Radula

Molluscs have a unique feeding structure called the radula, a ribbon of recurved up to thousands of chitinous teeth, designed to scrape food from a surface. The radula rests on a cartilaginous or hemocoelic base. Teeth are replaced as they wear and in some species 5 rows of teeth may be replaced daily

Radula and teeth shape vary with diet. Cone snails have modified radula that is used to deliver neurotoxins. Harpoon like teeth are discharged from the end of a long proboscis.

One of the most significant developments is from an omega-conotoxin of Conus geographus. Using the snail's natural conotoxin (MVIIA), a synthetic derivative called SNX-111 was produced. Chronic pain affects millions of people worldwide, and is worth potentially billions to pharmaceutical companies. Therefore the availability of newer, more effective analgesics is desirable. Studies showed SNX-111 to be 100-1000 times more powerful than morphine, without many of the associated side effects.

 

(Because you asked for it: an octopus radula) )

 

 

Other features

The typical mollusc uses gills for respiration and a circulatory system for internal transport.

 

 

In most arranged so the flow is countercurrent. Fluids flow in opposite directions in separate tubes with the possibility of exchange. The fluid entering one tube highest in oxygen at that end, while that entering the second tube is lowest in oxygen at the other end. Although the oxygen differential between the two fluids is small at any point along the length of the countercurrent system, almost all oxygen contained in initial tube is transferred to the low oxygen tube. Exchange of oxygen occurs by passive diffusion. There is a coelom but it is restricted to a small area around the heart and gonads. Most molluscs have an "open" circulatory system in which a heart pumps blood into cavities or a hemocoel within the body.

The cephalopods are adapted for a more active life style and have a closed circulatory system with separate hearts to pump blood to the gills.

 

Snails and slugs which live on land do not have gills. They have modified part of the mantle cavity to absorb oxygen from the air, functioning somewhat like a lung. Some snails of this type have returned to an aquatic habitat and must come to the surface the breath air.

Digestion is mainly extracellular, except in bivalves where the tiny particles obtained by filter-feeding are digested intracellularly.

 

 

Molluscs also have an excretory organ that filters blood and expels both excess water and metabolic wastes (basically a metanephridia but not called that).

The basic nervous system of most molluscs is similar to that of annelids. In molluscs with a definite head end, ganglia are larger and communicate with anterior sense organs. In cephalopods, the head ganglia have grown to form a large brain. This generates some surprisingly complex behavior, especially in the octopus. Among its many talents, the octopus is a master at mimicking other sea creatures.

•

Molluscs, as a group, are well endowed with sense organs. The chiton has only scattered sensory patches to detect light and chemicals. However, many molluscs are equipped with statocysts, sensory tentacles and eyes. In snails, the eyes are mounted on extendible stalks. In some scallops sixty primitive tiny bright blue eyes eyes reside in rows along a scallop’s mantle edge to detect motion, light and dark. A scallop can easily regrow any lost or injured eyes. Although these eyes may or may not produce clear images, the ability to sense an object moving with the speed of one of the scallop’s predators allows the scallop to save its skin (or to be scientifically correct, its shell) by either shutting immediately or swimming away. A bivalve's eye is represented by diagram b. They can also be found in chitons and limpets. Gastropods can have pinhole or vesicular eyes. In vesicular eyes the liquid filled bubble of some pinhole eyes has developed into a lens.

Most cephalopods have a camera-type eye, almost identical to that of vertebrates.

The diagram below compares the eye found in vertebrates and most cephalopods.

1: Sense cells (retina); 2: nerve cells; 3: optical nerve; 4: blind spot. During the development of the mollusc eye, the outer epithelium caves in and forms the eye ball. The light sense cells develop as a part of the eye wall. Only later are they connected to the nervous system. In contrary, the inverse vertebrate eye is formed first and invaded later by sensory cells and nerve cells from the brain.

 

In the ancestral mollusc, gonads in animals of separate sexes produced eggs and sperm that were shed into the sea. Following external fertilization, a swimming larva developed. This trochophore settled on the surface and developed into the adult form.

Chitons have retained this primitive reproductive pattern, but it has been modified in most other molluscs.

Most marine bivalves and aquatic gastropods produce eggs, in which the trochophore stage is passed within the egg capsule, and the hatchlings emerge as veliger larvae.

After a few weeks, the veligers develop into the adult form.

In most freshwater bivalves, fertilization is internal. The resulting larvae must attach to the gills of a fish, where they live as ectoparasites before developing the bivalve body form.

In some species females sport a lure to attract fish that will carry their larvae to other places.

 

Sea slugs and air-breathing gastropods are mostly hermaphroditic. Courtship rituals proceed copulation during which bundles of sperm are exchanged. Fertile eggs may be attached to underwater objects or laid in moist soil. Eggs of land snails and slugs develop without a larval stage; tiny snails or slugs emerge at hatching.

The cephalopods also court before mating, but the sexes are separate. During copulation, large sperm bundles are inserted into the female's mantle cavity.

In some species, males can be distinguished by the modified sucker discs found at the tips of his longer two tentacles. The male uses these long arms to remove a sperm packet from his mantle cavity and to then insert this packet into the female's mantle cavity. This arm is autonomous and sometimes can get broken off while mating and is retained within the female's cavity. If this happens, the male simply grows a new one. Eggs are laid on the sea bed and some cephalopods, especially octopi, stand guard until they hatch. The eggs have large food stores that allow the young cephalopods to develop without a true larval stage.

In summary the key characteristics of mollusks are: Body bilaterally symmetrical; unsegmented; usually with a definite head.
muscular foot, used chiefly for locomotion.
Dorsal body wall forms the mantle, which encloses the mantle cavity, is modified into gills or a lung, and secretes the shell (shell absent in some).
Surface epithelium usually ciliated, bearing mucous glands and sensory nerve endings.
Coelom mainly limited to area around heart.
Complex digestive system; rasping organ (radula) usually present.
Open circulatory system of heart, blood vessels and sinuses.
Gaseous exchange by gills, lung, mantle or body surface.
Sensory organs of touch, smell, taste, equilibrium, and vision (in some); eyes highly developed in cephalopods. Come in a wide diversity of form.

The earliest shelled mollusk? Also see Monoplacophora in your textbook.