Phylum Mollusca.pptx

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1. Phylum Mollusca General characteristics and classification Numreen Nazir Ph.D. Zoology

2. A large, muscular foot variously modified for locomotion, digging, attachment, and prey capture. Mantle, a highly modified epidermis that covers and protects the soft body. In most species, the mantle also secretes a shell of calcium carbonate. A visceral mass housing the internal organs. A mantle cavity, the space between the mantle and viscera. Gills, when present, are suspended within this cavity. A radula , a protrusible , rasp-like feeding organ present in most, but not all, species. In herbivorous mollusks (e.g., chitons and snails), the radula is used for scraping algae from rocks. In carnivores, the radula can be fang-like and is used for piercing prey (e.g., squids and octopods), or may be pointed and used for drilling through shells (e.g., some snails).

3. This is the second largest animal phylum. Molluscs are terrestrial or aquatic (marine or fresh water) having an organ-system level of organization. They are bilaterally symmetrical, triploblastic and coelomate animals. Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump. A soft and spongy layer of skin forms a mantle over the visceral hump. The space between the hump and the mantle is called the mantle cavity in which feather like gills are present. They have respiratory and excretory functions. The anterior head region has sensory tentacles. The mouth contains a file-like rasping organ for feeding, called radula .

4. Molluscs are a  clade  of organisms that all have soft bodies which typically have a "head" and a "foot" region. Often their bodies are covered by a hard exoskeleton, as in the shells of snails and clams or the plates of chitons . A part of almost every ecosystem in the world, molluscs are extremely important members of many ecological communities. They range in distribution from terrestrial mountain tops to the hot vents and cold seeps of the deep sea, and range in size from 20-meter-long giant squid to microscopic  aplacophorans , a millimeter or less in length, that live between sand grains.

5. Morphology Despite their amazing diversity, all molluscs share some unique characteristics that define their body plan. The body has a head, a foot and a visceral mass. This is all covered with a mantle (also known as a pallium ) that typically secretes the shell. In some groups, like slugs and octopuses, the mantle is secondarily lost, while in others, it is used for other activities, such as respiration. The freshwater Sinistral Pond Snail ( Physella  sp.) scrapes algae from the glass with its radula , the two "toothy" arcs you can see lining the mouth. The buccal cavity, at the anterior of the mollusc , contains a radula (lost in bivalves) — a ribbon of teeth supported by an odontophore , a muscular structure. The radula is generally used for feeding. The ventral foot is used in locomotion. This foot propels the mollusc by utilizing muscular waves and/or cilia in combination with mucus.

6. Typically, at least in the more primitive members of each group, there are one or more pairs of gills (called ctenidia ) which lie in a posterior cavity (the pallial cavity) or in a posterolateral groove surrounding the foot. The pallial cavity typically contains a pair of sensory osphradia (for smelling) and is the space into which the kidneys, gonads, and anus open. Molluscs are coelomate , although the  coelom is reduced and represented by the kidneys, gonads, and pericardium, the main body cavity which surrounds the heart.

7. Life history and ecology Molluscs occur in almost every habitat found on Earth, where they are often the most conspicuous organisms. While most are found in the marine environment, extending from the intertidal to the deepest oceans, several major gastropod clades live predominantly in freshwater or terrestrial habitats. Remarkably, one study found around 3000 species within a single locality at a coral reef in New Caledonia. In terrestrial communities, gastropods can achieve reasonably high diversity and abundance: as many as 60-70 species may coexist in a single habitat and abundance in leaf litter can exceed more than 500 individuals in four liters of litter.

8. After their initial appearance, molluscan taxonomic diversity tended to remain low until the Ordovician, when gastropods, bivalves, and cephalopods show large increases in diversity. For bivalves and gastropods this diversification increases throughout , with relatively small losses at the end-Permian and end-Cretaceous extinction events. Cephalopod diversity is much more variable through the Phanerozoic , whereas the remaining groups ( monoplacophorans , rostroconchs , polyplacophorans , and scaphopods ) maintain low diversity over the entire Phanerozoic or became extinct.

9. There are four major groups within the phylum Mollusca : Class  Polyplacophora  consists of chitons , snail-like molluscs with eight-part overlapping scale shells . Class  Gastropoda  are true snails and slugs. They represent the most diverse class within phylum Mollusca with 60,000 to 80,000 extant species in marine, freshwater, and terrestrial habitats. Class  Bivalvia  are molluscs with hinged two-part shells. Examples include clams, oysters, mussels, and scallops. Class  Cephalopoda  are molluscs with large heads, large eyes, and grasping tentacles. Examples include octopus, squid, cuttlefish, and nautiloids .

10. The  foot  is a muscular organ found in all molluscs . Polyplacophorans and gastropods have a single flat foot used for crawling. Some bivalves, such as clams, have a paddle-shaped foot adapted for digging into soft sediments.

11. Because a sea slug’s stomach is in its foot, it is named  Gastropoda , “stomach-foot” (from the Greek root words  gastro  meaning  stomach  and  pod  meaning  foot ; The foot in octopus and squid is modified into many tentacles that are attached to the animal’s head. That feature gave the class its name  Cephalopoda  (from the Greek root word  cephal - meaning  head ), or the “head-foot” molluscs . Octopus and squid use their tentacles for moving and for grasping and holding the prey they capture for food.

12. In most molluscs , the  mantle  produces a hard protective shell. The mantle also creates patterns of color on a shell. The shell is an exoskeleton, even though it is completely surrounded by soft tissue in some molluscs . The shell is continually produced and grows with the animal. Chitons are in the class  Polyplacophora  ( poly  meaning  many ;  placo  meaning  plate  or  shell ;  phora  meaning  bearing ). A chiton’s mantle produces eight shell-like plates that cover the body. Joints between the plates allow the chiton to curl up in a ball and to move flexibly. The class of molluscs called  Bivalvia  (from Latin root words  bi - meaning  two  and - valv  meaning  folding door ) includes clams, oysters, mussels, and scallops. Bivalves produce two shells that are hinged at the top 

13. The mouth structures of many molluscs include a specially adapted rasp-like tongue called a radula . The  radula  is a hard ribbon-shaped structure covered in rows of teeth. Herbivorous snails have a mouth with a radula of usually five to seven complex teeth. There is a great diversity of radula forms in the mollusca . The snail uses its radula like a file, rasping it back and forth over the substrate to scrape off small bits of food

14. As radular teeth wear down or break off, new teeth are formed to replace them. The tooth patterns of snail radulas are distinctive to species, and scientists can identify snails by looking at their radulas . Some radulas are highly specialized. A group of gastropods called cone snails are carnivorous (meat-eating) hunters that produce venom in glands near the mouth. Their radulas are shaped into long, hollow teeth, which they thrust one at a time into their prey like harpoons

15. Molluscs breathe with gills called  ctenidia  that sit in a cavity between the mantle and body mass. In some molluscs , most notably bivalves like oysters and mussels, the ctenidia are also used as filter feeding apparatus to strain particulate food from the water. Molluscs have a complete digestive tract surrounded by a small coelom .

16. The molluscan circulatory system is composed of a series of blood  sinuses  or cavities, rather than closed, discrete vessels. This is referred to as an  open circulatory system. Molluscs display a large diversity of nervous systems, from the rudimentary nervous system of the brainless bivalves to the complex systems of the cephalopods, who have well-developed brains and are considered the most intelligent of invertebrates.

17. Chitons  ( Polyplacophora ) are basal relative to other extant molluscs . Their soft bodies are covered with a series of eight shell plates. The joints between these shell plates enable to chitons to roll up for protection. Chitons are mobile and contract their muscular foot in waves to move about. The primarily herbivorous chitons have a well-developed radula .