Mollusks are soft bodied animals that have an internal or external shell, a similar body plan consisting of four basic parts: a foot, mantle, shell,

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1 Mollusca

2 Mollusks are soft bodied animals that have an internal or external shell, a similar body plan consisting of four basic parts: a foot, mantle, shell, and visceral mass. Mollusks also possess a common development in that they all exist as a trochophore larva (present when the larva hatch and become a microscopic swimmer). In most mollusks, this larva changes into a veliger larva which is unique to mollusks. Eg. Snail, clam, octopus, squid.

3 Trochophore larva Veliger larva

4 Structure Consists of a muscular foot, a thin tissue layer called the mantle which secretes a calcium carbonate shell, and a visceral mass containing the internal organs. Mollusks, and all higher phyla possess a true coelom. (Eg pericardial cavity, and the cavity inside the gonad are coelomic lined with mesoderm).

5 Ingesting Food Every mode of feeding exists in this group. Most are carnivores, herbivores, or filter feeders. Many feed with a tongue-shaped structure called a radula (a rasplike structure of tiny teeth used for scraping food particles off a surface and drawing them into the mouth). Some mollusks have sharp jaws (beaks) to ea prey. Eg octopuses and squid (they also possess a radula). Filter feeders use their gills to sift food from the water Eg. Clams, oysters

6 Digesting Food Have a stomach and intestine where both physical and chemical digestion occurs. A large digestive gland surrounds the stomach and serves as the main organ of digestion and absorption.

7 Elimination Undigested food is eliminated through the anus.

8 Internal Transport Slow moving or sessile mollusks have an open circulatory system, where a simple heart pumps colorless blood containing dissolved nutrients and oxygen through blood vessels and open spaces called sinuses. Fast moving mollusks such as octopuses and squid have a closed circulatory system where blood is always contained in blood vessels. This is much more efficient.

9 Respiration Marine and freshwater mollusk breathe with gills. Terrestrial mollusks, (land snails and slugs) lack gills; instead their mantle cavity has formed a simple lung filled with air and open to the outside by a small pore called the pneumostome. The lung is richly supplied with blood sinuses and raised into folds to increase surface area.

10 Excretion Tube shaped excretory organs called nephridia remove waste products of metabolism form the blood (eg. Ammonia) and release it to the outside.

11 Locomotion Many methods of locomotion: the foot can be used for burrowing as in clams, jet propulsion is found in octopuses and squid as well as scallops when they slam their shells shut, crawling along using the foot as in snails and slugs, swimming with their foot as in sea hares.

12 Sensing Environment Mollusks leading simple inactive lives have simple sense organs chemical and touch receptors, statocysts for balance, and ocelli (eyespots). Active mollusks such as squid and octopuses are in need of more complex sense organs cephalopod eyes are very advanced and their sense of touch is very acute.

13 Coordination Mollusks vary greatly in the complexities of their nervous system. Inactive mollusks have simple nervous systems with small ganglia and have a few nerve cords. Active mollusks like the cephalopods, have very complex nervous systems. Octopuses are the most intelligent invertebrates and possess a fairly complex brain they can learn quickly and remember things for weeks. They can be trained to perform tasks for rewards and avoid punishment.

14 Reproduction In most mollusks the sexes are separate and fertilization is external. In cephalopods fertilization is internal. Some mollusks are hermaphrodites (eg. Pulmonates snails and slugs) During early development, the fertilized egg (zygote) develops into a trochophore larva and then eventually into a veliger larva in most mollusks. The trochophore larva is also found in another group of invertebrates the annelid worms, which is evidence that mollusks and the annelids had a common ancestor over 500 million years ago.

Biology. Slide 1 of 43. End Show. Copyright Pearson Prentice Hall

Biology. Slide 1 of 43. End Show. Copyright Pearson Prentice Hall Biology 1 of 43 2 of 43 What Is a Mollusk? What are the defining features of mollusks? 3 of 43 What Is a Mollusk? What Is a Mollusk? Mollusks are soft-bodied animals that usually have an internal or external