37-1 Mollusks

 Invertebrates like clams, snails, slugs and octopuses

 Phylum mollusca

 112,000 species. Some are predators, others are filter feeders

Have bilateral symmetry

Common Features

Coelomates

 Have a true coelom (hollow fluid filled cavity)

Advantage is muscles of body wall are separate from those of the gut, so body wall muscles can contract without hindering the digestion process. Also transports blood

Trochophore  Shared by aquatic

mollusks and annelids during the larval stage

 Contains cilia at both ends and in the middle

Cilia contribute to dispersal of trochophore

Body Plan of Mollusks

2 regions 1.Head-foot 2.Visceral mass 1. Head-foot (fig. 37-2) head, mouth, sensory structures,foot for locomotion 2. Visceral mass (fig. 37-2) heart, digestive organs, excretory organs,

reproductive organs

 Mantle Covers visceral mass, which secretes 1 or

more hard shells made of calcium carbonate. Found in both sexes. Protects entire animal. The disadvantage is that the animal cannot exchange gases, so they had to evolve gills, which exchange gases with water. The gills are protected by the mantle cavity.

Ganglia   Located in the head-foot

region Connected by 2 pairs of

nerve cords

Radula A feeding adaptation. It is

a flexible, tongue-like strip covered with abrasive teeth. (fig. 37-3)

3 Classes of Mollusks

1. Gastropoda Snails, abalones, conchs 2. Bivalvia Clams, oysters, scallops 3. Cephalopodactopuses, squids, chambered nautilus *Only mollusk to have closed

circulatory system

1. Class Gastropoda

 Body undergoes torsion during larval stage. The visceral mass twists 180 degrees towards head, so everything like gills, mantle cavity and anus are near head. (fig. 37-4)

 Have open circulatory system. A circulatory fluid called hemolymph passes through body into tissue spaces called hemocoels then the hemolymph goes back to the heart.

Class Gastropoda Snails (land and

aquatic, fresh water and salt water)

 Aquatic snails respire through gills

 Land snails respire through mantle cavity and are hermaphrodites

Move around for food

Abalone

Conch

2. Class Bivalvia

 Shell is divided into 2 halves (or valves) connected by a hinge

 Use adductor muscles to close shell

Has open circulatory system

Valves  Each valve contains 3 layers of cells

secreted by mantle

a. Outer layer protects shell against acidic conditions

b. Middle layer is calcium carbonate

c. Inner layer is smooth and protects soft body. This is what forms a pearl.

Bivalves Bivalves are sessilehey use muscular foot to dig into sand

and become filter feeders Only mollusk not to have a radula  Have 3 pairs of ganglia (nerves) *near mouth *near digestive tract *near foot

Clams

 Buried in mud or sand Use siphons 1. Incurrent siphon allows food to enter,

sticks to mucus on gills which enters mouth. Gases are also exchanged and sperm also enters here.

2. Excurrent siphons allow all waste materials to exit the clam

Reproduction

Reproduction in fresh water clams vs. salt-water clams

1. Fresh water

 Sperm enters incurrent siphon and fertilization is internal

2. Salt-water Egg and sperm are released into the water.

Fertilization is external.

CephalopodaChambered

Nautilus

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