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Chapter 35. Mollusks Annelids. Mollusks. Describe the key characteristics of mollusks. Describe the body plan of mollusks. Name the characteristics of three major classes of mollusks. Compare the body plans of gastropods, bivalves, and cephalopods. Characteristics of Mollusks.
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Chapter 35 Mollusks Annelids
Mollusks • Describe the key characteristics of mollusks. • Describe the body plan of mollusks. • Name the characteristics of three major classes of mollusks. • Compare the body plans of gastropods, bivalves, and cephalopods. • .
Characteristics of Mollusks • PhylumMollusca is a diverse group of invertebrates such as clams, snails, slugs, squids, and octopuses. • They are called mollusks, from the Latin molluscus, which means “soft.” Many mollusks havesoft bodies and some have a hard shell.
Characteristics of Mollusks • Some mollusks are sedentary filter feeders, while others are fast-moving predators. • Mollusks arecoelomates. • Most aquatic mollusks and annelids have a larval stage called a trochophore.
Body Plan of Mollusks • The body of a mollusk is generally divided into: • The visceralmass, which contains the heart and the organs of digestion, excretion, and reproduction • the head-foot, which consists of: • the head, which contains the mouth and sensory structures • the foot, a large, muscular organ for locomotion
Body Plan of Mollusks • The coelom is limited to a space around the heart. • A layer of epidermis called the mantle coversthe visceral mass.
Body Plan of Mollusks • In most mollusks, the mantle secretes one or more hard shells containing calcium carbonate. • This disadvantage of a shell is offset by having gills. • The gills are protected within the mantle cavity.
Body Plan of Mollusks • Most mollusks are bilaterally symmetrical. • The nervous system consists of paired clusters of nerve cells called ganglia . • The main feeding adaptation of many mollusks is the radula.
Class Gastropoda • The largest and most diverse class of mollusks is Gastropoda, whose members are called gastropods. • Most gastropods, including snails, abalones, and conchs, have a single shell. Others, such as slugs and nudibranchs, have no shell.
Class Gastropoda • Gastropods have an open circulatory system in which a heart pumps hemolymph from gills or lungs into the hemocoel. abalones
Snails • Snails live in moist and aquatic environment. • Most snails locate food using eyes at the end of tentacles. • Snails survive dry periods by retreating into their shells and sealing the opening with a mucous plug. Fresh Water Snail Land (Common) Snail
Other Gastropods • Slugs are terrestrial and lack shells. • Pteropods, or “sea butterflies,” have a foot that is modified into a winglike flap and used for swimming rather than crawling. Grey Slug Sea Butterfly
Class Cephalopoda • The class Cephalopoda includes octopuses, squids, cuttlefishes (below), and chambered nautiluses. • They are called cephalopods, which means “head-foot.”
Class Cephalopoda • They are specialized as free-swimming predators. • They have tentacles and beaklike jaws on the head. • Their nervous system is very advanced. • They have a closed circulatory system. • Many use pigments to hide and disguise themselves chambered nautilus
Squids • Squids are cephalopods with ten tentacles. • They propels themselves by pumping jets of water with the mantle through an excurrent siphon. Deep Sea Squid
Octopuses • Octopuses have eight tentacles and are similar to squids. • They often crawl along the ocean bottom or lie in wait for prey.
Class Bivalvia • The class Bivalvia includes aquatic mollusks such as clams, oysters, and scallops. • They are called bivalves because their shell is divided into two valves connected by a hinge. • Each valve consists of three layers. Giant Clam
Class Bivalvia • Most bivalves are sessile filter feeders. • Bivalves lack a distinct head and have three pairs of nerve ganglia. Flame Scallop
Clams • Clams are bivalves that live buried in mud or sand. • The mantle cavity of a clam is sealed except for a pair of hollow, fleshy tubes called siphons. • Water enters through the incurrent siphon. • Water leaves through the excurrent siphon.
Annelids • Identify the structures that provide the basis for dividing annelids into three classes. • List the advantages of body segmentation. • Describe the structural adaptations of earthworms. • Compare the three classes of annelids.
Characteristics of Annelids • The phylum Annelida is made up of bilaterally symmetrical, segmented worms. • This phylum includes common earthworms, feather-duster worms, and bloodsucking leechs (below).
Characteristics of Annelids • They are called annelids,which means “little rings” and refers to the many body segments. • Annelids have a true coelom that is divided into separate compartments by partitions. Social Feather-duster Worms
Characteristics of Annelids • Most annelids have external bristles called setae (singular, seta), and some have fleshy protrusions called parapodia (singular, parapodium).
Class Oligochaeta • Annelids of the class Oligochaeta generally live in the soil or in fresh water and have no parapodia. • Oligochaeta means “few bristles”; these annelids have a few setae on each segment. • The most familiar member is the earthworm.
Class Oligochaeta Structure and Movement • An earthworm’s body has over 100 nearly-identical segments. • Circular and longitudinal muscles line the interior body wall. • Locomotion is made possible by segmentation.
Class OligochaetaFeeding and Digestion • Earthworms ingest soil as they burrow through it. • Soil is moved through these structures: • mouth • pharynx • esophagus • crop • gizzard • intestine • includes the typhlosole • anus
Class OligochaetaCirculation • Contractions of the aorticarches and the dorsal blood vessel force blood through the closed circulatory system.
Class OligochaetaRespiration and Excretion Respiration and Excretion • Oxygen and carbon dioxide diffuse through the skin, which contains many small blood vessels and must be kept moist. • Cellular wastes and excess water are excreted through nephridia.
Class OligochaetaNeural Control • The nervous system of an earthworm consists of a chain of ganglia connected by a ventral nerve cord. • Sensory structures are found in all segments but are concentrated at the anterior end.
Class OligochaetaReproduction • Earthworms are hermaphrodites, but an individual worm cannot fertilize its own eggs. • During mating, earthworms press their ventral surfaces together. • They are held together by their setae and by a film of mucus secreted by each worm’s clitellum.
Class OligochaetaReproduction • The sperm from each worm move through the mucus to the seminal receptacle of the other. • The clitellum secretes a tube of mucus and chitin. • Fertilization occurs inside the tube, which forms a protective case for the young worms.
