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Chapter 29 Mollusks and Annelids Section 29.1 Mollusks Mollusks Snails Slugs Oysters Clams Scallops Octopuses Squids A True Coelem Mollusks and annelids are likely the first animal groups to have true coelems Body Cavity The gut and other organs are suspended from the body wall and cushioned by fluid Mollusks and Annelids Also contain trochophores Larval stage developed from fertilized eggs Key Characteristics Body cavity True coelem Symmetry Mostly bilateral 3-part body plan Visceral mass Central section containing the organs Mantle heavy fold of tissue forming the outer layer of the body Foot Muscular region used for locomotion Organ systems Excretion Circulation Respiration Digestion Reproduction Shell 1 or 2 shells that serve as exoskeleton Radula Tongue-like organ Organ Systems Excretion Beating cilia pulls fluid from coelom into nephridia Respiration Gills (aquatic) Primitive lung (terrestial) Recover useful molecules from the coelomic fluid Reproduction Circulation 3-chambered heart Most have male and female organs Snails and slugs are hermaphrodites Body Plans of Mollusks Differ in each class of mollusk Shell and foot are adapted to different living conditions Snails and slugs Gastropods Single shell Foot for locomotion Terrestrial gastropods secrete mucus from the base of their foot Most are herbivores Some are predators Radula may contain poison Bivalves Most are marine Hinged shell (valves) Adductor muscles connect valves Do NOT have distinct head region or radula Male and female Filter feeders Use foot to dig into sand Siphons Used to draw in sea water Capable of producing pearls Nacre Cephalopods Squids, octopus, cuttlefish, and nautiluses Large head attached to tentacles Foot divided into many parts Squids have 10, octopus has 8 Nautilus has outer shell squids and cuttlefish have internal shells Most intelligent of all invertebrates Well-developed brain Draw water into mantle cavity and expel it through a siphon Propulsion Secrete fluid for protection Active predators Tentacles Section 29.2 Annelids The First Segmented Animals 2/3 of Annelids live in the sea Range in size from 1 mm to 3 meters Easily recognized by their segments Contain digestive, excretory, circulatory and locomotor organs Some segments are modified for reproduction, feeding or sensation Cerebral ganglion Primitive brain Septa Internal body walls that separate segments Characteristics of Annelids Coelom Large fluid-filled cavity Organ systems Show high degree of specialization The gut has different regions to perform different functions in digestion Bristles Setae Found in pairs on segments of annelids Allow worm to crawl and anchor itself Parapodia Fleshy appendages Annelid Groups Groups differ in number of setae found on each segment Not all groups have parapodia These are the major characteristics used to classify Annelids Marine Worms Largest group of Annelids Iridescent colors Contain fleshy, paddle-like parapodia Many are burrowing species Some are free-swimming predators Earthworms Contain setae on each segment Have no eyes They do have light-sensitive and touch-sensitive organs located at the ends of their bodies Other sensory cells detect moisture Scavengers Eat their way through soil Specialized digestive system Pharynx – esophagus – crop – gizzard – intestine Hydrostatic Skeleton Created by the fluid-filled coelom Supports each segment Segments become longer when circular muscles contract Segments bunch up when longitudinal muscles contract These muscle allow them to crawl Leeches Contain suckers at both ends of their body Most species are predators or scavengers Some are parasites Lack setae and parapodia Segments are not separated internally
