Chapter 28 Mollusks & Annelids BIOLOGY II Miss. Loulousis
Mollusks have a true coelom Snails, slugs, oysters, clams, scallops, octopuses, and squids are all mollusks Annelid earthworm next section Mollusks and Annelids Probably first group of organisms to develop a true coelom (body cavity) Larval stage called a trochophore, develops into fertilized egg In some species is free-swimming and propels itself through the water by movement of cilia on its surface
**Trochophore larva The microscopic trochophore larva has a belt of cilia that circles its body. The beating of the cilia propels the trochophore through the water
Phylum Mollusca Mollusks, cont. 2 nd largest animal phyla (1 st Arthropoda) Abundant in almost all marine, freshwater, and terrestrial habitats Terrestrial mollusks often go unnoticed who searches for snails? Seven classes of mollusks 3 major classes Gastrpoda snails and slugs Bivalvia clams, oysters, and scallops Cephalopoda octopuses and squids
**Key Characteristics of Mollusks Body cavity Symmetry Three-part body plan Organ systems Shell Radula Despite the visual differences between mollusks, all share these common characteristics
Body Cavity True coelom In most species, it is very small and only around the area immediately surrounding the heart
Symmetry Most exhibit bilateral symmetry
Three-part body plan Visceral mass Central section that contains the mollusk s organs Mantle Wrapped around the visceral mass like a cape Forms the outer layer of the body Muscular foot Used primarily for locomotion (movement)
Three-part body plan
Organ systems Systems for excretion, circulation, respiration, digestion, and reproduction
Shell Many have either one or two shells that serve as an exoskeleton For protection Shell is composed of protein that is strengthened by calcium carbonate Not to be confused with the mantle The mantle is made of tissues which secrete the shell
Radula All except bivalves have a radula Tongue-like organ located in the mouth Has thousands of pointed, backward-curving teeth arranged in rows When it feeds it pushes the radula out of its mouth The radula scrapes across surfaces collecting fragments of food Mollusks that are predators use the radula for attacking prey
**Mollusks on the menu A delicacy in many restaurants Oysters Clams Scallops Snails (escargot) Squid Octopuses (calamari) Abalone Mussels
Mollusk Body Plan
Organ Systems Only coelomates without segmented bodies One way digestive system EXCRETION Coelom collects waste-filled body fluids Cilia pulls the fluid from the coelom into tiny tubular structures called nephridia Recover useful molecules (sugars, salts, and water) Remaining fluid leaves the mollusk through a pore that opens in the mantle cavity CIRCULATION Digestive tube is surrounded by mesoderm, which prevents diffusion of nutrients into the cells Blood carries nutrients and oxygen to tissues and removes waste and carbon dioxide Most have a 3-chambered heart and an open circulatory system Octopuses and squids have a closed circulatory system
Organ Systems, cont. RESPIRATION Most respire with gills located in the mantle cavity Freshwater snails Ciliated gills beat on the inner surface of mantle causing a continuous stream of water to pass over the gills Terrestrial snails Most have no gills Instead, the thin membranes that line their empty mantle cavity functions like a primitive lung Must be kept moist for oxygen to diffuse across it Therefore, most are active at night or after it rains During dry times, they pull back into their shell and plug the opening with a wad of mucus to keep from drying out Sea snails Lack gills Gas exchange takes place through their skin
Organ systems, cont. REPRODUCTION Most have distinct male and female individuals Some snails and slugs are hermaphrodites Some species of oysters and sea slugs can change from one sex to the other and back again Marine mollusks are moved from place to place via the trochophore larvae drifting in ocean currents Octopuses, squids, freshwater snails, and some freshwater mussels have no free-swimming larvae Larval stage occurs within the egg and a juvenile-stage mollusk hatches from the egg
**Body Plans of Mollusks Gastropods Bivalves Cephalopods
Gastropods Snails and slugs Range from microscope to almost 40 inches Most have a single shell Foot is adapted for locomotion Terrestrial species secrete mucus from the base of their foot, forming a slimy path Most have a pair of tentacles on their heads with eyes often located at the tips Varied feeding habits Many are herbivores some are garden pests some are active predators Source of food for humans Land snails (genus Helix) are raised on snail farms and consumed in great quantities
Gastropods
Bivalves Most are marine Oysters, mussels, and clams Important food source for humans Have a two-part hinged shell Valves are secreted by the mantle Two thick muscles called the adductor muscles connect the valves Most are sessile, some can move quite fast if necessary Do not have a distinct head region or a radula nerve ganglion above their food serves as simple brain Have simple sense organs Most are male or female (some hermaphroditic) Reproduce sexually Most are filter feeders Many produce pearls (oyster) Only a few produce gem quality pearls
Bivalves
Cephalopods Squid, octopuses, cuttlefish, and nautiluses Foot is modified into tentacles Most of body is made up of a large head attached to tentacles Squid have 10, octopuses 8, nautilus 80-90 Used for seizing prey Most lack external shell Nautilus is only living species that still has an external shell Squid and cuttlefish have internal shell Most intelligent of all invertebrates Complex nervous system & well developed brain Eye is similar to that of vertebrates some have color vision Squids and octopuses can release a dark fluid that clouds the water to conceal their escape Active marine predators
1.Excretion in sea snails 2.Feeding in clams 3.Swimming in oyster larvae-trochophores Algal blooms occur when aquatic microorganisms called algae suddenly become more numerous in a particular area of the ocean or a lake. These rapid increases in algal populations can have a variety of effects on other aquatic organisms. For example, some species of algae produce poisonous chemicals, or toxins. The concentration of these toxins in the water can rise to dangerous levels during an algal bloom. One toxin in particular causes cilia to stop beating. We will refer to this toxin as toxin A.
28.2 Annelids The First Segmented Animals Coelomates Phylum Annelida Fossils can be found in rock that is 530 myo Evolved in the sea, where 2/3 live today Most other annelids are terrestrial earthworms Range in size from less than 1 mm to more than 3 m (10ft) long Easily recognized by their segments Each segment contains digestive, excretory, circulatory, and locomotor organs Some are modified for reproduction, feeding, or sensation
Annelids, cont. Cerebral ganglion, or primitive brain, is located in one anterior segment Connected to nerve cord that runs along the underside of the worm Sensory info is delivered by nerve cord Septa, internal body walls, separate the segments of most annelids Nutrients pass via the circulatory system
Characteristics of Annelids Coelom Large and located entirely within the mesoderm Organ systems High degree of specialization closed circulatory system Excretory structures called nephridia Bristles External bristles called setae Provide traction as the annelid crawls Some have parapodia fleshly appendages used for locomotion or gas exchange
Classification of Annelids External char. that help classify Differ in number of setae (bristles) they have on each segment Not all annelids have parapodium
**Insect Larvae Many people mistake grub, maggots, and caterpillars for worms These animals are arthropods, not annelids Larval stage for beetles, flies, and moths or butterflies
Marine Worms Members of class Polychaeta (PAHL ih keet uh) Largest group of annelids Live in all ocean habitats Beautiful, showing unusual forms and iridescent colors Distinct char. is the pair of fleshy, paddle-like parapodia that occur on most segments Usually have setae Used to swim, burrow, or crawl Greatly increase surface area making gas exchange more efficient
Marine worms, cont. Many are burrowing species, some live in protective tubes formed by secretions of their glands Some like the feather duster, live with only their head stuck out of the tube Traps food particles from the water
Nereis Free-swimming Predators that use their strong jaws to capture and eat small animals
**Importance of Polychaetes Most important in benthic (bottom-dwelling) communities Few people notice them, or know of their existence but they exert a strong influence on marine benthic food webs Some of the 10,000 species so far described eat animals, other eat plants, and many more provide food for commercially important fish
Earthworms Members of the class Oligochaeta (AHL ih goh keet uh) No parapodia and only a few setae Lack distinctive head region, no eyes Have light-sensitive and touch-sensitive organs located at each end of body Other sensory cells that detect moisture
Earthworms, cont. Highly specialized scavengers Literally eat their way through the soil, consuming their own weight in soil everyday Soil moves through one-way gut via the pharynx, down the esophagus, and into the crop (storage container). From here, it moves into the gizzard, to the intestine, and out the anus Food is absorbed into the intestinal wall Remaining material passes out the anus in a form called castings Tunneling activity allows air to penetrate soil, and their casting fertilize it Rich, organic soil may contain thousands of earthworms per acre
Anatomy of Earthworm
Hydrostatic Skeleton Fluid creates hydrostatic skeleton that supports each segment Each segment contains muscles that pull against this hydrostatic skeleton Circular muscles wrap around each segment Longitudinal muscles span the length When the circular muscles contract, the worm becomes longer When the longitudinal muscles contract, the segments bunch up Move by alternately contracting these muscles
Leeches Has suckers at both ends of its body Most species are predators or scavengers, but some are parasites or vertebrates and crustaceans Only members of the class Hirudinea (hihr yoo DIHN ee uh) Lack both setae and parapodia Body is flattened, and unlike other annelids, its segments are not separate internally
Leeches