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Mollusks-Segmented wormsArthropods Mollusks-Segmented wormsArthropods • • • • Organ systems Triploblastic Bilateral True coelom Mollusca • Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids • Most molluscs are marine, though some inhabit fresh water and some are terrestrial • Molluscs are soft-bodied animals, but most are protected by a hard shell • All molluscs have a similar body plan with three main parts: – Muscular foot – Visceral mass – Mantle • Many molluscs also have a water-filled mantle cavity, and feed using a rasplike radula • Most molluscs have separate sexes with gonads located in the visceral mass • The life cycle of many molluscs includes a ciliated larval stage called a trochophore • There are four major classes of molluscs: – Polyplacophora (chitons) – Gastropoda (snails and slugs) – Bivalvia (clams, oysters, and other bivalves) – Cephalopoda (squids, octopuses, cuttlefish, and chambered nautiluses) Table 33-3 Chitons • Class Polyplacophora consists of the chitons, ovalshaped marine animals encased in an armor of eight dorsal plates Fig. 33-16 Gastropods • About three-quarters of all living species of molluscs are gastropods Fig. 33-17 (a) A land snail (b) A sea slug • Most gastropods are marine, but many are freshwater and terrestrial species • Most have a single, spiraled shell • Slugs lack a shell or have a reduced shell • The most distinctive characteristic of gastropods is torsion, which causes the animal’s anus and mantle to end up above its head Fig. 33-18 Mantle cavity Anus Mouth Stomach Intestine Bivalves • Molluscs of class Bivalvia include many species of clams, oysters, mussels, and scallops • They have a shell divided into two halves Fig. 33-19 • The mantle cavity of a bivalve contains gills that are used for feeding as well as gas exchange Fig. 33-20 Mantle Hinge area Coelom Gut Heart Adductor muscle Digestive gland Anus Mouth Excurrent siphon Shell Palp Foot Mantle cavity Gonad Gill Water flow Incurrent siphon Cephalopods • Class Cephalopoda includes squids and octopuses, carnivores with beak-like jaws surrounded by tentacles of their modified foot • Most octopuses creep along the sea floor in search of prey Fig. 33-21 Octopus Squid Chambered nautilus • Squids use their siphon to fire a jet of water, which allows them to swim very quickly • One small group of shelled cephalopods, the nautiluses, survives today • Cephalopods have a closed circulatory system, welldeveloped sense organs, and a complex brain Annelida • Annelids have bodies composed of a series of fused rings • The phylum Annelida is divided into three classes: – Oligochaeta (earthworms and their relatives) – Polychaeta (polychaetes) – Hirudinea (leeches) Table 33-4 Oligochaetes • Oligochaetes (class Oligochaeta) are named for relatively sparse chaetae, bristles made of chitin • They include the earthworms and a variety of aquatic species • Earthworms eat through soil, extracting nutrients as the soil moves through the alimentary canal • Earthworms are hermaphrodites but cross-fertilize Fig. 33-22 Cuticle Epidermis Coelom Circular muscle Septum (partition between segments) Metanephridium Longitudinal muscle Anus Dorsal vessel Chaetae Intestine Fused nerve cords Ventral vessel Nephrostome Metanephridium Clitellum Esophagus Crop Pharynx Giant Australian earthworm Intestine Gizzard Cerebral ganglia Mouth Subpharyngeal ganglion Blood vessels Ventral nerve cord with segmental ganglia Polychaetes • Members of class Polychaetes have paddle-like parapodia that work as gills and aid in locomotion Fig. 33-23 Parapodia Leeches • Members of class Hirudinea are blood-sucking parasites, such as leeches • Leeches secrete a chemical called hirudin to prevent blood from coagulating Fig. 33-24 Arthropoda • Two out of every three known species of animals are arthropods • Members of the phylum Arthropoda are found in nearly all habitats of the biosphere Arthropod Origins • The arthropod body plan consists of a segmented body, hard exoskeleton, and jointed appendages, and dates to the Cambrian explosion (535–525 million years ago) • Early arthropods show little variation from segment to segment General Characteristics of Arthropods • The appendages of some living arthropods are modified for many different functions Fig. 33-29 Cephalothorax Antennae (sensory reception) Head Abdomen Thorax Swimming appendages (one pair located under each abdominal segment) Walking legs Pincer (defense) Mouthparts (feeding) • The body of an arthropod is completely covered by the cuticle, an exoskeleton made of layers of protein and the polysaccharide chitin • When an arthropod grows, it molts its exoskeleton • Arthropods have an open circulatory system in which fluid called hemolymph is circulated into the spaces surrounding the tissues and organs • A variety of organs specialized for gas exchange have evolved in arthropods CLASS ARACHNIDA • • • • • Spiders-Scorpion-Tick Cephalothorax and Abdomen 4 pairs of legs Simple eyes No antennae Fig. 33-31 50 µm Scorpion Dust mite Web-building spider Class Crustaceans • While arachnids and insects thrive on land, crustaceans, for the most part, have remained in marine and freshwater environments • Crustaceans typically have branched appendages that are extensively specialized for feeding and locomotion • Most crustaceans have separate males and females • Cephalothorax and abdomen-compound eyes-tow pair of antennae-5 pairs of legs-mostly marine and freshwater Fig. 33-38a (a) Ghost crab Class Diplopoda • Millipedes –terrestrial, and have jaw-like mandibles – Each trunk segment has two pairs of legs – Elogate---dorsal convexed body—many segments— one pair of antennae—simple eyes---terrestrial – Scavengers Fig. 33-33 • Centipedes, class Chilopoda, are carnivores – They have one pair of legs per trunk segment – Flattened body – Simple-eyes – One pair of antennae Fig. 33-34 Class Insecta • more species than all other forms of life combined • They live in almost every terrestrial habitat and in fresh water • Head thorax and abdomen • Three pairs of walking legs • Often with wins on thorax • Simple and compound eyes • One pair of antennae Fig. 33-35 Abdomen Thorax Head Compound eye Antennae Heart Cerebral ganglion Dorsal artery Crop Anus Vagina Malpighian tubules Ovary Tracheal tubes Nerve cords Mouthparts • Insects diversified several times following the evolution of flight, adaptation to feeding on gymnosperms, and the expansion of angiosperms • Insect and plant diversity declined during the Cretaceous extinction, but have been increasing in the 65 million years since • Flight is one key to the great success of insects • An animal that can fly can escape predators, find food, and disperse to new habitats much faster than organisms that can only crawl • Many insects undergo metamorphosis during their development • In incomplete metamorphosis, the young, called nymphs, resemble adults but are smaller and go through a series of molts until they reach full size • Insects with complete metamorphosis have larval stages known by such names as maggot, grub, or caterpillar • The larval stage looks entirely different from the adult stage • Most insects have separate males and females and reproduce sexually • Individuals find and recognize members of their own species by bright colors, sound, or odors • Some insects are beneficial as pollinators, while others are harmful as carriers of diseases, or pests of crops • Insects are classified into more than 30 orders