Chapter 33 Notes
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Date Created: 04/05/16
Biology Chapter 33 Notes Life without a backbone o Invertebrates are animals that lack a backbone o They account for more than 95% of known animal species o Invertebrates are morphologically diverse and occupy almost every habitat on Earth Concept 33.1: Sponges are basal animals that lack true tissues o Animals in the phylum Porifera are known informally as sponges o They are sedentary and live in marine waters or fresh water o Sponges are filter feeders, capturing food particles suspended in the water that passes through body Water is drawn through pores into a cavity called the spongocoel and out through an opening called the osculum o Sponges lack true tissue and organs o Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food o Sponges consist of a gelatinous noncellular mesohyl layer between two cell layers Amoebocytes are found in the mesohyl and play roles in digestion and structure o Most sponges are hermaphrodites: Each individual functions as both male and female Concept 33.2: Cnidarians are an ancient phylum of eumetazoans o All animals except sponges and a few other groups belong to the clade Eumetazoa, animals with true tissues o Phylum Cnidaria is one of the oldest groups in this clade o Cnidarians have diversified into a wide range of both sessile and motile forms including jellies, corals, and hydras o They exhibit a relatively simple diploblastic, radial body plan o The basic body plan of a cnidarian is a sac with a central digestive compartment, the gastrovascular cavity o A single opening functions as mouth and anus o There are two variations on the body plan: the sessile polyp and motile medusa A polyp adheres to the substrate by the aboral end of its body A medusa has a bell-shaped body with its mouth on the underside Medusae do not attach to the substrate but move freely Cnidarians are carnivores that use tentacles to capture prey o The tentacles are armed with cnidocytes, unique cells that function in defense and capture of prey o Nematocysts are specialized organelles within cnidocytes that eject a stinging thread Phylum Cnidaria diverged into two major clades, Medusozoa and Anthozoa, early in its evolutionary history o Medusozoans Medusozoans include all cnidarians that produce a medusa Scyphozoans (jellies) Cubozoans (box jellies) Hydrozoans Most hydrozoans alternate between polyp and medusa forms Hydra, a freshwater cnidarian, exists only in polyp form and reproduces asexually by budding The medusa is the predominant stage in the life cycle of most scyphozoans and cubozoans For example, coastal scyphozoans have a brief polyp stage, whereas open ocean species generally have no polyp stage In cubozoans the medusa is box-shaped Cubozoans often have highly toxic cnidocytes For example, the sting of the sea wasp off the coast of northern Australia can lead to respiratory failure, cardiac arrest, and death within minutes o Anthozoans The clade Anthozoa includes the corals and sea anemones Anthozoans occur only as polyps Corals often form symbioses with algae and secrete a hard exoskeleton (external skeleton) Each generation grows on the skeletal remains of the previous generation, forming “rocks” that provide habitat for other species Concept 33.3: Lophotrochozoans, a clade identified by molecular data, have the widest range of animal body forms Bilaterian animals have bilateral symmetry and triploblastic development Most have a coelom and a digestive tract with two openings The clade Bilateria contains Lophotrochozoa, Ecdysozoa, and Deuterostomia o The clade Lophotrochozoa was identified by molecular data Some develop a lophophore for feeding, others pass through a trochophore larval stage, and a few have neither feature o Lophotrochozoa includes the flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids o Flatworms Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats Although flatworms undergo triploblastic development, they are acoelomates Flatworms have a gastrovascular cavity with one opening Gas exchange takes place across the surface, and protonephridia regulate the osmotic balance Their dorsoventrally flattened shape maximizes surface area for gas exchange Flatworms are divided into two lineages Catenulida, or “chain worms,” reproduce asexually by budding Rhabditophora are more diverse and include both free-living and parasitic species o Free-Living Species The best-known rhabditophorans are planarians Planarians live in fresh water and prey on smaller animals Planarians have light-sensitive eyespots and centralized nerve nets The planarian nervous system is more complex and centralized than the nerve nets of cnidarians Planarians are hermaphrodites and can reproduce sexually, or asexually through fission o Parasitic species Parasitic rhabditophorans live in or on other animals Two important groups of parasitic rhabditophorans are the trematodes and the tapeworms Tapeworms are parasites of vertebrates and lack a digestive system Tapeworms absorb nutrients from the host’s intestine Trematodes parasitize a wide range of hosts, and most have complex life cycles with alternating sexual and asexual stages Trematodes that parasitize humans spend part of their lives in snail hosts They produce surface proteins that mimic their host and release molecules that manipulate the host’s immune system o Rotifers Rotifers, phylum Rotifera, are tiny animals that inhabit fresh water, the ocean, and damp soil Rotifers are smaller than many protists but are truly multicellular and have specialized organ systems Rotifers have an alimentary canal, a digestive tube with a separate mouth and anus that lies within a fluid-filled pseudocoelom § Rotifers reproduce by parthenogenesis, in which females produce offspring from unfertilized eggs Some species are unusual in that they lack males entirely o Lophophorates: Ectoprocts and Brachiopods Lophophorates have a lophophore, a crown of ciliated tentacles around their mouth Lophophorates have a true coelom Lophophorates include two phyla: Ectoprocta and Brachiopoda Ectoprocts are sessile colonial animals that superficially resemble plants A hard exoskeleton encases the colony, and some species are reef builders Brachiopods superficially resemble clams and other hinge-shelled molluscs, but the two halves of the shell are dorsal and ventral rather than lateral as in clams Brachiopods are marine and most attach to the seafloor by a stalk o Molluscs Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids Most molluscs are marine, though some inhabit fresh water and some snails and slugs 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, but many snails are hermaphrodites The life cycle of many molluscs includes a ciliated larval stage called a trochophore Four of the major classes of molluscs are: Polyplacophora (chitons) Gastropoda (snails and slugs) Bivalvia (clams, oysters, and other bivalves) Cephalopoda (squids, octopuses, cuttlefish, andchambered nautiluses) Chitons Chitons are oval-shaped marine animals encased in an armor of eight dorsal plates They use their foot like a suction cup to grip rock, and their radula to scrape algae off the rock surface Gastropods About three-quarters of all living species of molluscs are gastropods Most gastropods are marine, but many are freshwater and terrestrial species Gastropods move slowly by a rippling motion of the foot or by cilia Most have a single, spiraled shell that functions in protection from injury, dehydration, and predation Most gastropods are herbivores, but some species use modified radula to feed on prey Bivalves Bivalves are aquatic and include many species of clams, oysters, mussels, and scallops They have a shell divided into two halves drawn together by adductor muscles Some bivalves have eyes and sensory tentacles along the edge of their mantle The mantle cavity of a bivalve contains gills that are used for feeding as well as gas exchange Most species are sedentary, but some have limited motility Cephalopods Cephalopods are carnivores with beak-like jaws surrounded by tentacles of their modified foot They are able to immobilize prey with a poison present in their saliva Most octopuses creep along the sea floor in search of prey 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, well-developed sense organs, and a complex brain Shelled cephalopods called ammonites were common but went extinct at the end of the Cretaceous 65.5 million years ago Protecting freshwater and terrestrial molluscs Molluscs are the animal group with the largest number of recent extinctions The most threatened groups are: o Freshwater bivalves, including pearl mussels o Terrestrial gastropods, including Pacific island land snails These molluscs are threatened by habitat loss, pollution, and non-native species o Annelids Annelids are coelomates with bodies composed of a series of fused rings The phylum Annelida is divided into two clades Errantia Sedentaria Errantians Most members of clade Errantia are mobile, marine organisms Many errantians have a pair of paddle-like or ridge-like structures called parapodia (“beside feet”) on each body segment Each parapodium has numerous chaetae, bristles made of chitin Parapodia are not unique to this clade Sedentarians Sedentarians tend to be less mobile than errantians Some species burrow into the substrate, while others live in protective tubes Tube-dwelling sedentarians often have elaborate gills or tentacles used for filter feeding This clade also contains the leeches and the earthworms o Leeches Most species of leeches live in fresh water; some are marine or terrestrial Leeches include predators of invertebrates, and parasites that suck blood Leeches secrete a chemical called hirudin to prevent blood from coagulating o Earthworms Earthworms eat through soil, extracting nutrients as the soil moves through the alimentary canal Earthworms are hermaphrodites but cross-fertilize Some reproduce asexually by fragmentation Concept 33.4: Ecdysozoans are the most species-rich animal group o Ecdysozoans are covered by a tough coat called a cuticle o The cuticle is shed or molted through a process called ecdysis o The two largest phyla are nematodes and arthropods o Nematodes Nematodes, or roundworms, are found in most aquatic habitats, in the soil, in moist tissues of plants, and in body fluids and tissues of animals They have an alimentary canal, but lack a circulatory system Body wall muscles are all longitudinal, and their contraction produces a thrashing motion o Arthropods 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 This body plan dates to the Cambrian explosion (535–525 million years ago) Early arthropods show little variation from segment to segment Arthropod evolution is characterized by a decrease in the number of segments and an increase in appendage specialization These changes may have been caused by changes in Hox gene sequence or regulation General characteristics of arthropods The appendages of some living arthropods are modified for functions such as walking, feeding, sensory reception, reproduction, and defense o These modified appendages are jointed and come in pairs The body of an arthropod is completely covered by the cuticle, an exoskeleton made of layers of protein and the polysaccharide chitin o When an arthropod grows, it molts its exoskeleton Arthropods have eyes, olfactory receptors, and antennae that function in touch and smell Arthropods have an open circulatory system in which hemolymph is circulated into the spaces surrounding the tissues and organs A variety of organs specialized for gas exchange have evolved in arthropods Morphological and molecular evidence suggests that living arthropods consist of three major lineages that diverged early in the phylum’s evolution o Chelicerates (sea spiders, horseshoe crabs, scorpions, ticks, mites, and spiders) o Myriapods (centipedes and millipedes) o Pancrustaceans (lobsters and other crustaceans, as well as insects and their relatives) Chelicerates o Chelicerates, clade Chelicerata, are named for clawlike feeding appendages called chelicerae o They have an anterior cephalothorax and a posterior abdomen o The earliest chelicerates were eurypterids (water scorpions) o Most marine chelicerates (including eurypterids) are extinct, but some species survive today, including horseshoe crabs o Most modern chelicerates are arachnids, which include spiders, scorpions, ticks, and mites o Arachnids have six pairs of appendages: the chelicerae, the pedipalps, and four pairs of walking legs o Gas exchange in spiders occurs in respiratory organs called book lungs o Many spiders produce silk, a liquid protein, from specialized abdominal glands Myriapods o The clade Myriapoda includes millipedes and centipedes o All living myriapods are terrestrial o They have a pair of antennae and three pairs of appendages modified as mouthparts o Millipedes eat decaying leaves and plant matter o Millipedes have many legs, with two pairs per trunk segment o Centipedes are carnivores o Centipedes have one pair of legs per trunk segment o Poison claws on the foremost trunk segment paralyze prey and aid in defense Pancrustaceans o Recent evidence indicates that terrestrial insects are more closely related to crustaceans than myriapods o Some lineages of crustaceans are more closely related to insects than other crustaceans o Together, insects and crustaceans form the clade Pancrustacea o Crustaceans Crustaceans live in marine and freshwater environments Many crustaceans have highly specialized appendages Small crustaceans exchange gases through the cuticle; larger crustaceans have gills Most crustaceans have separate males and females Isopods include terrestrial, freshwater, and marine species Pill bugs are a well-known group of terrestrial isopods Decapods are all relatively large crustaceans and include lobsters, crabs, crayfish, and shrimp Planktonic crustaceans include many species of copepods, which are among the most numerous of all animals They are rivaled in abundance by shrimplike krill Barnacles are a group of mostly sessile crustaceans They have a cuticle that is hardened into a calcium carbonate shell o Insects Hexapoda is an enormous clade including insects and their relatives Insects live in almost every terrestrial habitat and in fresh water The internal anatomy of an insect includes several complex organ systems § 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 has 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 Insect wings are an extension of the cuticle 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 Concept 33.5: Echinoderms and chordates are deuterostomes o Echinoderms (phylum Echinodermata) include sea stars and sea urchins § Chordates (phylum Chordata) include the vertebrates o Echinoderms and chordates constitute the clade Deuterostomia o Deuterostomes share developmental characteristics: Radial cleavage Formation of the anus from the blastopore o However, some other animals also share these o developmental characteristics o Deuterostomes are defined primarily by DNA similarities o Echinoderms Sea stars and most other echinoderms are slowmoving or sessile marine animals A thin epidermis covers an endoskeleton of hard calcareous plates Echinoderms have a unique water vascular system, a network of hydraulic canals branching into tube feet that function in locomotion and feeding Males and females are usually separate, and sexual reproduction is external Most adult echinoderms have radial symmetry with multiples of five Echinoderm larvae have bilateral symmetry Living echinoderms are divided into five clades Asteroidea (sea stars and sea daisies) Ophiuroidea (brittle stars) Echinoidea (sea urchins and sand dollars) Crinoidea (sea lilies and feather stars) Holothuroidea (sea cucumbers) Asteroidea: Sea Stars and Sea Daisies Sea stars have multiple arms radiating from a central disk The undersurface of each arm bears tube feet, which grip substrate with adhesive chemicals Sea stars feed on bivalves by prying them open with their tube feet, everting their stomach, and digesting their prey externally with digestive enzymes Sea stars can regrow lost arms Sea daisies are a group of armless species in the clade Asteroidea; only three species are known Sea daisies live on submerged wood and absorb nutrients through a membrane that surrounds their body Ophiuroidea: Brittle Stars Brittle stars have a distinct central disk and long, flexible arms, which they use for movement Some species are suspension feeders, while others are predators or scavengers Echinoidea: Sea Urchins and Sand Dollars Sea urchins and sand dollars have no arms but have five rows of tube feet Their spines are used for locomotion and protection Sea urchins feed on seaweed using a jaw- like structure on their underside Crinoidea: Sea Lilies and Feather Stars Sea lilies live attached to the substrate by a stalk Feather stars can crawl using long, flexible arms Both are suspension feeders Crinoidea have changed little over the course of evolution Holothuroidea: Sea Cucumbers Sea cucumbers lack spines, have a very reduced endoskeleton, and do not look much like other echinoderms Sea cucumbers have five rows of tube feet; some of these are developed as feeding tentacles o Chordates Phylum Chordata consists of two basal groups of invertebrates as well as vertebrates Chordates are bilaterally symmetrical coelomates with segmented bodies Chordates did not evolve from echinoderms, but have evolved separately from them for at least 500 million years
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