Exam 3 Study Guide: Chapters 18, 19, 20, and 35
Describe the key derived traits of the chordates and the chordate subgroups. (how is a chordate defined?)
All chordates share a set of derived characters Some species have some of these traits only during embryonic development Four key characters of chordates: Notochord, dorsal hollow nerve cord, pharyngeal slits or clefts, and a muscular, post-anal tail.
Define altruism and kin selection and describe examples of each.
Altruism is defined as behavior that reduces an individual's fitness while increasing the fitness of others in the population. Natural selection favoring altruistic behavior that benefits relatives is called kin selection. The concept of inclusive fitness describes an individual's success at perpetuating its genes by producing its own offspring and helping close relatives, who likely share many of those genes, to produce offspring. Altruistic behavior almost always benefits relatives!
Describe the levels of organization in an animal's body. (cell, tissue, organ, etc.).
Anatomy is the study of structure. Physiology is the study of function. Animals consist of a hierarchy of levels of organization. Tissues are an integrated group of similar cells that perform a common function. Organs perform a specific task and consist of two or more tissues. Organ systems consist of multiple organs that together perform a vital body function.
Describe the defining characteristics of animals.
Animals are eukaryotes, are multicellular heterotrophs, and have cells that lack cell walls. Animals also use ingestion, the eating of food. Fungi, in contrast, absorb nutrients after digesting food outside their body.
Describe the attributes that define animals.
Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers. Animals are eating, moving machines.
Describe the four main types of animal tissues, noting their structures and functions.
Animals have four main categories of tissues: epithelial tissue, connective tissue, muscle tissue, and .nervous tissue. Epithelial tissues, or epithelial, are sheets of closely packed cells that cover body surfaces and line internal organs and cavities. Epithelial tissues are named according to the number of cell layers they have and the shape of the cells on their apical surface. Epithelial cells come in three shapes: squamous, like a fried egg, cuboidal, as tall as they are wide and columnar, taller than they are wide. Connective tissue can be grouped into six major types. Loose connective tissue is the most widespread, consists of ropelike collagen and elastic fibers that are strong and resilient, and helps to join skin to underlying tissues. Fibrous connective tissue has densely packed collagen fibers and forms tendons that attach muscle to bone. Adipose tissue stores fat in large, closely packed cells held in a matrix of fibers. Cartilage is a strong and flexible skeletal material and commonly surrounds the ends of bones. Bone has a matrix of collagen fibers embedded in a hard mineral substance containing calcium, magnesium, and phosphate. Blood transports substances throughout the body. Muscle tissue is the most abundant tissue in most animals. There are three types of vertebrate muscle tissue: skeletal muscle causes voluntary movements, cardiac muscle pumps blood, and smooth muscle moves walls of internal organs, such as the intestines. Nervous tissue senses stimuli and rapidly transmits information. Neurons carry signals by conducting electrical impulses. Other cells in nervous tissue insulate axons, nourish neurons, and regulate the fluid around neurons.
Distinguish between monkeys and apes. What are some examples of each group?
Anthropoids include monkeys and apes. They have a fully opposable thumb that functions in grasping. Monkeys have forelimbs about equal in length to their hind limbs, have tails; some have a long, prehensile (grasping) tail, while others lack a prehensile tail. Most apes have relatively long arms, short legs, and lack a tail. Monkeys include Old World (Africa and Asia) and New World (the Americas) monkeys. Many Old World monkeys are arboreal, but some ground dwelling, nostrils open downward, and they lack a prehensile tail. An example is a long-tailed macaque. New World monkeys are all arboreal, the nostrils are open to the side (far apart), and many have a long, prehensile (grasping) tail. An example is the golden lion tamarin. Apes include gibbons, orangutans, gorillas, chimpanzees, and humans. Compared to other primates, they have larger brains relative to body size; thus, their behavior is more flexible.
Define and distinguish between the proximate and ultimate causes of behavior.
Behavior encompasses a wide range of activities. Collectively, behavior is the sum of an animal's responses to internal and external environmental cues. Behavioral ecology is the study of behavior in an evolutionary context. The questions investigated by behavioral ecologists fall into two broad categories. Proximate questions concern the immediate reason for the behavior. How is it triggered by stimuli (environmental cues that cause a response)? What physiological or anatomical mechanisms play a role? What underlying genetic factors are at work? Proximate causes are the answers to such questions about the immediate mechanism for behavior. Ultimate questions address why a particular behavior occurs. Ultimate causes are evolutionary explanations for behavior.
Describe the adaptive advantage of innate behaviors (how do innate behaviors help animals survive?).
Biologists Lorenz and Tinbergen were among the first to demonstrate the importance of innate behavior, behaviors that are under strong genetic control and are performed in virtually the same way by all individuals of a species. Some examples of innate behaviors include predator defense, nest building, and parental care.
Define social behavior and sociobiology, providing examples of each.
Biologists define social behavior as any kind of interaction between two or more animals, usually of the same species. Sociobiology applies evolutionary theory to the study and interpretation of social behavior to explain how social behaviors are adaptive and could have evolved by natural selection.
Explain how courtship rituals are adaptive (what is a courtship ritual? How do they enhance reproductive success?)
Careful communication is an essential prerequisite for mating. In many species, prospective mates must perform an elaborate courtship ritual. The ritual confirms that individuals are of the same species, of the opposite sex, physically primed for mating, and not threats to each other.
Explain how negative feedback is used to regulate internal body temperature. (what is negative feedback?)
Control systems detect change and direct responses. Negative-feedback mechanisms keep internal variables steady and permit only small fluctuations around set points. Example: temperature regulation.
Explain how organ systems work together to perform life's functions.
Each organ performs specific functions. The heart has: extensive muscle that generates contractions, epithelial tissues that line the heart chambers, prevent leaks, and form a smooth surface for blood flow, connective tissues that make the heart elastic and strong, and neurons that regulate contractions. The small intestine: is lined by a columnar epithelium, includes connective tissues that contain blood vessels, and has two layers of smooth muscle that help propel food. The inner surface of the small intestine has many finger-like projections that increase the surface area for absorption.
Describe the systems that help an animal exchange materials with its environment. (what materials do animals have to exchange with the environment?)
Every organism is an open system that must exchange matter and energy with its surroundings. What kinds of things do cells need to take up or get rid of? Size and shape make a difference: cells in small and flat animals can exchange materials directly with the environment. As organisms increase in size, the surface area is too small for the corresponding volume and is too far away from the deepest cells of the body. In these larger organisms, evolutionary adaptations consist of extensively branched or folded internal surfaces, which increase the surface area, and provide for sufficient environmental exchange. The respiratory system exchanges gases between the external environment and blood. The digestive system acquires food and eliminates wastes. The urinary system eliminates metabolic waste. The circulatory system distributes gases, nutrients, and wastes throughout the body and exchanges materials between blood and body cells through the interstitial fluid that bathes body cells.
Define optimal foraging and provide examples.
Food-obtaining behavior, or foraging, includes eating and any mechanism an animal uses to search for, recognize, and capture food. According to the predictions of optimal foraging theory, an animal's feeding behavior should provide maximal energy gain with minimal energy expense and minimal risk of being eaten while foraging.
Describe the characteristics of and distinguish between each of the following vertebrate groups: hagfishes, lampreys, chondrichthyans, ray-finned fishes, lobe-finned fishes, amphibians, reptiles, birds, and mammals. (What is unique about each group? What are some examples of each group?) Also, explain the significance of the evolution of the backbone, jaws, four limbs, and the amniotic egg. (What is the function of each of these traits? How do they help an animal survive?)
Hagfishes and lampreys are craniates, have a notochord, but lack hinged jaws and paired fins. Lampreys but not hagfishes have rudimentary vertebral structures. Thus, lampreys are vertebrates, but hagfishes are not vertebrates. Jawed vertebrates appeared in the fossil record about 470 million years ago and quickly diversified, using their paired fins and tail to chase a wide variety of prey. Jaws may have evolved by modifications of skeletal supports of the anterior pharyngeal (gill) slits. The remaining gill slits remained as sites of gas exchange. Three lineages of jawed vertebrates with gills and paired fins are commonly called fishes: chondrichthyans—sharks and rays, ray-finned fishes—for example, tuna, trout, and goldfish, and lobe-finned fishes—coelacanths, lungfishes, and tetrapods. Chondrichthyans have a flexible skeleton made of cartilage, electrosensors on their heads, and a lateral line system that helps them locate prey. Most sharks are fast-swimming predators, with sharp vision and a keen sense of smell. Most rays are adapted for life on the bottom, with dorsoventrally flattened bodies and eyes on the top of their heads. Ray-finned fishes have an internal skeleton reinforced with a hard matrix of calcium phosphate, flattened scales covered with mucus, an operculum that covers a chamber of gills, and a buoyant swim bladder (derived from an ancestral lung). With more than 27,000 species, ray-finned fishes are the most diverse group of vertebrates. Lobe-fins have muscular pelvic and pectoral fins that are supported by rod-shaped bones. Today, three lineages of lobe-fins survive: coelacanths, living deep in the oceans, were once thought to be extinct, lungfishes, which can gulp air into lungs, inhabit stagnant waters in the Southern Hemisphere, and tetrapods, adapted to life on land, include terrestrial vertebrates. During the late Devonian, a line of lobe-finned fishes gave rise to tetrapods, jawed vertebrates with limbs and feet that can support weight on land. Adapting to life on land was a key event in vertebrate history. All subsequent groups, amphibians, mammals, and reptiles (including birds), are descendants of these early land-dwellers. Like plants, vertebrates faced obstacles on land in regard to gas exchange, water conservation, structural support, and reproduction. Animal obstacles also included finding a means of locomotion and adapting sensory organs that worked well in water but not on land. Fossils reveal that the first tetrapod was a four-limbed fish that lived in shallow water and could breathe air. Amphibians include salamanders, frogs, and caecilians, use their moist skins to supplement their lungs for gas exchange, often have poison glands in their skins, usually, lay their eggs in water, undergo metamorphosis from a larval stage to the adult form, and were the first tetrapods to colonize land. Reptiles (including birds) and mammals are amniotes. The major derived character of this clade is an amniotic egg, with four internal membranes. The amnion is a fluid-filled sac surrounding the embryo. The yolk sac contains a rich store of nutrients for the developing embryo. The chorion (and allantois) enable the embryo to obtain oxygen from the air and dispose of carbon dioxide. The allantois also helps dispose of metabolic waste. Reptiles include lizards, snakes, turtles, crocodilians, birds, and extinct dinosaurs, have a skin covered with scales and waterproofed with keratin, obtain most of their oxygen using lungs, and are ectothermic, absorbing external heat rather than generating much of their own. Most birds can fly, and nearly every part of their bodies reflects adaptations that enhance flight. The forelimbs have been remodeled as feather-covered wings that act as airfoils. Large flight muscles anchored to a central ridge along the breastbone provide power. Many features help reduce weight for flight. Present-day birds lack teeth. The tail is supported by only a few small vertebrae. Feathers have hollow shafts. Bird bones have a honeycombed structure that makes them strong but light. Flight is very costly, and present-day birds have a high rate of metabolism. Unlike other living reptiles, birds are endothermic, using heat generated by metabolism to maintain a warm, steady body temperature. Birds have relatively large brains and display complex behaviors. They have acute senses, fine muscle control, and excellent eyesight. Birds evolved from a lineage of small, two-legged dinosaurs called theropods. Archaeopteryx is one of the oldest, most primitive known bird-like reptiles (150 million years old). It resembled a small bipedal dinosaur, with teeth, wing claws, and a long tail with many vertebrae. Mammals are endothermic amniotes with hair, which insulates their bodies, and mammary glands, which produce milk. Mammals have efficient respiratory and circulatory systems that support their high rate of metabolism. Mammalian teeth are differentiated for many kinds of diets.
Define the concept of homeostasis.
Homeostasis is the active maintenance of a steady state within the body. External environmental conditions may fluctuate wildly. Homeostatic mechanisms regulate internal conditions.
Compare the types of signals used by nocturnal and diurnal animals. (How do animals communicate?)
In behavioral ecology, a signal is a stimulus transmitted by one animal to another animal. The sending of, reception of, and response to signals constitute animal communication, an essential element of interactions between individuals. Forms of communication vary considerably, and many animals use more than one type of signal simultaneously. Nocturnal mammals use odor and sound. Diurnal birds use visual and auditory signals. Fish may use visual signals, electrical signals, and/or sound. Honeybees "dance" to signal to other bees the location of a food source.
Define agonistic behavior and explain how agonistic behavior is adaptive.
In many species, conflicts that arise over limited resources, such as food, mates, or territories, are settled by agonistic behavior, which is social behavior that consists of threats, rituals, and sometimes combat that determines which competitor gains access to a resource and can directly affect an individual's evolutionary fitness.
Explain why invertebrate diversity is a valuable resource for humans.
Invertebrate animals are the primary consumers in most food webs, and in turn, serve as food for other animals Some invertebrate animals have served as models for important research in genetics (elegans and melanogaster, for example) A small percentage of invertebrates are human parasites or are vectors for disease Insects serve as critical pollinators for many of our food plants. Most flowering plants are pollinated by animals, chiefly insects. An estimated one-third of the world's food supply depends on pollinators. In the United States, the production of fruits and vegetables relies on pollination by bees, mostly non-native honeybees imported from Europe.
Define the major types of learning and note the adaptive advantages and examples of each. (i.e. habituation, imprinting, spatial learning etc.)
Learning is the modification of behavior as a result of specific experiences. Learning enables animals to change their behaviors in response to changing environmental conditions. There are various forms of learning, ranging from a simple behavioral change in response to a single stimulus to complex problem solving using entirely new behaviors. Habituation is one of the simplest forms of learning. An animal learns not to respond to a repeated stimulus that conveys little or no information. Habituation may increase fitness by allowing an animal's nervous system to focus on stimuli that signal food, mates, or real danger. Imprinting is generally irreversible learning and limited to a specific phase in an animal's life called a sensitive period. Examples include a young bird learning to identify its parents, newly hatched salmon imprinting on the complex mixture of odors unique to their stream, and song development in birds. In spatial learning, animals establish memories of landmarks in their environment that indicate the locations of food, nest sites, prospective mates, and potential hazards. Migration is the regular back-and-forth movement of animals between two geographic areas, enables many species to access food resources throughout the year, and allows them to breed or winter in areas that favor survival. Gray whales migrate annually between the coastlines of northern Alaska and Baja California and navigate using the coastline, the topography of the ocean floor, cues from the temperature and chemistry of the water, and magnetic sensing. Magnetic cues feature prominently in sea turtle migrations. Loggerhead sea turtles hatching on beaches from North Carolina to Florida enter the Atlantic Ocean to begin an incredible journey through thousands of miles of open ocean and return to the North American coast. Many birds migrate at night, navigating by the stars the way early sailors did. Birds that migrate during the daytime navigate by a combination of magnetic cues and visual cues such as landmarks and the position of the sun. Associative learning is the ability to associate one environmental feature with another. In one type of associative learning, an animal learns to link a particular stimulus to a particular outcome. For example, a dog may expect to go for a walk if the owner picks up the leash. Trial-and-error learning is an animal's ability to learn to associate one of its own behaviors with a positive or negative effect. Memory is the key to all associative learning. Social learning is learning by observing the behavior of others. Many predators, including cats, coyotes, and wolves, seem to learn some of their basic hunting tactics by observing and imitating their mothers. Vervet monkeys give distinct alarm calls when they see leopards, eagles, or snakes, all of which prey on vervets. Upon hearing a particular alarm call, other vervets in the group behave in an appropriate way, depending on the type of threat. Some animals have complex cognitive abilities that include problem-solving, the process of applying past experience to overcome obstacles in novel situations. Problem-solving behavior is highly developed in some mammals, especially dolphins and primates, and has been observed in some bird species.
Compare the characteristics of the four major arthropod lineages.
Living arthropods represent four major lineages. Chelicerates include horseshoe crabs and arachnids, such as spiders, scorpions, mites, and ticks. Millipedes and centipedes are terrestrial creatures identified by the number of jointed legs per body segment. Crustaceans are nearly all aquatic. They include crabs, shrimp, and barnacles, which feed with jointed appendages. Insects are the fourth and most diverse lineage of arthropods.
Define a territory and describe the ways in which territories are used, identified, and defended (what is the purpose of a territory?).
Many animals exhibit territorial behavior. A territory is an area, usually fixed in location, that one or more individuals defend and from which other members of the same species are usually excluded. Territories are usually used for feeding, mating, rearing young, or a combination of these activities. Individuals that have established a territory usually proclaim their territorial rights continually. Examples include the songs of bird, the noisy bellowing of sea lions, the chattering of squirrels, and scent markers used to signal a male coyote's territorial boundaries.
Explain how dominance hierarchies are maintained and identify their adaptive value.
Many animals live in social groups maintained by agonistic behaviors. Dominance hierarchy is the ranking of individuals based on social interactions. Examples include pecking order in chickens and hierarchies among the females within a wolf pack.
Distinguish between monotremes, marsupials, and placental mammals. (What is different about how they reproduce?)
Monotremes are egg-laying mammals. Living monotremes include the duck-billed platypus and echidnas. Unlike monotremes, the embryos of marsupials and eutherians are nurtured by a placenta, in which nutrients from the mother's blood diffuse into the embryo's blood. Marsupials have a brief gestation and give birth to tiny, embryonic offspring, that complete development while attached to the mother's nipples. Eutherians are mammals that bear fully developed live young and are commonly called placental mammals because their placentas are more complex than those of marsupials.
Describe the general animal life cycle and the basic animal body plan. (body symmetry, type of digestive tract, type of skeleton, etc.)
Most adult animals are diploid and reproduce sexually. The eggs and sperm are produced by meiosis, are the only haploid cells, and fuse during fertilization to form a zygote. The zygote divides by mitosis to form a hollow ball of cells called a blastula. One side of the blastula folds in and cells become rearranged to form a gastrula that establishes three embryonic layers. Endoderm forms a lining of the future digestive tract. Ectoderm forms an outer layer that will give rise to the skin and nervous system. Mesoderm forms a middle layer that will give rise to muscles and most internal organs. After the gastrula stage, many animals develop directly into adults. Other animals, such as the sea star, develop into one or more larval stages. A larva is an immature individual that looks different from the adult animal. A larva undergoes a major change in body form, called metamorphosis, and becomes a reproductively mature adult. Clusters of master control genes control the transformation of the zygote into an adult animal. Animal body plans vary in symmetry, the presence of true tissues, the number of embryonic layers, the presence of a body cavity, and details of their embryonic development.
Describe the common characteristics of insects, especially the ones that made this group so successful.
Nearly 75% of all identified animal species are insects. There may be as many as 30 million insect species. The body of an insect typically includes a head, thorax, abdomen, three sets of legs, and wings (with few exceptions). The extraordinary success of insects is due to body segmentation, an exoskeleton, jointed appendages, flight, a waterproof cuticle, and a complex life cycle with short generations and large numbers of offspring. More than 80% of insect species undergo complete metamorphosis, in which a free-living larva transforms from a pupa into an adult. Other insect species undergo incomplete metamorphosis, in which the transition from larva to adult is achieved through multiple molts, but without forming a pupa. Insect mouthparts are adapted for various types of feeding, such as chewing (grasshoppers), biting and tearing prey (mantids), lapping up fluids (houseflies), and piercing and sucking fluids of plants (aphids) and animals (mosquitoes). Insects have three pairs of legs, which are adapted for walking, jumping, grasping prey, digging in soil, or paddling on water. Most adult insects have one or two pairs of wings, allowing dispersal and escape from predators. Because wings are extensions of the cuticle, insects have acquired flight without sacrificing any legs. Many insects have protective color patterns and disguises, including modifications to antennae, wings, and bodies. Most adult insects have one or two pairs of wings, allowing dispersal and escape from predators. Because wings are extensions of the cuticle, insects have acquired flight without sacrificing any legs. Many insects have protective color patterns and disguises, including modifications to antennae, wings, and bodies.
Describe the general structures and functions of the major vertebrate organ systems. (i.e. circulatory system, digestive system, etc.).
The circulatory system delivers oxygen and nutrients to body cells, transports carbon dioxide to the lungs, and carries metabolic wastes to the kidneys. The respiratory system exchanges gases with the environment, supplying the blood with oxygen and disposing of carbon dioxide. The integumentary system protects against physical injury, infection, excessive heat or cold, and drying out. The skeletal system supports the body, protects organs such as the brain and lungs, and provides the framework for muscle movement. The muscular system moves the body, maintains posture, and produces heat. The urinary system removes waste products from the blood, excretes urine, and regulates the chemical makeup, pH, and water balance of blood. The digestive system ingests and digests food, absorbs nutrients, and eliminates undigested material. The endocrine system secretes hormones that regulate body activities. The lymphatic and immune systems protect the body from infection and cancer. The lymphatic system also returns excess body fluid to the circulatory system. The nervous system coordinates body activities by detecting stimuli, integrating information, and directing responses. The reproductive system produces gametes and sex hormones. The female reproductive system supports a developing embryo and produces milk.
Describe the derived characteristics of primates. (What do they have that other mammals don't?)
The earliest primates were probably small arboreal (tree-dwelling) mammals that arose before 65 million years ago when dinosaurs still dominated the planet. Primates include lorises, lemurs, tarsiers, and anthropoids(monkeys and apes) They have a short snout, eyes set close together on the front of their face, limber shoulder and hip joints, five highly mobile digits on hands and feet, and have flexible thumbs. Examples of primates are the slender loris and the coquerel's sifaka.
Describe the Cambrian "explosion" of animal diversity.
The oldest generally accepted animal fossils that have been found are 575-550 million years old. Animal diversification appears to have accelerated rapidly from 535 to 525 million years ago, during the Cambrian period, known as the Cambrian explosion. The most celebrated source of Cambrian fossils is the Burgess Shale, containing a cornucopia of perfectly preserved animal fossils. Of the 35 or so animal phyla, only one phylum includes vertebrates, animals with a backbone. Roughly 96% of animals are invertebrates, animals that lack a backbone.
Explain how genes and environmental factors contribute to human social behavior.
Variations in behavioral traits such as personality, temperament, talents, and intellectual abilities make each person a unique individual. Twins provide a natural laboratory for investigating the origins of complex behavioral traits. Twin studies compare identical twins, who have the same DNA sequence and are raised in the same environment, with fraternal twins, who share an environment but only half of their DNA sequence. Results from twin studies consistently show that for complex behavioral traits such as general intelligence and personality characteristics, genetic differences account for roughly half the variation among individuals. Genes do not dictate behavior but, instead, cause tendencies to react to the environment in a certain way.
Characterize the nine animal phyla discussed in this chapter in terms of the body plan, and name common examples. (what are the characteristics of each phylum, and what are some examples of animals in each phylum?) Also, define body segmentation, explain its functions, and note the animal phyla where it occurs.
•Porifera •Cnidaria •Platyhelminthes •Nematoda •Annelida •Mollusca •Arthropoda •Echinodermata •Chordata Sponges (phylum Porifera) are simple animals, do not have true tissues, usually, lack body symmetry, and are usually marine, although some are found in fresh water. Water enters through pores in the body wall into a central cavity and then flows out through a larger opening. The body of a sponge consists of two layers of cells separated by a gelatinous region. Sponges have skeletal fibers composed of flexible protein and mineralized particles called spicules. Adult sponges are sessile and cannot escape from predators. They deter pathogens, parasites, and predators by producing defensive toxins and antibiotics. Some of these compounds may prove useful to humans as new drugs. Cnidarians (phylum Cnidaria) are characterized by radial symmetry, have only two tissue layers, an outer epidermis, an inner cell layer lining the digestive cavity, and have a jelly-filled middle region that may contain scattered amoeboid cells. Cnidarians exhibit two kinds of radially symmetrical body forms. The sedentary polyp body is cylindrical with tentacles projecting from one end, as seen in hydras and sea anemones. The more mobile medusa form is exemplified by a marine jelly. Cnidarians are carnivores that use their tentacles to capture prey and to push prey into their mouths. The mouth leads to the gastrovascular cavity, which functions in digestion, in circulation, and as a hydrostatic skeleton. Cnidocytes are unique stinging cells that capture prey and function in defense. Flatworms (phylum Platyhelminthes) are the simplest bilaterians, have three tissue layers, but lack a body cavity. Flatworms live in marine, freshwater, and damp terrestrial habitats. Some are parasitic, and others are free-living. There are three major groups of flatworms. Free-living flatworms (planarians) have heads with light-sensitive eyecups, a simple brain and a pair of nerve cords that run the length of the body, and a branched gastrovascular cavity with a single opening. Flukes are parasitic flatworms with complex life cycles; many have suckers to attach to their hosts. Tapeworms are parasitic, inhabit the digestive tracts of vertebrates, and consist of a ribbon-like body with repeated units. Nematodes, or roundworms (phylum Nematoda), are abundant and diverse. Nematodes have bilateral symmetry, three tissue layers, a nonliving cuticle covering the body that prevents them from drying out, a fluid-filled body cavity that functions to distribute nutrients, and a complete digestive tract with a mouth and anus. Although about 25,000 species of nematodes have been named, estimates of the total number of species range as high as 500,000. Humans are host to at least 50 species of parasitic nematodes. Molluscs (phylum Mollusca) have three major body regions: a muscular foot, which functions in locomotion, a visceral mass containing most of the internal organs, a mantle, which may secrete a shell that encloses the visceral mass, and They have a circulatory system that pumps blood and distributes nutrients and oxygen throughout the body. Gastropods are the largest group of mollusks and include the snails and slugs. Bivalves include clams, oysters, mussels, and scallops. Cephalopods include squids, octopi, and nautiluses, are fast, agile predators, have large brains and sophisticated sense organs, including complex image-focusing eyes, and have a shell that is large in a nautilus, small and internal in a squid, or missing in an octopus. Annelids (phylum Annelida) have segmentation, the subdivision of the body along its length into a series of repeated parts, a nervous system that includes a simple brain and ventral nerve cord, and a closed circulatory system in which blood remains enclosed in vessels throughout the body. Many invertebrates, such as mollusks and arthropods, have an open circulatory system in which blood is pumped through vessels into open body cavities. Annelids are found in damp soil, the sea, and most freshwater habitats. The three groups of annelids are earthworms and their relatives, polychaetes, and leeches. Earthworms ingest soil and extract nutrients, aerating the soil and improving its texture. There are over a million species of arthropods (phylum Arthropoda), including crayfish, lobsters, crabs, barnacles, spiders, ticks, and insects. The diversity and success of arthropods are due to their segmentation, hard exoskeleton, and jointed appendages, for which the phylum is named. Arthropods have an open circulatory system and an exoskeleton, an external skeleton that protects the animal but must be shed in the process of molting to permit growth. The body of most arthropods includes a head, thorax, and abdomen, although these segments may be fused. Echinoderms (phylum Echinodermata) are a diverse group including sea stars, sand dollars, and sea urchins, slow-moving or sessile, all marine, radially symmetrical as adults, and deuterostomes (along with the chordates). Echinoderms have an endoskeleton of hard calcium-containing plates under thin skin, a water vascular system based on a network of water-filled canals that branch into extensions called tube feet, and the ability to regenerate lost arms. Sea urchins are spherical, have no arms, have tube feet that project through five rows in the animal's globe-like case, and usually, eat algae. Chordates (phylum Chordata) are defined by a dorsal, hollow nerve cord, a flexible, supportive, longitudinal rod, the notochord, pharyngeal slits, and a muscular post-anal tail. The chordates tunicates and lancelets do not have a backbone and use their pharyngeal slits for suspension feeding. Adult tunicates are stationary and attached, while the tunicate larva is a tadpole-like organism. Lancelets are small, bladelike chordates that live in marine sands.