Invertebrates and Vertebrates: Structure, Diversity, and Evolution

Invertebrates (Ch 33)

Overview of Invertebrates

Invertebrates are animals that lack a backbone and constitute approximately 95% of all animal species. They display a wide range of body plans, organ systems, and evolutionary adaptations. The study of invertebrates focuses on 14 major phyla within the kingdom Animalia.

• Eumetazoa: True animals with true tissues.

• Invertebrates: Animals without a backbone.

Animal Organ Systems

• Circulatory Systems:

  • Closed circulatory system: Blood is contained within vessels and pumped by a heart.

  • Open circulatory system: Hemolymph (mixture of blood and interstitial fluid) is pumped into a body cavity (hemocoel).

• Skeletal Systems:

  • Hydrostatic skeleton: Uses water in body cavities to maintain shape.

  • Exoskeleton: External skeleton for protection and support.

  • Endoskeleton: Internal skeleton.

• Digestive Systems:

  • Gastrovascular cavity: Single opening for ingestion and egestion; aids in circulation and excretion.

  • Complete digestive tract (alimentary canal): Separate mouth and anus for unidirectional food processing.

• Nervous Systems:

  • Nerve net: Simplest form, found in cnidarians.

  • Nerve ring and radial nerves: More complex, seen in echinoderms.

  • Central ganglia and nerve cords: Found in annelids and arthropods.

  • Brain, spinal cord, and nerves: Most complex, seen in vertebrates.

Phylum Porifera (Sponges)

• Basal animals lacking true tissues.

• Filter feeders: Capture particles from water passing through their bodies.

• Water flow: Drawn into the spongocoel and exits via the osculum.

• Choanocytes: Flagellated collar cells that engulf food by phagocytosis.

• Amoebocytes: Totipotent cells in the mesohyl that digest food, transport nutrients, and produce skeletal elements (spongin and spicules).

• Hermaphroditism: Most sponges function as both male and female, often sequentially.

Phylum Cnidaria

• Radial symmetry, diploblastic, gastrovascular cavity.

• Includes corals, jellies, and hydras.

• Body forms: Sessile polyps and motile medusae.

• Tentacles with cnidocytes: Specialized cells for defense and prey capture; contain nematocysts (stinging organelles).

• Nerve net: Noncentralized nervous system.

• Major clades:

  • Medusozoa: Includes scyphozoans (true jellies), cubozoans (box jellies), and hydrozoans.

  • Anthozoa: Includes sea anemones and corals; only polyp form, often with calcium carbonate exoskeletons.

Phylum Ctenophora (Comb Jellies)

• Radial symmetry.

• Eight rows of ciliary "combs" for locomotion.

• Phylogenetic placement is debated.

Bilateria

• Bilateral symmetry, triploblastic, coelom, and complete digestive tract.

• Most animal species are bilaterians.

Lophotrochozoans

• Some possess a lophophore (feeding structure), others have a trochophore larval stage.

Phylum Platyhelminthes (Flatworms)

• Bilateral symmetry, acoelomate, dorsoventrally flattened.

• Gastrovascular cavity branches throughout the body.

• Hydrostatic skeleton.

• Protonephridia: Excretory structures with flame bulbs for osmoregulation.

• Planarians: Free-living, freshwater, light-sensitive eyespots, centralized nervous system.

• Trematodes (flukes): Parasitic, complex life cycles with intermediate hosts (e.g., schistosomiasis).

• Tapeworms: Parasitic, absorb nutrients, have scolex (attachment organ) and proglottids (reproductive units).

Phylum Syndermata (Rotifers and Acanthocephalans)

• Rotifers: Microscopic, complex organ systems, crown of cilia, jaws (trophi), complete digestive tract.

• Parthenogenesis: Females produce offspring from unfertilized eggs.

Phylum Ectoprocta and Brachiopoda

• Ectoprocts: Colonial, exoskeleton, lophophore, U-shaped alimentary canal, no distinct head.

• Brachiopods: Resemble clams, dorsal and ventral shells, lophophore.

Phylum Mollusca

• Soft-bodied, often with calcium carbonate shell.

• Body plan: Muscular foot (movement), visceral mass (organs), mantle (secretes shell).

• Radula: Feeding organ (in many).

• Major clades:

  • Polyplacophora (chitons): Eight dorsal plates, marine grazers.

  • Gastropoda (snails, slugs): Single spiraled shell, head with tentacles, terrestrial forms use mantle for gas exchange.

  • Bivalvia (clams, oysters): Two hinged shells, sedentary filter feeders.

  • Cephalopoda (squid, octopus): Predators, foot modified into tentacles and siphon, closed circulatory system, well-developed senses.

Phylum Annelida (Segmented Worms)

• Segmented body, coelom, hydrostatic skeleton, closed circulatory system.

• Errantians: Mobile, parapodia, jaws.

• Sedentarians: Less mobile, includes earthworms (soil aeration, hermaphroditic) and leeches (parasitic, secrete hirudin).

Ecdysozoans

• Animals with a cuticle that is periodically shed (ecdysis).

Phylum Tardigrada (Water Bears)

• Microscopic, can survive extreme conditions in dormant state.

Phylum Nematoda (Roundworms)

• Cylindrical, unsegmented, tough cuticle, complete digestive tract.

• Many are parasitic; lack circulatory system, exchange by diffusion.

Phylum Arthropoda

• Segmented body, jointed appendages, exoskeleton of chitin.

• Largest animal phylum (over 2/3 of known species).

• Specialized body regions and appendages for various functions.

• Open circulatory system, diverse respiratory structures (gills, tracheae, book lungs).

• Major groups:

  • Chelicerates: Sea spiders, horseshoe crabs, arachnids (spiders, scorpions, ticks, mites); chelicerae (claw-like mouthparts), book lungs, silk production.

  • Myriapods: Centipedes (carnivorous, one pair of legs per segment) and millipedes (detritivores, two pairs per segment).

  • Pancrustaceans: Crustaceans (crabs, lobsters, shrimp, barnacles) and insects (hexapods, diverse metamorphosis types).

Deuterostomia

• Includes both vertebrate and invertebrate phyla.

Phylum Hemichordata

• Share traits with chordates (gill slits, dorsal nerve cord), hydrostatic skeleton.

Phylum Echinodermata

• Sea stars, sea urchins, sand dollars, sea cucumbers.

• Bilateral symmetry as larvae, radial as adults.

• Water vascular system with tube feet for movement and feeding.

• Endoskeleton of calcareous plates.

• Five major clades: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins, sand dollars), Crinoidea (sea lilies), Holothuroidea (sea cucumbers).

Phylum Chordata

• Bilaterally symmetrical, segmented, coelomate, endoskeleton, complete digestive tract.

Vertebrates (Ch 34)

Chordates

Chordates are bilaterian animals within the deuterostome clade. They include all vertebrates and two groups of invertebrates: urochordates and cephalochordates.

• Key characteristics (at some stage):

  • Notochord: Flexible rod for skeletal support.

  • Dorsal, hollow nerve cord: Develops into the central nervous system.

  • Pharyngeal slits or clefts: Openings for suspension feeding, gas exchange, or develop into head/neck structures in tetrapods.

  • Muscular, post-anal tail: Provides propulsion in aquatic species.

• Lancelets (Cephalochordata): Marine suspension feeders, retain chordate features as adults.

• Tunicates (Urochordata): Larvae display chordate traits; adults are sessile filter feeders.

Vertebrates

• Backbone, complex nervous system, enhanced predation and evasion abilities.

• Derived traits: Multiple sets of Hox genes, neural crest (gives rise to diverse structures).

Jawless Vertebrates (Cyclostomes)

• Hagfish (Myxini): Cartilaginous skull, reduced vertebrae, slime glands, scavengers.

• Lampreys (Petromyzontida): Parasitic, clamp onto host fish, feed on blood/tissue.

Jawed Vertebrates (Gnathostomes)

• Jaws with teeth, enlarged forebrain, lateral line system (in aquatic species).

• Major groups:

  • Chondrichthyes (cartilaginous fishes): Sharks, rays, skates; skeleton of cartilage, internal fertilization, various reproductive modes (oviparous, ovoviviparous, viviparous).

  • Osteichthyes (bony fishes and tetrapods): Bony skeleton, operculum covers gills, swim bladder for buoyancy, external fertilization.

  • Ray-finned fishes (Actinopterygii): Most familiar fishes, fins supported by bony rays.

  • Lobe-finned fishes (Sarcopterygii): Fins with rod-shaped bones and muscle; includes coelacanths, lungfishes, and tetrapods.

Tetrapods

• Four limbs with digits, neck, fusion of pelvic girdle to backbone, absence of gills (in most), ears for airborne sound.

• Key transitional fossil: Tiktaalik ("fishapod").

Amphibians

• Salamanders (Urodela): Tailed, some exhibit paedomorphosis.

• Frogs (Anura): Tailless, powerful hind legs, some called toads.

• Caecilians (Apoda): Legless, nearly blind, burrowing.

• Life cycle: Aquatic larvae, terrestrial adults; external fertilization, eggs lack shell.

Amniotes

• Tetrapods with amniotic egg (reptiles, birds, mammals).

• Amniotic egg membranes:

  • Amnion: Cushions embryo in fluid.

  • Chorion: Gas exchange.

  • Yolk sac: Nutrient supply.

  • Allantois: Waste storage.

• Rib cage for lung ventilation.

Reptiles

• Scaly skin (keratin), shelled eggs, internal fertilization.

• Ectothermic (except birds, which are endothermic).

• Major groups: Turtles, lepidosaurs (lizards, snakes), archosaurs (crocodilians, birds, extinct dinosaurs and pterosaurs).

• Birds: Adapted for flight (feathers, hollow bones, no bladder, single ovary), endothermic, evolved from theropod dinosaurs.

Mammals

• Mammary glands, hair, endothermy, efficient organs, large brain, differentiated teeth.

• Three major lineages:

  • Monotremes: Egg-laying (platypus, echidnas), no nipples.

  • Marsupials: Pouched mammals, short gestation, development continues in marsupium.

  • Eutherians: Placental mammals, complex placenta, long gestation.

Primates and Human Evolution

• Primates: Lemurs, tarsiers, monkeys, apes (including humans); grasping hands/feet, opposable thumbs, large brains, forward-facing eyes.

• Apes: Gibbons, orangutans, gorillas, chimpanzees, humans; no tail, long arms, large brain-to-body ratio.

• Humans (Homo sapiens): Bipedal, large brain, language, tool use, reduced jaw, short digestive tract, 99% genome similarity with chimpanzees.

• Hominins: Extinct species more closely related to humans than chimps; upright posture, reduced canines, flat faces.

Table: Comparison of Major Animal Phyla

Phylum	Symmetry	Body Cavity	Digestive System	Circulatory System	Skeletal System
Porifera	None	None	None	None	Spicules/Spongin
Cnidaria	Radial	None	Gastrovascular cavity	None	Hydrostatic/Exoskeleton (corals)
Platyhelminthes	Bilateral	Acoelomate	Gastrovascular cavity	None	Hydrostatic
Mollusca	Bilateral	Coelomate	Complete	Open (except cephalopods)	Exoskeleton (shell)
Annelida	Bilateral	Coelomate	Complete	Closed	Hydrostatic
Arthropoda	Bilateral	Coelomate	Complete	Open	Exoskeleton (chitin)
Echinodermata	Bilateral (larvae), Radial (adult)	Coelomate	Complete	Water vascular system	Endoskeleton (calcareous plates)
Chordata	Bilateral	Coelomate	Complete	Closed	Endoskeleton

Additional info: This table summarizes the main structural and functional features of major animal phyla for comparative study.