bio ch 28

Front: Why are sponges (Phylum Porifera) considered to lack "true" tissues despite having specialized cells?

They do not undergo true gastrulation during embryonic development and therefore lack true endoderm and ectoderm germ layers. Their cell layers act functionally like tissues (e.g., pinacoderm), but are not embryologically homologous to eumetazoan tissues.

Front: How do adult sponges differ from sponge larvae in terms of motility?

Adult sponges are sessile (permanently attached to a substrate), whereas sponge larvae (such as parenchymula and amphiblastula) are flagellated and free-swimming.

Front: Trace the path of water flow through a simple sponge body plan.

Water enters through numerous body wall pores (ostia) → passes into the central cavity (spongocoel) → exits through a large upper opening (osculum).

Front: What are the primary roles of choanocytes and amoebocytes in a sponge?

• Choanocytes ("collar cells"): Beat their flagella to generate water currents, trap food particles in their microvilli collars for phagocytosis, and differentiate into sperm.

• Amoebocytes: Move through the mesohyl to distribute nutrients, differentiate into specialized cells (sclerocytes, spongocytes) and eggs, and act as sponge stem cells.

Front: How are the four classes of sponges (Calcarea, Hexactinellida, Demospongia, Sclerospongiae) distinguished from one another?

• Calcarea: Calcium carbonate spicules, no spongin.

• Hexactinellida: Six-rayed siliceous (glass) spicules, no spongin, cells fused into a multinucleate syncytium.

• Demospongia: Contains spongin (may also have siliceous spicules).

• Sclerospongiae: Multilayered calcium carbonate skeletons (coralline sponges).

Front: How do sponges carry out digestion, gas exchange, and waste excretion without organ systems?

• Digestion: Strictly intracellular via phagocytosis by choanocytes and pinacocytes.

• Gas Exchange & Excretion: Direct diffusion of $\text{O}_2$, $\text{CO}_2$, and nitrogenous wastes between individual cells and passing water.

Front: What are gemmules, and which sponges produce them?

Dormant, environmentally resistant reproductive structures containing an inner layer of archaeocytes surrounded by a spicule-reinforced coat. They are produced by freshwater sponges (e.g., Spongilla) to survive freezing or drought and recolonize habitats later.

Front: How do Porifera (sponges) and Cnidaria differ in symmetry, embryonic germ layers, and defining specialized cell types?

• Porifera: Asymmetrical, lack true tissues/germ layers, and defined by flagellated choanocytes ("collar cells").

• Cnidaria: Radial or biradial symmetry, diploblastic (ectoderm and endoderm), and defined by stinging cnidocytes containing nematocysts.

Front: What are cnidocytes, nematocysts, and cnidocils, and how do they function?

• Cnidocytes: Specialized stinging cells around the mouth and tentacles.

• Nematocysts: Large intracellular organelles inside cnidocytes storing a coiled thread, barb, and toxins.

• Cnidocil: Touch-sensitive hairlike trigger on the outside of the cnidocyte that fires the nematocyst upon contact.

Front: What are the two tissue layers of a diploblastic cnidarian, and what non-living layer lies between them?

• Epidermis: Outer layer derived from ectoderm.

• Gastrodermis: Inner layer lining the digestive cavity, derived from endoderm.

• Mesoglea: Non-living, jelly-like matrix sandwiched between the epidermis and gastrodermis.

Front: How do cnidarians carry out digestion, gas exchange, and nervous coordination without complete organ systems?

• Digestion: Incomplete digestive system (gastrovascular cavity with one opening) performing extracellular and intracellular digestion, while also distributing nutrients.

• Gas Exchange & Excretion: Direct diffusion across cell layers into surrounding water.

• Nervous System: Rudimentary nerve net (and nerve rings in medusae) using peptide signaling molecules to coordinate movement and feeding.

Front: Compare the polyp and medusa body forms found in Cnidaria.

• Polyp: Sessile, cylindrical "stalk" form with the mouth/anus and surrounding tentacles facing up (e.g., Hydra, sea anemones).

• Medusa: Motile, umbrella-shaped "bell" form with the mouth and tentacles hanging down (e.g., sea jellies).

Front: What does it mean when a cnidarian (such as Obelia) is dimorphic, and what are the roles of gastrozooids and gonozooids?

• Dimorphic: Exhibits both polyp (asexual) and medusa (sexual) body forms during its life cycle (both are diploid).

• Gastrozooids: Feeding polyps adapted for capturing prey.

• Gonozooids: Reproductive polyps adapted for asexually budding off free-swimming medusae.

Front: What are the defining features of Class Anthozoa (corals, sea anemones)?

• Exist only as sessile polyps (no medusa stage).

• Possess a flattened pharynx with siphonoglyphs and internal partitions (mesenteries/septa) that increase surface area.

• Corals often form mutualistic relationships with photosynthetic zooxanthellae algae.

Front: What are the defining features of Class Scyphozoa (true sea jellies)?

• Prominent medusa stage (lacks a velum; called a scyphomedusa) with a brief polyp stage.

• Branched gastrovascular cavity (diverticuli and radial canals), sensory rhopalia around the bell margin, and gonads derived from gastrodermis.

Front: What makes Class Cubozoa (box jellyfish) unique among cnidarians in terms of morphology and sensory organs?

• Box-shaped medusa with muscular pads (pedalia) at the four corners where tentacles attach.

• Possess true organs: complex, image-forming eyes (with a cornea, lens, and retina) alongside simple eye spots.

Front: How does Class Hydrozoa differ from Scyphozoa and other cnidarians in medusa structure, gonad origin, and colonial forms?

• Hydromedusae possess a muscular velum under the bell margin.

• Gonads are derived from epidermal tissue (rather than gastrodermal tissue).

• Includes solitary polyps (Hydra), dimorphic colonies (Obelia), and floating polymorphic colonies (siphonophores like the Portuguese man o' war, Physalia physalis).

Front: What general traits define Superphylum Lophotrochozoa, and where does its name come from?

• General traits: Triploblastic (3 germ layers), bilaterally symmetrical with cephalization, and protostomes ("mouth first").

• Name origin: Combines two defining structures found across member phyla: a ciliated feeding crown (lophophore) and a free-swimming larval stage (trochophore).

Front: What are the key anatomical and physiological features of Phylum Platyhelminthes (flatworms)?

• Body cavity & gas exchange: Acoelomate (solid mesoderm); lack circulatory and respiratory systems, relying on diffusion across their flat body.

• Digestive & excretory systems: Incomplete gastrovascular cavity (one opening); flame cells and tubules regulate salts and excrete nitrogenous waste.

• Nervous & reproductive systems: Ladder-like nervous system with anterior cerebral ganglia; mostly monoecious (hermaphroditic).

Front: What are the unique anatomical and reproductive features of Phylum Rotifera ("wheel-bearers")?

• Body plan: Microscopic pseudocoelomates divided into a head, trunk, and sticky foot.

• Feeding structures: A ciliated corona (resembling rotating wheels) sweeps food into the mastax (muscular pharynx with toothy jaws).

• Reproduction: Many exhibit haplodiploidy; Class Bdelloidea reproduces exclusively via parthenogenesis (all-female) and imports up to 10% of its genome from other species.

Front: What are the distinguishing anatomical features of Phylum Nemertea (ribbon/proboscis worms)?

• Unsegmented, flattened-tube marine carnivores with remarkable contractility.

• Possess a complete digestive tract (mouth to anus), a closed circulatory system (derived from the coelom), and a brain of four anterior ganglia (sometimes containing hemoglobin).

Front: What is the rhynchocoel in Phylum Nemertea, and how does it function?

An extracoelomic, fluid-filled cavity located dorsal to the gut that houses an eversible proboscis. Contraction of rhynchocoel muscles creates hydrostatic pressure to shoot out the proboscis (often barbed) like a harpoon to capture prey, while a retractor muscle pulls it back in.

Front: What are the two monophyletic lineages of Platyhelminthes, and what are rhabdites?

• Lineages: Catenulida ("chain worms" reproducing via incomplete asexual budding) and Rhabditophora (most free-living and all major parasitic flatworms).

• Rhabdites: Rodlike structures discharged in mucus by free-living flatworms for defense and traction during ciliary gliding.

Front: Compare the three major parasitic flatworm groups: Monogenea, Trematoda, and Cestoda.

• Monogenea: Fish ectoparasites with a single host; attach to gills using anchors/hooks.

• Trematoda (Flukes): Internal parasites with complex life cycles (usually a mollusk intermediate host and vertebrate primary host); attach via suckers (e.g., Schistosoma spp.).

• Cestoda (Tapeworms): Intestinal endoparasites (e.g., Taenia spp.) that lack a digestive system and absorb nutrients across their body wall.

Front: How does the blood fluke Schistosoma spp. cause severe human disease (schistosomiasis)?

Free-swimming larvae (released from aquatic snails) penetrate human skin and mature in blood vessels. However, barbed parasite eggs cause the main damage—tearing blood vessels (causing ulcers and bloody diarrhea) or washing into the liver, where they trigger severe inflammation and cirrhosis.

Front: Describe the body structure and transmission cycle of a tapeworm (Class Cestoda, e.g., Taenia spp.).

• Structure: Anchors to the host intestine via a hooked/suckered scolex, followed by a chain of reproductive segments (proglottids) filled with fertilized eggs.

• Transmission: Egg-filled proglottids pass in feces, infect livestock/fish muscle tissue when eaten, and infect humans who consume raw or undercooked meat.

Front: What shared developmental and body plan traits unite Phylum Mollusca and Phylum Annelida within Superphylum Lophotrochozoa?

Both are bilaterally symmetrical, cephalized, triploblastic, schizocoelous eucoelomates (protostomes) whose aquatic members often share a free-swimming trochophore larval stage.

Front: What are the four defining anatomical features shared across most members of Phylum Mollusca?

• Muscular foot: Ventral organ for locomotion and anchorage.

• Visceral mass: Central region containing digestive, nervous, excretory, and reproductive organs.

• Mantle (pallium): Dorsal fold of tissue covering the visceral mass that usually secretes a calcium-carbonate shell.

• Radula: Rasping, tongue-like feeding organ with chitinous teeth (absent in bivalves).

Front: What are the defining anatomical features of Phylum Annelida?

• True body segmentation (metamerism) separated internally by membranous septa.

• Thin, flexible collagenous cuticle (not shed via ecdysis) and chitinous bristles called setae (chaetae).

• Closed circulatory system (in most), paired metanephridia in each segment, and a ventral nerve cord with segmental ganglia.

Front: How is the true coelom formed in mollusks and annelids, and how does the adult coelom differ in mollusks?

Both form their coelom via schizocoely (splitting of solid mesodermal masses). However, in adult mollusks, the true coelom is greatly reduced to a small cavity surrounding the heart (pericardial cavity), gonads, and parts of the kidneys/intestine.

Front: What is the primary extracoelomic body cavity in mollusks, and what structures does it house?

The mantle cavity (formed between the mantle and visceral mass). It serves as the major internal body chamber housing the gills (ctenidia) or terrestrial "lung" lining, as well as openings for the anus, nephridia, and gonads.

Front: Compare the eight classes of Phylum Mollusca based on their shell, foot, and key adaptations.

• Caudofoveata & Solenogastres: Shell-less, worm-like benthic marine mollusks with calcareous spicules; reduced/absent foot.

• Monoplacophora: Single cap-like dorsal shell; serially repeated gills and excretory organs (e.g., Neopilina).

• Polyplacophora (Chitons): Eight dorsal calcareous plates; broad suction foot; ultra-hard magnetite radula teeth; light-sensing esthetes.

• Bivalvia (Clams, mussels, scallops): Two hinged valves; laterally flattened wedge foot; no radula; filter-feed using gills and siphons.

• Gastropoda (Snails, slugs): Coiled shell (or shell-less); crawling foot; undergo embryonic torsion; some (Conus) modify the radula into a venomous harpoon.

• Cephalopoda (Squids, octopuses, nautiluses): Shell reduced/internal or chambered (Nautilus with siphuncle); foot modified into arms, tentacles, and a jet-propulsion siphon; closed circulatory system, beak, ink gland, chromatophores, and direct development.

• Scaphopoda (Tusk shells): Single conical shell open at both ends; burrow in sand; lack gills (respire through mantle cavity) and capture prey with ciliated tentacles (captacula).

Front: What is torsion in gastropods, and what anatomical challenge does it create?

During embryonic development, the visceral mass and mantle rotate 180 degrees counterclockwise, bringing the anus and mantle cavity forward above the head (twisting nerve cords into a figure-eight). While this lets the head retract into the shell first, it creates a sanitation risk of waste washing over the gills (fouling).

Front: What is metamerism, how do annelid segments grow, and what are its evolutionary advantages?

Metamerism is true segmentation where internal and external structures repeat across body compartments. Segments arise sequentially from a growth zone just anterior to the pygidium (teloblastic growth). It allows animals to grow larger by adding compartments and enables more efficient, localized locomotion.

Front: How do the two major annelid groups—Polychaeta and Oligochaeta (including leeches)—differ?

• Polychaeta (Marine worms): Possess fleshy, paired lateral appendages (parapodia) bearing numerous setae; lack a clitellum; usually dioecious with external fertilization and a trochophore larva.

• Oligochaeta (Earthworms & Leeches): Lack parapodia; possess a mucus/cocoon-secreting clitellum (permanent in earthworms, seasonal in leeches) and undergo direct development. Earthworms have few setae, whereas leeches (Hirudinea) lack setae and have anterior/posterior suckers.

Front: What defining physiological process and protective structure unite all members of Superphylum Ecdysozoa (nematodes, tardigrades, arthropods)?

All members secrete a tough, flexible cuticle (exoskeleton) and grow by periodically molting and shedding it in a process called ecdysis.

Front: Describe the body plan, body cavity, and digestive tract of Phylum Nematoda (roundworms).

• Body plan: Triploblastic, bilaterally symmetrical, unsegmented cylindrical body (external cuticle rings are not true internal segments).

• Body cavity: Pseudocoelomate.

• Digestive tract: Complete digestive system (separate mouth with lips/teeth, muscular pharynx, intestine, rectum, and anus).

Front: Why do nematodes move with a characteristic whiplike, thrashing motion?

They possess longitudinal muscles only (lacking circular muscles) that attach directly to the dorsal and ventral nerve cords, contracting against their tough collagen-and-chitin cuticle.

Front: How do nematodes carry out excretion, osmoregulation, and nervous coordination?

• Excretion/Osmoregulation: Ammonia diffuses directly across the body wall; specialized excretory glands/canals (called renette cells in marine nematodes) regulate salt balance.

• Nervous system: Four longitudinal nerve cords (largest on the ventral side) fuse anteriorly into a pharyngeal nerve ring ("brain") and posteriorly into a tail ganglion.

Front: What are dauer larvae, copulatory spicules, and eutely in nematode reproduction and development?

• Dauer larva: An arrested, stress-resistant intermediate larval stage formed during overcrowding or food scarcity.

• Copulatory spicules: Tail structures in male nematodes used to hold open the female's vulva during internal fertilization.

• Eutely: A rigid developmental pathway in which every adult individual of a species possesses a fixed, exact number of cells.

Front: What practical laboratory features make Caenorhabditis elegans an ideal model organism?

• Microscopic (~1 mm), easy to culture on agar plates feeding on E. coli.

• Transparent body allowing direct live observation of internal cells and organelles.

• Rapid 3-day life cycle (egg to egg) and short 2–3 week lifespan (great for genetics and aging research).

• Small genome (~20,000 genes) amenable to molecular genetic manipulation.

Front: How is cell number and sex determination organized in C. elegans, and what major biological discoveries has it enabled?

• Cell constancy (eutely): Adult hermaphrodites (XX) have exactly 959 somatic cells in identical positions (almost one-third are neurons); rare males are XO.

• Key discoveries: Clarified cell-fate determination, apoptosis (programmed cell death), neuronal signaling, lipid regulation (obesity/diabetes), and cilia function (polycystic kidney disease).

Front: What are tardigrades ("water bears"), and what is cryptobiosis?

• Tardigrades: Microscopic (<1 mm), plump, segmented-cuticle ecdysozoans with four pairs of clawed, stubby legs found in marine, freshwater, and damp terrestrial habitats.

• Cryptobiosis: A state of suspended metabolic activity allowing them to survive extreme freezing, vacuum, radiation, high pressure, and up to 99% water loss using specialized protective proteins that shield DNA and cell structures.

Front: Describe the feeding, internal transport, excretion, and development of Phylum Tardigrada.

• Feeding: Pierce plant, algal, or small animal cells with a chitinous stylet and suck out contents with a muscular pharynx.

• Transport & Excretion: Main body cavity is a hemocoel (no heart or respiratory organs); Malpighian tubules empty metabolic wastes into the gut.

• Development: Mostly dioecious with external fertilization during molting; direct development exhibiting eutely (fixed cell count; further growth occurs via cell enlargement rather than cell division).

Front: What are the defining structural features of Phylum Arthropoda, and how do they grow?

• Features: Eucoelomate protostomes with a segmented body fused into functional regions (tagmata), jointed appendages, and a tough chitinous exoskeleton (outer waxy epicuticle + chitinous procuticle made of exocuticle and endocuticle).

• Growth: Periodic physiological molting followed by ecdysis (shedding the old exoskeleton).

Front: Compare the circulatory, excretory, and nervous systems across arthropods.

• Circulatory: Open circulatory system where dorsal tubular "hearts" pump hemolymph into a central blood cavity (hemocoel); aquatic species often use hemocyanin (copper-based) for oxygen transport.

• Excretory: Crustaceans use green glands, whereas insects use Malpighian tubules (working with the hindgut to conserve water).

• Nervous: Anterior brain/ganglia connected to a ventral nerve cord with segmental ganglia.

Front: Contrast the respiratory structures used by terrestrial insects/myriapods, aquatic crustaceans, and chelicerates.

• Insects & Myriapods: Branching tracheal tubes opening via spiracles to deliver air directly to tissues.

• Aquatic Crustaceans: Filamentous gills.

• Chelicerates: Terrestrial arachnids use stacked book lungs (or tracheae); aquatic horseshoe crabs use external book gills.

Front: Compare the five subphyla of Arthropoda based on tagmata, antennae, and specialized appendages.

• Trilobita (Extinct): Three longitudinal lobes; biramous appendages; prominent compound eyes.

• Chelicerata (Spiders, scorpions, horseshoe crabs, ticks): Two tagmata (prosoma and opisthosoma); no antennae; first appendages are claw/fang-like chelicerae, followed by sensory/feeding pedipalps and 4 pairs of walking legs.

• Myriapoda (Centipedes & Millipedes): Head and trunk; 1 pair of antennae; mandibles; uniramous legs. Centipedes (Chilopoda) are predators with 1 leg pair/segment and venomous forcipules; millipedes (Diplopoda) are detritivores/herbivores with 2 leg pairs per fused diplosegment.

• Crustacea (Crabs, lobsters, shrimp, pill bugs): Usually cephalothorax (covered by a carapace) + abdomen; 2 pairs of antennae; mandibles; biramous (two-branched) appendages; calcium-reinforced cuticle; larvae include nauplius, zoea, and cypris.

• Hexapoda (Insects): Three tagmata (head, thorax, abdomen); 1 pair of antennae; mandibles/maxillae/labium; 3 pairs of uniramous legs on the thorax, plus 2 pairs of wings (on mesothorax and metathorax) in pterygotes.

Front: Why are arthropods considered the most abundant and successful animal phylum on Earth?

• Account for ~85% of all described animal species (>1.1 million species, led by insects).

• Key drivers of success: Protective yet flexible jointed exoskeleton; specialized tagmata and appendages; efficient tracheal/book-lung respiration for land colonization; de novo evolution of wings (originally aiding thermoregulation before flight); metamorphosis separating juvenile and adult niches; and eusociality coordinated via pheromones.

Front: How do arthropods contribute ecologically and environmentally across ecosystems?

• Pollination & Coevolution: Insects act as primary pollinators for flowering plants, driving mutualistic diversification.

• Nutrient Cycling & Food Webs: Detritivores (millipedes, terrestrial isopods) break down decaying plant matter; aquatic crustaceans (krill, zooplankton) form the base of marine food chains; terrestrial insects sustain birds, amphibians, reptiles, and mammals.

Front: What developmental characteristics define Superphylum Deuterostomia ("mouth second")?

• Blastopore fate: The blastopore becomes the anus, while the mouth forms secondarily at the opposite end of the archenteron.

• Cleavage: Radial, regulative, and indeterminate (early cell fates are not fixed, allowing identical twins or cloning).

• Coelom formation: Enterocoely—mesodermal pouches bud off from the endodermal archenteron and fuse to form an enterocoelom.

Front: What are the two major clades of Deuterostomia, and which phyla belong to each?

• Ambulacraria: Includes Echinodermata (sea stars, urchins, sea cucumbers) and Hemichordata (acorn worms, pterobranchs).

• Chordata: Includes the vertebrate classes plus two invertebrate subphyla: Urochordata (tunicates/salps) and Cephalochordata (lancelets).

Front: Describe the body symmetry, endoskeleton, and surface projections of Phylum Echinodermata ("prickly skin").

• Symmetry: Bilaterally symmetrical free-swimming larvae (e.g., bipinnaria) metamorphose into adults with pentaradial symmetry (arranged in multiples of five around an oral-aboral axis).

• Endoskeleton: Internal skeleton of calcareous ossicles covered by an epidermis, often bearing defensive spines.

• Surface projections: Tiny stalked claws (pedicellaria) keep the surface clean of debris, while dermal projections (papulae) act as gills for gas exchange.

Front: How does the water vascular (ambulacral) system of an echinoderm work, and what are its functions?

Seawater enters through an aboral madreporite → flows through the stone canal into a central ring canal → branches into radial canals along each arm → fills bulb-like ampullae that hydraulically extend or retract rows of tube feet (podia). It functions in locomotion, prey capture, tactile/chemical sensation, and gas exchange.

Front: Describe the digestive, circulatory, nervous, and excretory features of echinoderms.

• Digestive: Mouth leads to a bag-like stomach (sea stars can evert their cardiac stomach into bivalve shells to digest prey externally), branching into pyloric caeca in each arm and ending at an aboral anus.

• Circulatory: A hemal system (oral, gastric, and aboral rings) works alongside the water vascular system and spacious coelom to circulate nutrients.

• Nervous: Simple circumoral nerve ring and five radial nerves (lacks a brain or large ganglia).

• Excretory: Specialized podocytes filter fluid near the disc center, while ammonia diffuses out through the papulae.

Front: Compare the five extant classes of Phylum Echinodermata.

• Asteroidea (Sea stars): Thick arms radiating from a central disk; tube feet in oral ambulacral grooves; eversible stomach for preying on mollusks.

• Ophiuroidea (Brittle stars): Long, thin, flexible arms sharply distinct from the central disk; move by lashing arms (water vascular system is not used for locomotion).

• Echinoidea (Sea urchins & sand dollars): Armless, spherical or flattened body with a continuous internal shell (test) and a complex chewing apparatus called Aristotle's lantern.

• Crinoidea (Sea lilies & feather stars): Suspension feeders with branched, feather-like arms; sea lilies attach via a stalk, while feather stars crawl/swim using aboral cirri.

• Holothuroidea (Sea cucumbers): Elongated oral-aboral axis lying horizontally ("functional" bilateral symmetry); reduced tube feet; breathe via posterior respiratory trees; some have hemoglobin in circulating cells.

Front: What are the five key anatomical features shared by all members of Phylum Chordata at some stage of development?

• Notochord

• Dorsal hollow nerve cord

• Pharyngeal slits

• Post-anal tail

• Endostyle or thyroid gland (secretes iodinated hormones)

Front: Which two subphyla make up the "invertebrate" chordates, and how do they feed?

• Urochordata (tunicates, salps, larvaceans)

• Cephalochordata (lancelets)

  Both are marine suspension feeders (filtering phytoplankton and microorganisms from water).