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Complete Guide to All Animal Phyla

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1. Acanthocephala

• Name meaning: Spiny head
• Common name: Spiny-headed worms / Thorny-headed worms
• Species count: ~1,150
• Body plan: Bilateral symmetry, no digestive tract (entirely parasitic), anterior proboscis covered in recurved hooks used to anchor into host intestinal walls. Pseudocoelomate.
• Habitat: Exclusively endoparasites - adults live in vertebrate intestines; larvae develop in arthropod intermediate hosts
• Reproduction: Dioecious (separate sexes); complex two-host life cycle
• Key traits: No mouth, no gut - absorb nutrients directly through tegument. Ligament sacs enclose reproductive organs. Related to Rotifera.
• Examples: Macracanthorhynchus hirudinaceus (pigs), Moniliformis moniliformis (rats)

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2. Acoelomorpha

• Name meaning: Without coelom/body cavity
• Common name: Acoels
• Species count: ~400
• Body plan: Bilateral, flatworm-like, no coelom, no gut cavity (food digested in syncytial mass), simple nerve net
• Habitat: Marine - mostly interstitial (between sand grains) or on the seafloor; some symbiotic with cnidarians
• Reproduction: Hermaphroditic
• Key traits: Among the simplest bilaterians; debated whether they are basal bilaterians or secondarily simplified. Statocyst for balance.
• Examples: Convolutriloba retrogemma, Symsagittifera roscoffensis (harbors symbiotic algae)

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3. Annelida

• Name meaning: Little rings
• Common name: Segmented worms
• Species count: ~22,000
• Body plan: Bilateral, segmented (metameric), true coelom (eucoelomate), closed circulatory system, ventral nerve cord. Setae (bristles) present in most.
• Habitat: Marine, freshwater, terrestrial - extremely diverse environments
• Diet: Detritivores, predators, filter feeders, parasites
• Major classes:
  • Polychaeta - bristle worms (mostly marine)
  • Clitellata - earthworms (Lumbricus) and leeches (Hirudo)
  • Sipuncula - peanut worms (sometimes now included here)
• Key traits: Setae, clitellum in Clitellata, nephridia for excretion, well-developed brain
• Examples: Earthworms, ragworms, leeches, tube worms, Christmas tree worms

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4. Arthropoda

• Name meaning: Jointed feet
• Common name: Arthropods
• Species count: 1,200,000+ described (possibly 5-10x more undescribed)
• Body plan: Bilateral, segmented, hard chitinous exoskeleton, jointed appendages, open circulatory system, ventral nerve cord. Grow by molting (ecdysis).
• Habitat: Every ecosystem on Earth - land, sea, freshwater, air
• Diet: Every feeding strategy known
• Major subphyla/classes:
  • Chelicerata - spiders, scorpions, mites, horseshoe crabs
  • Myriapoda - centipedes, millipedes
  • Crustacea - crabs, lobsters, barnacles, shrimp, copepods
  • Hexapoda - insects (over 1 million species alone)
• Key traits: Compound eyes (most), tagmatization (fusion of body segments into functional units), diverse appendages
• Examples: Insects, spiders, crabs, lobsters, centipedes, barnacles, mites

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5. Brachiopoda

• Name meaning: Arm foot
• Common name: Lamp shells
• Species count: ~390 living (~30,000 fossil species - were far more diverse)
• Body plan: Bilateral, two unequal calcified shells (dorsal and ventral, unlike bivalve mollusks which are left/right), lophophore feeding organ, coelomate
• Habitat: Marine, sessile, attached to substrate by a pedicle (stalk)
• Diet: Filter feeders via lophophore
• Key traits: Extremely ancient lineage - essentially unchanged for hundreds of millions of years ("living fossils"). Distinguished from bivalves by shell orientation.
• Examples: Lingula (inarticulate, has survived since Cambrian), Terebratula

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6. Bryozoa (Ectoprocta)

• Name meaning: Moss animal
• Common name: Moss animals / Sea mats
• Species count: ~6,000 living (~20,000 fossil)
• Body plan: Colonial, sessile, each individual (zooid) has a lophophore, U-shaped gut, coelomate. Colonies form encrusting, branching, or leafy structures.
• Habitat: Mostly marine; some freshwater. Grow on rocks, shells, seaweed, and ship hulls
• Diet: Filter feeders
• Key traits: Colonies are modular - individuals specialized for feeding, defense (avicularia), or reproduction. Protective exoskeleton around each zooid.
• Examples: Bugula, Plumatella (freshwater), Membranipora

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7. Chaetognatha

• Name meaning: Long-hair jaw
• Common name: Arrow worms
• Species count: ~130
• Body plan: Bilateral, transparent, torpedo-shaped, chitinous grasping spines flanking mouth, fins, true coelom, deuterostome-like development (debated)
• Habitat: Marine - almost entirely planktonic; dominant component of marine zooplankton
• Diet: Carnivores - seize copepods, fish larvae, other plankton with their jaw spines; some produce neurotoxins (tetrodotoxin)
• Key traits: Remarkably abundant despite small species count. Transparent body. Unique among invertebrates - their phylogenetic position has long been debated.
• Examples: Sagitta, Spadella

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8. Chordata

• Name meaning: Cord (notochord)
• Common name: Chordates
• Species count: ~100,000+
• Body plan: Bilateral, deuterostome, four defining traits at some point in life: notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail. True coelom.
• Habitat: All habitats on Earth
• Diet: Every known feeding strategy
• Major subphyla:
  • Cephalochordata - lancelets (Amphioxus) - most primitive, ~30 species
  • Urochordata (Tunicata) - sea squirts, salps, ~3,000 species
  • Vertebrata - fish, amphibians, reptiles, birds, mammals (~66,000 species)
    • Jawless fish (Cyclostomata): lampreys, hagfish
    • Cartilaginous fish (Chondrichthyes): sharks, rays
    • Bony fish (Osteichthyes): ~30,000 species
    • Amphibia: frogs, salamanders, caecilians
    • Reptilia: lizards, snakes, turtles, crocodilians
    • Aves (Birds): ~10,000 species
    • Mammalia: ~6,400 species
• Examples: Humans, whales, eagles, frogs, tuna, sea squirts, lancelets

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9. Cnidaria

• Name meaning: Stinging nettle (cnida = nettle)
• Common name: Cnidarians
• Species count: ~13,000
• Body plan: Radial symmetry (some biradial), diploblastic (two tissue layers), no true coelom, cnidocytes (stinging cells with nematocysts), nerve net, no brain
• Habitat: Mostly marine; some freshwater (Hydra)
• Diet: Carnivores - use nematocysts to capture prey; some are mixotrophic (host photosynthetic algae)
• Major classes:
  • Anthozoa - corals, sea anemones, sea fans (~7,500 spp)
  • Medusozoa - jellyfish (Scyphozoa), box jellies (Cubozoa), hydroids (Hydrozoa)
  • Myxozoa - microscopic parasites (sometimes included here)
• Two body forms: Polyp (sessile) and medusa (free-swimming); some have only one
• Examples: Jellyfish, corals, sea anemones, Hydra, Portuguese man-o-war, box jellyfish

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10. Ctenophora

• Name meaning: Comb bearer
• Common name: Comb jellies
• Species count: ~200
• Body plan: Biradial symmetry, diploblastic or triploblastic (debated), eight rows of ciliary plates (ctenes/combs) used for locomotion, colloblasts (sticky cells) for prey capture - NOT nematocysts
• Habitat: Exclusively marine, mostly planktonic; some benthic
• Diet: Carnivores - capture small plankton, zooplankton, fish larvae
• Key traits: Bioluminescent. Phylogenetically controversial - some analyses place them as the earliest-diverging animal lineage, before even sponges. No anus (most) - food waste exits through mouth.
• Examples: Mnemiopsis leidyi (an invasive species in Black Sea), Beroe, Pleurobrachia

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11. Cycliophora

• Name meaning: Circular bearer
• Common name: Cycliophorans
• Species count: 3 (extremely recently discovered - first described 1995)
• Body plan: Tiny (<1 mm), sessile, circular mouth surrounded by a ring of cilia, complex life cycle with multiple body forms (feeding stage, sessile dwarf male, mobile female)
• Habitat: Marine - live exclusively on the lips of Norway lobsters (Nephrops) and other clawed lobsters
• Diet: Filter feeders - eat food particles in the water currents generated by the host's feeding
• Key traits: Discovery caused major excitement in zoology - first new animal phylum found in decades. Relationships unclear - possibly related to Entoprocta or Bryozoa.
• Examples: Symbion pandora (the only species known for years)

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12. Echinodermata

• Name meaning: Spiny skin
• Common name: Echinoderms
• Species count: ~7,500 living (~13,000 fossil)
• Body plan: Secondary pentaradial (5-fold) symmetry in adults (larvae are bilateral), deuterostome, water vascular system (hydraulic tube feet), calcareous ossicles in skin, true coelom
• Habitat: Exclusively marine, all depths from intertidal to deep sea
• Diet: Filter feeders, herbivores, predators, detritivores depending on class
• Major classes:
  • Asteroidea - sea stars (~2,000 spp)
  • Ophiuroidea - brittle stars and basket stars (~2,000 spp)
  • Echinoidea - sea urchins and sand dollars (~1,000 spp)
  • Holothuroidea - sea cucumbers (~1,700 spp)
  • Crinoidea - sea lilies and feather stars (~700 spp)
• Key traits: High regenerative ability; no centralized brain. Tube feet used for locomotion, feeding, gas exchange.
• Examples: Starfish, sea urchins, sea cucumbers, sand dollars, brittle stars, feather stars

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13. Entoprocta (Kamptozoa)

• Name meaning: Inner anus
• Common name: Goblet worms
• Species count: ~200
• Body plan: Small (~5mm), cup-shaped body (calyx) on a stalk, ring of tentacles surrounding both mouth AND anus (unlike Bryozoa where anus is outside tentacle ring), acoelomate or pseudocoelomate
• Habitat: Mostly marine, some freshwater; sessile, often epizoic on sponges or polychaetes
• Diet: Filter feeders using ciliated tentacles
• Key traits: Superficially similar to Bryozoa but anus placement and body plan differ. Can be solitary or colonial.
• Examples: Loxosoma, Urnatella (freshwater)

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14. Gastrotricha

• Name meaning: Hairy stomach
• Common name: Gastrotrichs
• Species count: ~800
• Body plan: Microscopic (0.06-3 mm), bilateral, flattened ventrally with cilia on underside for gliding locomotion, adhesive tubes for attachment, pseudocoelomate
• Habitat: Aquatic - marine and freshwater, interstitial (in sediments); among algae and detritus
• Diet: Bacteria, algae, diatoms, organic particles - microbivores
• Key traits: Hermaphroditic. Transparent. Very fast life cycle (some live only 3 days as adults).
• Examples: Chaetonotus, Macrodasys

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15. Gnathostomulida

• Name meaning: Jaw mouth
• Common name: Jaw worms
• Species count: ~100
• Body plan: Microscopic (0.3-3.5 mm), bilateral, acoelomate, unique paired jaw apparatus (made of cuticle) in pharynx, monociliated epithelium (each cell bears only one cilium - unique among animals)
• Habitat: Marine interstitial - live in the fine spaces between sand grains in low-oxygen sediments; often found below wave-washed beaches
• Diet: Scrape bacteria and fungi from sediment grain surfaces using jaws
• Key traits: Can survive low-oxygen (even anoxic) conditions. No circulatory or respiratory system. Related to Rotifera and Micrognathozoa (all in Gnathifera clade).
• Examples: Gnathostomula, Austrognathia

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16. Hemichordata

• Name meaning: Half chord
• Common name: Acorn worms and pterobranchs
• Species count: ~130
• Body plan: Bilateral, deuterostome, three-part body (proboscis/collar/trunk), pharyngeal slits, stomochord (short notochord-like structure - not a true notochord), open circulatory system
• Habitat: Marine - acorn worms burrow in sediment; pterobranchs are colonial and sessile on rocks
• Diet: Acorn worms: deposit feeders; pterobranchs: filter feeders via lophophore-like tentacles
• Key traits: Considered a key link between invertebrates and chordates - share pharyngeal slits and deuterostome development. Some produce brominated compounds (produce iodine-like smell).
• Examples: Balanoglossus (acorn worm), Rhabdopleura (pterobranch)

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17. Kinorhyncha

• Name meaning: Moving snout
• Common name: Mud dragons
• Species count: ~250
• Body plan: Microscopic (0.1-1 mm), bilateral, 13 segments with spines and plates (but no cilia for locomotion - unique), pseudocoelomate, retractable head with spines used to anchor during movement
• Habitat: Marine - exclusively interstitial in muddy and sandy sediments, from intertidal to deep sea
• Diet: Diatoms, bacteria, organic matter in sediment
• Key traits: Move by anchoring head spines and pulling body forward. No external cilia. Part of the Ecdysozoa (molting animals), related to nematodes.
• Examples: Echinoderes, Pycnophyes

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18. Loricifera

• Name meaning: Corset bearer
• Common name: Brush heads
• Species count: ~40 described (many more suspected)
• Body plan: Microscopic (250 µm), bilateral, enclosed in a rigid lorica (case), retractable head with spines (scalids) and mouth cone, pseudocoelomate
• Habitat: Marine interstitial - in coarse marine sediment; some discovered in completely anoxic, brine-filled deep-sea sediments - first multicellular animals known to live without oxygen
• Diet: Bacteria and organic particles
• Key traits: Described only in 1983 (Reinhardt Kristensen). The anaerobic species found in deep Mediterranean hypersaline basins are extraordinary - appear to have hydrogenosomes instead of mitochondria.
• Examples: Nanaloricus mysticus (type species), Rugiloricus

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19. Micrognathozoa

• Name meaning: Tiny jaw animals
• Common name: Micrognathozoans
• Species count: 1 (Limnognathia maerski - only known species, described 2000)
• Body plan: Microscopic (~150 µm), bilateral, complex jaw apparatus (most complex relative to body size of any animal), accordion-like extensible thorax, acoelomate
• Habitat: Freshwater - found in Greenland springwater and Antarctic streams; interstitial
• Diet: Bacteria and diatoms, scraped with complex jaws
• Key traits: Newest animal phylum - described in 2000. Only hermaphroditic females found to date; males unknown. Jaws are structurally complex with 15+ elements. Related to Gnathostomulida and Rotifera.
• Examples: Limnognathia maerski (sole species)

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20. Mollusca

• Name meaning: Thin shell / soft
• Common name: Mollusks
• Species count: ~85,000 (second most diverse animal phylum)
• Body plan: Bilateral (except Gastropoda which undergo torsion), unsegmented, muscular foot, mantle (secretes shell), visceral mass, radula (rasping tongue) in most; true coelom
• Habitat: Marine, freshwater, terrestrial - ubiquitous
• Diet: Herbivores, carnivores, filter feeders, detritivores - highly varied
• Major classes:
  • Gastropoda - snails and slugs (~70,000 spp) - only class to undergo torsion
  • Bivalvia - clams, oysters, mussels, scallops (~20,000 spp) - no radula, filter feeders
  • Cephalopoda - octopus, squid, cuttlefish, nautilus (~800 spp) - most intelligent invertebrates
  • Polyplacophora - chitons (~1,000 spp) - 8 shell plates
  • Scaphopoda - tusk shells (~900 spp)
  • Monoplacophora - once thought extinct, rediscovered 1952; primitive deep-sea forms
  • Aplacophora - worm-like, no shell
• Examples: Octopus, giant squid, oysters, snails, nudibranchs, nautilus, clams

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21. Nematoda

• Name meaning: Thread-like
• Common name: Roundworms / Nematodes
• Species count: ~30,000 described (estimates up to 1,000,000+ total)
• Body plan: Bilateral, cylindrical, unsegmented, pseudocoelomate, cuticle shed during growth (Ecdysozoa), hydrostatic skeleton, unique excretory system with renette cells
• Habitat: Every habitat on Earth - soil, marine sediment, freshwater, parasites of plants and animals. Among the most abundant animals - billions per square meter of soil.
• Diet: Bacteria, fungi, other nematodes, plant tissues; many are parasites
• Key traits: C. elegans is the most studied model organism in biology - every cell mapped. Parasitic forms include major human pathogens.
• Notable parasites: Ascaris lumbricoides (roundworm), Wuchereria bancrofti (lymphatic filariasis), Trichinella (trichinosis), Necator (hookworm), Onchocerca volvulus (river blindness)
• Examples: C. elegans, hookworms, filarial worms, pinworms, root-knot nematodes

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22. Nematomorpha

• Name meaning: Thread form
• Common name: Horsehair worms / Gordian worms
• Species count: ~360
• Body plan: Very long (up to 1 m) and thin (1-3 mm diameter), bilateral, adults have degenerate gut (do not feed), pseudocoelomate, thread-like body
• Habitat: Adults in freshwater and moist soil; larvae are internal parasites of arthropods (crickets, beetles, grasshoppers)
• Life cycle: Larvae parasitize arthropods, consuming host tissue except vital organs; when mature, they manipulate the host into jumping into water, then emerge
• Key traits: Mind-control of host is one of nature's most dramatic examples of parasitic manipulation - infected crickets are driven to drown themselves so the worm can complete its life cycle in water.
• Examples: Gordius, Paragordius, Nectonema (marine genus)

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23. Nemertea (Rhynchocoela)

• Name meaning: Sea nymph (Nemertea) / Beak hollow (Rhynchocoela)
• Common name: Ribbon worms
• Species count: ~1,400
• Body plan: Bilateral, unsegmented, acoelomate to eucoelomate (debated), unique eversible proboscis (often armed with stylets for prey capture), closed circulatory system (one of simplest animals with one), complete gut (mouth to anus)
• Habitat: Mostly marine benthic; some freshwater, terrestrial (humid tropics), and commensal/parasitic
• Diet: Carnivores - use proboscis to capture annelids, crustaceans, mollusks; some scavenge
• Key traits: Some species extremely long - Lineus longissimus may exceed 55 meters (longest animal recorded). Remarkable regenerative ability. Proboscis is separate from digestive system.
• Examples: Lineus, Cerebratulus, Malacobdella (commensal in bivalve mantle cavities)

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24. Onychophora

• Name meaning: Claw bearer
• Common name: Velvet worms
• Species count: ~200
• Body plan: Bilateral, soft-bodied, worm-like with multiple stubby unjointed legs (lobopods) with claws, thin permeable cuticle (molted), open circulatory system, tracheal respiration (like insects)
• Habitat: Terrestrial - tropical and southern hemisphere forests, under bark, in leaf litter; require humid conditions due to water-permeable cuticle
• Diet: Carnivores - immobilize prey (insects, spiders) by spraying adhesive slime from oral papillae up to 30 cm
• Key traits: Living evolutionary intermediate between annelids and arthropods - considered a "missing link." Bear live young in most species. Poorly known despite their fascinating biology.
• Examples: Peripatus, Euperipatoides, Opisthopatus

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25. Orthonectida

• Name meaning: Straight swimmer
• Common name: Orthonectids
• Species count: ~25
• Body plan: Microscopic, parasitic, ciliated; two life stages: sexual (free-swimming, male and female) and plasmodium (multinucleate, within host tissue). Simplest body plan among animals - possibly secondarily simplified.
• Habitat: Marine - endoparasites in tissues of flatworms, nemerteans, polychaetes, bivalves, echinoderms
• Diet: Absorb nutrients directly from host tissues
• Key traits: Together with Rhombozoa formerly grouped as "Mesozoa" - now thought to be separately derived from more complex ancestors. Genomic data suggests they are simplified bilaterian worms.
• Examples: Rhopalura ophiocomae (parasite of brittle stars), Intoshia

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26. Phoronida

• Name meaning: Bearer of Pheronis (a sea nymph)
• Common name: Horseshoe worms
• Species count: ~14
• Body plan: Worm-like, bilateral, coelomate, live in self-secreted chitinous tube, anterior lophophore (horseshoe-shaped ring of tentacles for filter feeding), U-shaped gut, closed circulatory system with hemoglobin
• Habitat: Marine, sessile - in tubes buried in sand/mud or bored into rock/shell, intertidal to 400 m depth
• Diet: Filter feeders - cilia on lophophore tentacles drive food particles to mouth
• Key traits: Extremely small phylum with only ~14 species, but cosmopolitan. Related to Brachiopoda and Bryozoa (all in Lophophorata clade). Have red blood (hemoglobin).
• Examples: Phoronis australis, Phoronopsis harmeri, Phoronis hippocrepia

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27. Placozoa

• Name meaning: Plate animal
• Common name: (none)
• Species count: 1 formally described (Trichoplax adhaerens) + several cryptic species
• Body plan: Flat, disc-shaped, ~1-2 mm, no organs, no nervous system, no muscles, no mouth - two cell layers (dorsal epithelium, ventral epithelium) separated by fiber cells. Simplest known animal body plan.
• Habitat: Marine - tropical to subtropical coastal waters, crawl on hard substrates
• Diet: External digestion - settle over food (algae, bacteria), secrete digestive enzymes, and absorb nutrients through ventral cells
• Key traits: Possibly represents the simplest living animal body plan. Their phylogenetic position is debated - possibly sister to all other animals or to Cnidaria. Reproduce by fission and budding; sexual reproduction rarely observed.
• Examples: Trichoplax adhaerens

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28. Platyhelminthes

• Name meaning: Flat worm
• Common name: Flatworms
• Species count: ~25,000
• Body plan: Bilateral, dorsoventrally flattened, acoelomate (no body cavity), no circulatory system (gas exchange by diffusion - hence why they must be flat), protonephridia (flame cells) for excretion, complex hermaphroditic reproductive system
• Habitat: Marine, freshwater, terrestrial; many are parasitic
• Major classes:
  • Turbellaria - free-living flatworms (planarians); remarkable regeneration
  • Trematoda - flukes; endoparasites with complex life cycles
  • Monogenea - ectoparasites of fish gills
  • Cestoda - tapeworms; no gut, absorb nutrients through tegument; can be meters long
• Notable parasites: Schistosoma (bilharzia - infects 240 million people), Taenia (tapeworms), Fasciola (liver fluke)
• Examples: Planarians (Dugesia), tapeworms, liver flukes, blood flukes, monogenean fish parasites

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29. Porifera

• Name meaning: Pore bearer
• Common name: Sponges
• Species count: ~9,000
• Body plan: No true tissues, no nervous system, no muscles - loose aggregation of totipotent cells. Body perforated by pores (ostia) and channels; water pumped through by flagellated choanocytes. Supported by spicules (silica or calcium carbonate) or spongin fibers.
• Habitat: Mostly marine (all depths), a few freshwater; sessile, attached to substrate
• Diet: Filter feeders - choanocytes trap bacteria, phytoplankton, dissolved organic matter from water current
• Major classes:
  • Demospongiae - ~90% of all sponges; spongin and/or siliceous spicules; include bath sponges
  • Calcarea - calcium carbonate spicules; exclusively marine
  • Hexactinellida - glass sponges; fused silica spicules; deep sea
  • Homoscleromorpha - small, simple; recently elevated to class
• Key traits: No symmetry (most). Can reassemble from dissociated cells. Ancient lineage - 600+ million years old. Some species can live for thousands of years.
• Examples: Bath sponge (Spongia), Venus flower basket (Euplectella), Oscarella, freshwater sponge (Spongilla)

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30. Priapulida

• Name meaning: Penis (after Priapus, Greek fertility god)
• Common name: Priapulid worms / Cucumber worms
• Species count: ~17 living (~50 fossil)
• Body plan: Cylindrical, bilateral, spiny retractable proboscis (introvert) at front end, pseudocoelomate; caudal appendage (tail) present in some
• Habitat: Marine - bury in soft sediment (mud and sand) in cold seas; Arctic and Antarctic especially well-represented; some deep sea
• Diet: Carnivores - evert proboscis to engulf polychaetes and other soft-bodied prey
• Key traits: Ancient lineage - abundant in Cambrian fossil beds (~500 mya). Part of Ecdysozoa (molt their cuticle). Played a much larger ecological role in ancient seas.
• Examples: Priapulus caudatus, Maccabeus tentaculatus, Halicryptus spinulosus

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31. Rhombozoa (Dicyemida)

• Name meaning: Rhombus animal
• Common name: Rhombozoans / Dicyemids
• Species count: ~75
• Body plan: Microscopic, parasitic; extremely simple - single elongated axial cell surrounded by a jacket of ciliated peripheral cells; no organs
• Habitat: Marine - endoparasites in renal (kidney) sacs of cephalopod mollusks (octopus, squid, cuttlefish)
• Life cycle: Asexual (vermiform larvae produced within axial cell) and sexual (infusoriform larvae, which escape to the sea) phases
• Key traits: Formerly grouped with Orthonectida as "Mesozoa" - now thought independently derived. Their simplified body plan is likely secondary reduction from more complex ancestors. Every specimen of certain cephalopod species is infected.
• Examples: Dicyema japonicum, Conocyema polymorpha

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32. Rotifera

• Name meaning: Wheel bearer
• Common name: Rotifers / Wheel animals
• Species count: ~2,000 (plus ~700 Acanthocephala, sometimes included)
• Body plan: Microscopic (0.1-0.5 mm), bilateral, pseudocoelomate, defining feature: corona (crown of cilia at head that beats rhythmically like a spinning wheel, used for feeding and locomotion), muscular pharynx with jaw-like mastax containing trophi
• Habitat: Mostly freshwater; marine, moist soil, mosses; some parasitic
• Diet: Bacteria, algae, detritus, other microscopic organisms - filtered by corona and processed by mastax
• Major orders: Bdelloidea (no males - reproduce entirely by parthenogenesis; extraordinarily resistant to desiccation), Monogononta (cyclical parthenogenesis + sex), Seisonidea (ectoparasites of crustaceans)
• Key traits: Bdelloid rotifers are famous for surviving complete desiccation for decades and for evolving without sex for millions of years. Can be revived from museum specimens 100 years old.
• Examples: Brachionus, Philodina, Seison

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33. Sipuncula

• Name meaning: Little tube
• Common name: Peanut worms
• Species count: ~160
• Body plan: Bilateral, unsegmented, coelomate, retractable introvert (front portion of body that inverts inside the trunk when disturbed - making it look like a peanut), simple nervous system, U-shaped gut in some
• Habitat: Marine - burrow in sand/mud, bore into coral, hide in crevices; intertidal to deep sea, tropical to polar
• Diet: Deposit feeders and some suspension feeders
• Key traits: Now often included within Annelida by molecular analyses. When disturbed, they retract the introvert dramatically. No respiratory system - gas exchange through body wall.
• Examples: Sipunculus nudus, Phascolosoma, Themiste

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34. Tardigrada

• Name meaning: Slow step
• Common name: Water bears / Moss piglets
• Species count: 1,300+
• Body plan: Microscopic (0.1-1.5 mm), bilateral, four pairs of stubby legs each with claws or adhesive discs, cuticle molted periodically (Ecdysozoa), no circulatory or respiratory system
• Habitat: Every habitat on Earth - mosses, lichens, leaf litter, marine sediment, freshwater, Antarctic ice, hot springs, deep sea. One of the most environmentally resistant animals known.
• Diet: Fluids from algae, bacteria, other small invertebrates (pierce cell walls with stylets)
• Key traits: Cryptobiosis - can enter a state of suspended animation (tun formation) surviving:
  • Temperatures from -272°C to +150°C
  • Pressure up to 6,000 atm
  • Desiccation for decades
  • High radiation doses
  • Vacuum of outer space (survived unshielded exposure in 2007 ESA experiment)
• Examples: Ramazzottius, Milnesium tardigradum, Echiniscus

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35. Xenoturbellida

• Name meaning: Strange flatworm
• Common name: (none)
• Species count: 6
• Body plan: Bilateral, simple worm-like, ~1-20 cm, ciliated epidermis, no true organs - no coelom, no brain, no nephridia, no gonads (gametes released through body wall rupture), nerve net only, simple statocyst for balance
• Habitat: Marine - deep-sea mud (200-3,700 m); some species found off coasts of Sweden, Pacific, and elsewhere
• Diet: Thought to feed on bivalve mollusks (eggs and tissues found in gut)
• Key traits: Long misidentified as flatworms or mollusks. Now placed in Xenambulacraria (with echinoderms and hemichordates) as a deuterostome. Genomic evidence suggests surprisingly close relationship to deuterostomes despite simple appearance.
• Examples: Xenoturbella bocki, Xenoturbella profunda, X. monstrosa

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Summary Table

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Major Evolutionary Groupings

• Porifera - sponges (basal, no true tissues)
• Ctenophora - may be earliest-diverging (disputed)
• Placozoa - simplest body plan
• Eumetazoa - all animals with true tissues:
  • Cnidaria - radially symmetrical
  • Bilateria - bilateral symmetry:
    • Deuterostomes - anus forms first in development: Chordata, Echinodermata, Hemichordata, Xenoturbellida
    • Protostomes - mouth forms first:
      • Ecdysozoa (molt their cuticle): Arthropoda, Nematoda, Tardigrada, Onychophora, Kinorhyncha, Loricifera, Priapulida, Nematomorpha
      • Lophotrochozoa (lophophore or trochophore larva): Annelida, Mollusca, Platyhelminthes, Nemertea, Brachiopoda, Bryozoa, Phoronida, Rotifera, Acanthocephala, Entoprocta, Gastrotricha, Gnathostomulida, Micrognathozoa, Sipuncula

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