Topic 14: Ecdysozoans: Nematodes, Arthropods, & some minor groups (Ch. 33) Ecdysozoa




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BIOL 1030 – TOPIC 14 LECTURE NOTES

Topic 14: Ecdysozoans: Nematodes, Arthropods, & some minor groups (Ch. 33)


  1. Ecdysozoa (clade)

    1. defined primary by molecular evidence

    2. synapomorphy is ecdysis, or molting, of cuticle

      1. cuticle is a (relatively) tough external coat that is often chitinous and often serves as an exoskeleton

      2. growth requires partial dissolving and then shedding of the cuticle (ecdysis) and depositions of a new cuticle

      3. process of molting (ecdysis) usually controlled by molting hormone, or ecdysone

      4. new cuticle is relatively soft and expandable for a while

        • animal usually “puffs up” to expand new cuticle, then grows into it

        • animal is vulnerable while the new cuticle hardens

    3. clade includes the following phyla and organizational taxons that we will cover:

      1. Scalidophora

        • Phylum Priapulida (clade)

        • Phylum Loricifera (clade)

        • Phylum Kinorhyncha (clade)

      2. Nematoida

        • Phylum Nematoda (clade)

        • Phylum Nematomopha (clade)

      3. Panarthropoda

        • Phylum Onychophora (clade)

        • Phylum Tardigrada (clade)

        • Phylum Arthropoda (clade)




  1. Scalidophora

    1. clade of three phyla: Priapulida, Loricifera, and Kinorhyncha

    2. basal branch within Ecdysozoa

    3. all are pseudocoelomate or acoelomate

    4. have spiny, evertable proboscis that is used for feeding




  1. Phylum Priapulida (clade) – penis worms

    1. marine; 16 living species, all with phallic appearance

    2. range from near microscopic to about 20 cm in length

    3. fossil record back to the Cambrian period, were likely major predators during Cambrian

    4. named for Greek fertility god Priapos




  1. Phylum Loricifera (clade)

    1. ~100 living species

    2. live in marine sediment

    3. tiny (less than 3 mm long)

    4. can telescope most of body into lorica, a protective 6-plate pocket




  1. Phylum Kinorhyncha (clade)

    1. ~150 living species

    2. live in marine sediment

    3. tiny (less than 1 mm long)

    4. segmented body, with head, neck, and trunk with 11 segments




  1. Nematoida

    1. clade containing two phyla: Nematoda and Nematomorpha

    2. sister group to Panarthropoda

    3. pseudocoelomate




  1. Phylum Nematoda – roundworms or nematodes

    1. "If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable…we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a thin film of nematodes." -N.A. Cobb, 1914, Yearbook of the United States Department of Agriculture

    2. ~25,000 living species (maybe as many as 500,000)

    3. most soil-dwelling and microscopic (< 1 mm)

    4. covered with flexible, thick cuticle

    5. muscles extend along length (longitudinal), not around

    6. feeding:

      1. many are parasites

      2. mouth often has stylets for piercing

      3. muscular chamber in throat (pharynx) used for sucking up food

    7. many are important plant parasites

    8. about 50 species parasitize humans

    9. Trichinella - causes trichinosis, females in digestive tract of pigs produce young which make their way to muscle tissue where they form cysts - cook well and you are fine, but 2.4% of people in U.S. carry the worm (don’t eat raw pork!)

    10. Caenorhabditis elegans – important lab animal; adult has exactly 959 cells; complete developmental cellular anatomy known

    11. no cilia or flagellae, even on sperm

    12. reproduction – sexual, with separate sexes (dioecious)




  1. Phylum Nematomorpha – horsehair worms or nematomorphs

    1. ~320 living species

    2. similar to nematodes in physiology

    3. adults average ~1 meter long

    4. all are parasitic

      1. famous example – the cricket brain takeover

      2. larva develops within Orthopteran host

      3. when the nematomorph reaches adulthood, it causes the host to drown itself

      4. adult emerges and lives the rest of its life in water




  1. Panarthropoda

    1. clade containing three phyla: Tardigrada, Onychophora, and Arthropoda

    2. all have legs, claws, ventral nervous system, and segmented body

    3. all have coelom, but it is mostly associated with gonads; main cavity is instead the hemocoel

    4. all have open circulatory system with hemolymph (associated with hemocoel)




  1. Phylum Onychophora – velvet worms

    1. ~150 described living species

    2. once marine, forms living today are all terrestrial

    3. mostly limited to humid forests in the Southern hemisphere

    4. (segmented) worms with legs

      1. many repetitious body segments, some similarity to caterpillars

      2. legs are internally hollow and not jointed; sac-like

      3. 13-43 pairs of legs

    5. average 5 cm long as adults; some as long as 20 cm

    6. thin, chitinous cuticle

    7. predatory, mainly on arthropods and mollusks

      1. often squirt fast-drying glue-like slime to catch and trap larger prey

      2. toxic saliva injected into victim to kill it and begin digestion




  1. Phylum Tardigrada – water bears

    1. over 1000 living species

    2. aquatic or on other organisms; easiest to find on lichens and mosses

      1. many feed on plants or algae

      2. some feed on bacteria

      3. some hunt smaller animals

    3. mostly microscopic

    4. body with head, then 4 segments

      1. covered by a chitinous cuticle

      2. 4 pairs of lobe-like legs with claws

      3. rounded, stubby appearance and slow, lumbering gait

    5. very hardy animals; able to enter a state of dormancy where they can

      1. can survive extreme cold or heat

      2. can survive very high radiation doses

      3. can survive up to a decade without water




  1. Phylum Arthropoda - arthropods

    1. includes spiders, insects, lobsters, and others

    2. very diverse and important group

      1. ~1 million named species (about 2/3 of all named species)

      2. probably at least 10 million living species still unnamed

      3. dramatic impact (for good and bad) on environment

      4. major impact (for good and bad) on economy and human health

    3. characteristics

      1. jointed appendages (arthros – jointed, podes – feet); may be modified as mouth parts, antennae, and legs

      2. exoskeleton – cuticle composed primarily of chitin

        • provides protection

        • strong but brittle; must be thicker for larger insects

        • insects cannot be too large because the exoskeleton would be too heavy

      3. segmented – often fused into sections or tagmata (tagmatization important in arthropod evolution)

      4. all have a distinct head with a brain; sometimes head and thorax are fused into a cephalothorax, and sometimes several segments fused into an abdomen

    4. four subphyla with living members

      1. Cheliceriformes or Chelicerata (spiders, horseshoe crabs, sea spiders, etc.)

        • cephalothorax and abdomen

        • have one pair of chelicerae (pinchers or fangs); after that, one pair of pedipalps (sensory appendages)

        • 4 pairs of walking legs (from cephalothorax)

      2. Myriapoda (millipedes and centipedes)

        • head with and multisegmented trunk

        • head has one pair of antennae, then one pair of mandibles and other mouthparts

        • trunk has pairs of walking legs

        • appendages are unbranched (uniramous)

        • tracheal system for respiration

      3. Crustacea (lobsters, sowbugs, barnacles, etc.)

        • cephalothorax and abdomen

        • head has two pair of antennae, then one pair of mandibles and other mouthparts

        • cephalothorax has various numbers of pairs of walking legs

        • abdomen also often has appendages

        • appendages are branched (biramous)

      4. Hexapoda (insects and their relatives)

        • head, trunk, and abdomen

        • head has one pair of antennae, then one pair of mandibles and other mouthparts

        • trunk has 3 pairs of walking legs and often 2 pairs of wings

        • appendages are unbranched (uniramous)

        • tracheal system for respiration

      5. the trilobites are an extinct group (would be a different subphylum) that are abundant in the fossil record

Adapted from the Tree of Life website http://tolweb.org/tree?group=Arthropoda&contgroup=Bilateria




    1. internal features

      1. no cilia

      2. limited coelom

      3. open circulatory system

        • longitudinal dorsal vessel is heart

        • opens at brain end and into hemocoel

      4. respiratory system

        • respiration across the cuticle is common, especially in the smaller arthropods

        • myriapods and hexapods have system of branched, cuticle-lined air ducts called tracheae

          • smallest tracheoles in direct contact with cells

          • all body cells must be near a tracheole to get enough oxygen (this is another limit to arthropod body size)

          • air gets in through openings in the exoskeleton called spiracles

        • some chelicerates have book lungs or book gills, series of sheets in a chamber used for gas exchange

        • some crustaceans have gills

      5. excretory system

        • varies depending on group

        • most unique are Malpighian tubules in terrestrial myriapods and hexapods

          • slender projections of digestive tract

          • between midgut and hindgut

          • as fluid passed through them, nitrogenous wastes are precipitated as uric acid or guanine

          • wastes are emptied into hindgut

          • most of the water and salts reabsorbed, rest is discarded

      6. nervous system

        • two chains of ganglia along ventral surface

        • front has three fused ganglia pairs, forming brain

        • ventral ganglia control most functions; many can live and even reproduce for some time without a brain

        • compound eyes found in many arthropods, composed of several ommatidia

        • simple eyes (ocelli) also present in some




  1. Subphylum Chelicerata

    1. spiders, horseshoe crabs, and sea spiders

    2. lack jaws; first pair of appendages are mouthparts called chelicerae (fangs in some)

    3. second pair of appendages are pinchers or feelers (pedipalps)

    4. rest of appendages are legs; 4 pairs of legs

    5. three classes: Merostomata, Arachnida, and Pycnogonida

    6. we will only cover the first two; leaving out Pycnogonida (sea spiders)




  1. Class Merostomata (grade if fossils are included; clade for living members) – horseshoe crabs

    1. 4 living species considered “living fossils”; same form in fossil record back to 220 MYA

    2. pedipalps appear like legs (as do chelicerae to some extent) – appear to have extra legs

    3. 5 pairs of book gills

    4. shell called a carapace

    5. long “tail” spine called telson




  1. Class Arachnida (clade) – arachnids

    1. include harvestmen, scorpions, spiders, mites, and ticks

    2. their chelicerae are fangs, often attached to a poison gland

    3. two body segments: prosoma (cephalothorax) and opisthosoma (abdomen)

    4. usually with book gills or book lungs

    5. four pair of walking legs (8 legs) from prosoma

    6. feeding

      1. most are carnivorous

      2. exception: mites are mostly herbivorous

      3. can ingest only liquefied food

      4. typically inject some digestive juices into food and then suck up partially digested fluid

    7. classification within the class is in a state of flux

      1. most traditional classifications have 11 extant orders

      2. clade/grade nature of each of most of these is in questions, revisions certain to come

      3. we will cover 4 orders, each of which is probably at least a grade if not a clade

    8. Order Opiliones – harvestmen, or daddy longlegs

      1. over 6000 living species

      2. oval, compact body with extremely long, slender legs

      3. most are predators of insects and arachnids

      4. females have an ovipositor for laying eggs

    9. Order Scorpiones – scorpions

      1. ~2000 living species

      2. pedipalps are pinchers

      3. stinger in last segment of abdomen

      4. most ancient group of terrestrial arthropods (425 MYA)

      5. young born alive

    10. Order Araneae – spiders

      1. ~40,000 living species

      2. produce silk from a protein fluid forced out of spinnerets

        • modified appendages at rear of opisthosoma

        • up to six pairs

      3. predators

        • important in controlling insect populations

        • hunt or use webs for prey capture, or in some cases both

        • venom glands are connected to fangs, used to paralyze prey

        • black widow and brown recluse venom particularly lethal to humans

      4. webs used for many purposes, capturing prey, wrapping prey or sperm or eggs, gossamer threads as parachutes

      5. mating

        • many have elaborate courtship where male puts female into a trance

        • pedipalps used in copulation to transfer and deposit sperm

        • after mating, male is often eaten

    11. Order Acari – mites and ticks

      1. ~30,000 described living species, estimates are that there are over 1 million actual living species

      2. chelicerae fused into a capitulum; used for piercing

      3. most mites have cephalothorax and abdomen fused, many live on us

      4. some mites are serious pests of crop and house plants

      5. ticks are blood-feeding ectoparasites that transmit many diseases in vertebrates




  1. Subphylum Myriapoda

    1. millipedes and centipedes and their relatives

    2. head with and multisegmented trunk

    3. uniramous mandibulates

      1. have paired jaws (mandibles) as mouthpart appendages after the antennae

      2. all appendages are basically uniramous (“single-branched”) appendages

    4. have a tracheal respiratory system

    5. have Malpighian tubules for excretion Myriapoda form a clade with Chelicerata

    6. four classes; we will cover two: Chilopoda and Diplopoda




  1. Class Chilopoda (clade) – centipedes

    1. ~3000 living species described

    2. body with head and multisegmented trunk

    3. carnivorous, eat mainly insects

    4. venomous

    5. poison claws on foremost trunk segment

    6. one pair of legs per trunk body segment




  1. Class Diplopoda (clade) – millipedes

    1. ~8000 living species described

    2. body with largely inconspicuous head and multisegmented trunk

    3. two pairs of legs per trunk body segment starting with the 2nd segment after the head

    4. each segment is a tagma that is a fusion of two ancestral segments

    5. most millipedes are herbivores or detritivores; NOT venomous

    6. secrete a variety of noxious fluids and gases to ward off attackers




  1. Subphylum Crustacea – crustaceans

    1. include shrimp, lobsters, crayfish, crabs, isopods, barnacles, etc.

    2. have two pair of sensory antennae as first appendages

    3. biramous mandibulates

      1. have paired jaws (mandibles) as mouthpart appendages after the antennae

      2. all appendages are basically biramous (“two-branched”)

    4. ~35,000 described living species - most marine, many freshwater, some (sowbugs) terrestrial

    5. nauplius – similar larva shared by all (unifies group)

    6. decapods – shrimp, lobsters, crayfish, crabs

      1. 10 walking legs

      2. exoskeleton usually reinforced with calcium carbonate

      3. cephalothorax covered in carapace (a dorsal shield)

      4. some have swimmerets on abdomen (for swimming, reproduction)

      5. some have uropods and/or telson at end of abdomen (for swimming)

    7. others include sowbugs (terrestrial isopods), Daphnia (freshwater plankton), barnacles (sessile shelled filter-feeders), etc.




  1. Subphylum Hexapoda

    1. insects and their relatives

    2. head, trunk, and abdomen

    3. three body segments: head, thorax, abdomen

      1. head has one pair of sensory antennae

      2. three pairs of legs on thorax only (never head or on abdomen)

    4. uniramous mandibulates

      1. have paired jaws (mandibles) as mouthpart appendages on head after the antennae

      2. all appendages are basically uniramous (“single-branched”) appendages

    5. have a tracheal respiratory system

    6. have Malpighian tubules for excretion

    7. insects plus some minor groups (Collembola, Protura, and Diplura) of uncertain classification within the subphylum

    8. we will focus only on insects (class Insecta)




  1. Class Insecta – insects

    1. terrestrial and freshwater, few marine; extremely diverse and successful group

    2. date back in fossil record to 300 MYA; considerable coevolution with flowering plants likely

    3. ½ of all named species; about a 1018 individuals alive at any given time

    4. size range from 0.1 mm to 30 cm

    5. external anatomy:

      1. mouthparts are often specialized, such as for piercing, sucking, or sopping

      2. wings, if present, attach to middle and posterior segment of thorax (2 pair) or on middle segment only (1 pair) – when there is only 1 pair, the second pair is actually present as greatly reduced halteres

      3. sensory hairs – cover body; responsive to mechanical stimulation

    6. fat body in hemocoel

      1. group of cells that serves in part as a food reservoir

      2. also has some functions like a vertebrate liver

    7. sound production often well developed

      1. produced by various means

      2. detected by sensory hairs or, in some, by thin membrane called tympanum

    8. nearly all communicate within their species using pheromones

      1. chemical signals used for recognition and attracting mates

      2. may even control development (queen ant pheromones keep workers under control)

    9. life histories

      1. stages between molts called instars

      2. process of molting (ecdysis) controlled by molting hormone, or ecdysone

      3. variety in pattern of metamorphosis, or form changes

      4. types of metamorphosis

        • ametabolous – two stages (egg, adult)

          • babies just small copies of adults

          • examples: silverfish, springtails

        • hemimetabolous – three stages (egg, nymph, adult)

          • also called simple or incomplete metamorphosis

          • have nymphs that become more like adults with every molt

          • usually differ mainly in wing size

          • examples: grasshoppers, dragonflies

        • holometabolous – four stages, (egg, larva, pupa, adult)

          • also called complete metamorphosis

          • consists of a tremendous change from the wormlike larva to the adult stage after crossing through a sedentary pupa stage

          • larvae - usually no eyes, usually no legs, feeders

          • pupa or chrysalis – transformational stage, sedentary

          • adult - may feed (maybe on something completely different than larvae) or may not feed, breeders

          • larvae and adults typically have very different lifestyles and habitats (no competition between adults and larvae)

          • occurs for >90% of insect species

          • examples: butterflies, beetles, bees, ants, flies

      5. metamorphosis is under control of juvenile hormone

        • high level = remain larva or nymph after molting

        • medium level = become pupa or near-adult nymph after molting

        • low level = become adult after molting

        • in some cases, can be used as a sort of insecticide or insect population control treatment

    10. classification within Insecta is an area of very active research and revisions; we won’t try to cover it extensively

    11. instead, learn about these selected, important insect orders – listed in descending order by number of named, living species

      1. Coleoptera – beetles (~350,000 living species) – very diverse; winged; hard forewings; holometabolous

      2. Diptera – flies (~150,000 living species) – winged; hind wings reduced to small halteres used only for steering and stability; holometabolous

      3. Hymenoptera – bees, wasps, ants (~125,000 living species) – winged, although some sterile castes lack wings; often highly social; holometabolous

      4. Lepidoptera – butterflies and moths (~120,000 living species) – winged; wings have scales; coiled proboscis for feeding; holometabolous

      5. Hemiptera – true bugs (~85,000 living species) – winged; many sucking insects like assassin bugs; hemimetabolous

      6. Orthoptera – grasshoppers, crickets, and their relatives (~13,000 living species) – winged; many with great leaping ability; hemimetabolous

      7. Odonata – dragonflies and damselflies (~5,000 living species) – two pairs of wings that cannot be folded flat onto back (ancient wing form); hemimetabolous

      8. Isoptera – termites (~2,000 living species) – winged, although some sterile castes lack wings; social; one of the few animals to eat and digest wood (although they do need some microbial help with that); hemimetabolous




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