Smaller Ecdysozoans

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Smaller Ecdysozoans

• Chapter 18

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Ecdysozoans

• Many protostomes possess a cuticle.
• Non-living outer layer secreted by epidermis.
• Cuticle restricts growth and must be molted via
  ecdysis.
• Regulation of molting achieved by the hormone
  ecdysone.

• Members of Ecdysozoa molt cuticle as they grow.

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Ecdysozoan Phyla

• Phylum Nematoda
• Phylum Nematomorpha
• Phylum Kinorhyncha
• Phylum Loricifera
• Phylum Priapulida
• All have a nonliving cuticle that is shed as the
  organism grows.

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Diversity

• Ecdysozoans do not share the same body plan.
• Members of Nematoda, Nematomorpha, and
  Kinorhyncha are pseudocoelomate.
• Members of Priapulida are presumed to be
  pseudocoelomate, but have not been carefully
  studied yet.
• Loricifera species may be pseudocoelomate or
  acoelomate.

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Phylum Nematoda Roundworms

• About 25,000 species of nematodes are described,
  but as many as half a million may exist.
• Many prefer the name Nemata for this phylum.
• Found in virtually all habitats in all biomes.
• Topsoil may contain billions per acre.

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Phylum Nematoda Roundworms

• Free-living nematodes feed on bacteria, yeasts,
  fungal hyphae, and algae.
• Predatory nematodes eat rotifers, tardigrades,
  small annelids, and other nematodes.
• Important as food for mites, insects, larvae, and
  fungi.

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Phylum Nematoda Roundworms

• Caenorhabditis elegans is an important model for
  studies of genomics and cell development and
  differentiation.

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Phylum Nematoda

• Some species of nematodes are important parasites
  of plants and animals.
• Nematode parasites exist in nearly all animal and
  plant species.

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Phylum Nematoda

• Triploblastic three embryonic germ layers.
• Pseudocoel used as hydrostatic skeleton.
• Cylindrical shape
• Lack of motile cilia or flagella
• Nonsegmented
• Nonliving cuticle

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Phylum Nematoda

• The cylindrical bodies of nematodes are covered
  by a tough coat called a cuticle.

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Phylum Nematoda

• Complete digestive system.
• Circulatory and respiratory systems are lacking.
• Ring of nerve tissue and ganglia around the
  pharynx lead to dorsal and ventral nerve cords.

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Phylum Nematoda

• Nematodes have separate sexes.
• Dioecious, females are larger.
• Fertilization is internal.
• Eggs are stored in uterus until deposited.
• Cuticle is shed between each of four juvenile
  stages.

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Phylum Nematoda Roundworms

• Sensory papillae at head and tail.
• Amphids, pair of sensory organs on head, lead
  into a deep cuticular pit with modified cilia.

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Phylum Nematoda

• Some larvae are free-living, some require and
  intermediate host.
• Parasitic nematodes of humans
• Intestinal roundworm (Ascaris)
• Hookworm
• Pinworm
• Trichina worm
• Whipworm

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Phylum Nematomorpha

• Phylum Nematomorpha, the horsehair worms, are
  free living as adults, and parasites in
  arthropods as juveniles.
• Currently placed as the sister taxon to nematodes
• About 320 species are known.
• Occur worldwide
• Pseudocoelomate
• Triploblastic

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Phylum Nematomorpha

• Horsehair worms leaving the body of a katydid
  found dead near a river in Illinois.
• Photos by Bryan Suson

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Phylum Nematomorpha

• Adults utilize stored nutrients.
• Recent studies reveal that they can absorb
  organic molecules through vestigial gut and body
  wall.
• Circulatory, respiratory and excretory systems
  are absent.
• Nematomorphs are dioecious.
• Females discharge eggs into water in long strings.

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Phylum Nematomorpha

• Juveniles may encyst on vegetation and are eaten
  by an arthropod such as a grasshopper.
• Larval stages have hooks that may be used to bore
  into a host.
• May also cause infection via drinking water.
• Larvae encyst within host.
• Marine nematomorphs infect hermit crabs and other
  crabs.

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Phylum Nematomorpha

• After months in an arthropod host, mature worm
  emerges into nearby water or during rainfall.
• Parasite stimulates terrestrial insects to seek
  water.

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Phylum Kinorhyncha

• Kinorhynchs are usually under 1 mm long.
• About 179 species are known.
• Found worldwide, from intertidal areas to 6000 m
  deep.
• Most live in mud, but some have been found in
  algae, sponges, and other invertebrates.

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Phylum Kinorhyncha

• Spines (scalids) function in locomotion,
  chemoreception, and mechanoreception.
• Retractile head has a circle of spines and a
  retractile proboscis (introvert).
• Body wall made of a cuticle and syncytial
  epidermis.

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Phylum Priapulida

• The phylum Priapulida contains 16 species of
  marine worms that occur in colder waters.
• Found from intertidal zones to deep ocean floors,
  several thousand meters deep.
• Some are tube dwellers and feed on detritus.

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Phylum Priapulida

• Cylindrical bodies under 15 cm long.
• Burrow by body contractions and orient mouth at
  the surface.
• Retractable introvert has papillae and rows of
  curved spines to sample and capture prey.
• Chitinous cuticle covers body and is molted
  regularly.
• Sexes are separate.

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Phylum Loricifera

• Nine circlets of scalids on the introvert,
  similar to those of kinorhynchs.
• Entire forepart can be retracted into the
  circular lorica.
• Diet is unknown, possibly feed on bacteria.
• Brain fills the head and nerves innervate
  scalids.
• Dioecious with dimorphic males and females.
• Body cavity is a pseudocoel.

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Clade Panarthropoda

• Clade Panarthropoda contains Arthropoda and two
  allied phyla, Onychophora and Tardigrada.
• In onychophorans and arthropods coelom develops
  by schizocoely, but enterocoelic in tardigrades.
• A new cavity, hemocoel, forms from fusion of the
  main coelomic cavity with the blastocoel.
• Blood from open circulatory system enters the
  hemocoel and surrounds the internal organs.
• Contains a muscular heart but limited muscular
  blood vessels.

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Phylum Onychophora

• About 70 living species of velvet worms in the
  phylum Onychophora.
• 1.4 to 15 cm in length.
• Limited to moist, leafy rain forest habitat in
  tropical and subtropical regions.
• Changed little over 500 million years.
• Fossil Aysheaia similar to modern forms.
• Share traits with annelids and arthropods and
  were considered a missing link.
• Most are predaceous, some live in termite nests.

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Phylum Onychophora

• No external segmentation except for paired
  appendages.
• Skin is soft - cuticle contains protein and
  chitin but does not harden as in arthropods.
• Body covered with tiny tubercles bearing sensory
  bristles.
• Minute scales on tubercles impart an iridescent
  and velvety appearance.
• Head has one pair of large antennae and an
  annelid-like eye at base.

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Phylum Onychophora

• 14 to 43 pairs of unjointed legs.
• Legs move by waves of body contractions.
• Slime glands on each side of body cavity open on
  oral papillae.
• Mouth leads to a straight digestive tract.
• Each segment contains a pair of nephridia, each
  containing a vesicle, ciliated funnel, and duct.

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Phylum Onychophora

• Tracheal system provides respiration to all parts
  of body.
• Cannot close spiracles, so they are restricted to
  moist habitats.
• Important differences suggest the tracheal system
  evolved independently from that of arthropods.

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Phylum Onychophora

• Open circulatory system.
• Nervous system organized much like a ladder.
• Sense organs include pigment cup ocelli, taste
  spines, tactile papillae, and hygroscopic
  receptors that orient the animal toward water
  vapor.

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Phylum Onychophora

• With exception of one parthenogenetic species,
  all are dioecious with paired reproductive
  organs.
• Males deposit spermatophores on back of female.
• White blood cells dissolve the skin and sperm
  migrate to ovaries.
• May be oviparous, ovoviviparous, or viviparous.

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Phylum Tardigrada

• Tardigrades, or water bears, are less than a
  millimeter in length.
• Freshwater or marine
• Live in spaces between sand grains.

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related
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33
Phylum Tardigrada

• Trunk bears four pairs of short, stubby,
  unjointed legs, each with four to eight claws.
• Body covered by non-chitinous cuticle that is
  molted four or more times during lifetime.
• Mouth leads to a muscular pharynx adapted for
  sucking.
• Two stylets protrude to pierce integument of
  nematodes or walls of plant cells and allow them
  to suck juices.

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Phylum Tardigrada

• They share many characteristics with arthropods.
• But legs are unjointed.
• Non-chitinous cuticle that is molted.

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Phylum Tardigrada

• No circulatory or respiratory systems.
• Gas exchange occurs at body surface.
• Muscles are all longitudinal.
• Uses hydrostatic pressure as a skeleton.
• Brain is relatively large.

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Phylum Tardigrada

• Sexes are separate.
• In parthenogenetic freshwater and moss-dwelling
  species, males are unknown.
• Egg-laying, like defecation, occurs at molting.
• Eggs may be highly ornate.

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Phylum Tardigrada

• Tardigrades can enter a state called
  cryptobiosis, where metabolism is imperceptible.
• Tardigrades can dehydrate from 85 water to only
  3 water.
• In this state they can resist extreme
  temperatures, ionizing radiation, oxygen
  deficiency, etc. for years.
• When water is available, they become
  metabolically active again.

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Phylogeny

• Evolutionary relationships among ecdysozoans are
  not well-understood.
• Members of this clade do not share a common
  cleavage pattern
• Nematodes and nematomorphs - cleavage is unique,
  not spiral or radial.
• Cleavage in kinorhynchs, lorificiferans, and
  tardigrades has yet to be studied.
• Priapulid cleavage is nearly radial.

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Phylogeny

• Recent phylogenies place Nematoda and
  Nematomorpha as sister taxa since they share a
  collagenous cuticle.
• Phylum Kinorhyncha and Priapulida are considered
  sister taxa because they share a two-layered
  pharynx.
• Velvet worms, phylum Onychophora, are the sister
  taxon to tardigrades.

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Phylogeny

• Onychophorans share a number of characteristics
  with annelids
• Metamerically arranged nephridia, muscular body
  wall, pigment cup ocelli, and ciliated
  reproductive ducts.
• Onychophorans also share features with the
  arthropods
• Including a cuticle, tubular heart, and hemocoel
  with open circulatory system, presence of
  tracheae (possibly not homologous), and large
  brain.

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Phylogeny

• Sequence analysis supports placement of
  Onychophora in clade Panarthropoda.
• Tardigrades and arthropods also share
  arthropod-type setae and muscles inserted on the
  cuticle.

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Adaptive Diversification

• Nematodes show impressive adaptation they are
  found in almost every habitat available to
  animals.
• Body structure is plastic enough to allow
  adaptation.
• Life cycle ranges from simple to complex.
• Have been known to survive in suboptimal
  conditions.