Smaller Ecdysozoans
Chapter 18
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.
Ecdysozoan Phyla
 Phylum Nematoda
 Phylum Nematomorpha
 Phylum Kinorhyncha
 Phylum Loricifera
 Phylum Priapulida
 All have a nonliving cuticle that is shed as the organism
grows.
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.
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.
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.
Phylum Nematoda: Roundworms
 Caenorhabditis
elegans is an
important model for
studies of genomics
and cell
development and
differentiation.
Phylum Nematoda
 Some species
of nematodes
are important
parasites of
plants and
animals.
 Nematode
parasites exist
in nearly all
animal and
plant species.
Phylum Nematoda
 Triploblastic – three embryonic germ layers.
 Pseudocoel – used as hydrostatic skeleton.
 Cylindrical shape
 Lack of motile cilia or flagella
 Nonsegmented
 Nonliving cuticle
Phylum Nematoda
 The cylindrical bodies of nematodes are
covered by a tough coat called a cuticle.
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.
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.
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.
Phylum Nematoda
 Some larvae are freeliving, some require and
intermediate host.
 Parasitic nematodes of
humans:
 Intestinal roundworm
(Ascaris)
 Hookworm
 Pinworm
 Trichina worm
 Whipworm
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
Phylum Nematomorpha
 Horsehair worms leaving the
body of a katydid found dead
near a river in Illinois.
Photos by Bryan Suson
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.
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.
Phylum Nematomorpha
 After months in an arthropod host, mature
worm emerges into nearby water or during
rainfall.
 Parasite stimulates terrestrial insects to seek
water.
Phylum Kinorhyncha
 Kinorhynchs are usually <1
mm long.
 Most live in mud, but some have
been found in algae, sponges,
and other invertebrates.
 Spines (scalids) function in
locomotion, chemoreception,
and mechanoreception.
 Retractile head has a circle of
spines and a retractile proboscis
(introvert).
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.
 Retractable introvert
has papillae and rows of
curved spines to sample
and capture prey.
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.
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.
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.
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.
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.
https://youtu.be/mrL2A7my1fc
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.
 Sense organs include pigment cup ocelli, taste spines,
tactile papillae, and hygroscopic receptors that orient
the animal toward water vapor.
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.
Phylum Tardigrada
 Tardigrades, or
water bears, are
less than a
millimeter in
length.
 Freshwater or
marine
 Live in spaces
between sand
grains.
http://www.youtube.com/watch?v=W9WWysGBAlU&feature=related
http://www.youtube.com/watch?v=blX7Ef1gxcM&feature=related
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.
Phylum Tardigrada
 They share many characteristics with
arthropods.
 But legs are unjointed.
 Non-chitinous cuticle that is molted.
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.
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.
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.
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.
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.
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.
Phylogeny
 Sequence analysis supports placement of
Onychophora in clade Panarthropoda.
 Tardigrades and arthropods also share
arthropod-type setae and muscles inserted on
the cuticle.
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.