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Chapter 33. Invertebrates. Overview: Life Without a Backbone. Invertebrates are animals that lack a backbone They account for 95% of known animal species. LE 33-2. Cnidaria. Echinodermata. Chordata. Porifera. Other bilaterians (including Nematoda, Arthropoda, Mollusca, and Annelida).

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Chapter 33

Chapter 33

Invertebrates


Overview life without a backbone
Overview: Life Without a Backbone

  • Invertebrates are animals that lack a backbone

  • They account for 95% of known animal species


Le 33 2

LE 33-2

Cnidaria

Echinodermata

Chordata

Porifera

Other bilaterians (including

Nematoda, Arthropoda,

Mollusca, and Annelida)

Deuterostomia

Bilateria

Eumetazoa

Ancestral colonial

choanoflagellate


Le 33 3a

LE 33-3a

A jelly

A sponge

0.5 mm

250 µm

A placozoan (LM)

A kinorhynch (LM)

A rotifer (LM)

A marine flatworm

Ectoprocts

Phoronids


Le 33 3b

LE 33-3b

A ribbon worm

A brachiopod

5 mm

A ctenophore, or comb jelly

An acanthocephalan

An octopus

A marine annelid

50 µm

A priapulan

A loriciferan (LM)


Le 33 3c

LE 33-3c

A roundworm

A scorpion

(an arachid)

100 µm

100 µm

Tardigrades (colorized SEM)

A cycliophoran (colorized SEM)

An acorn worm

An onychophoran

A sea urchin

A tunicate


Concept 33 1 sponges are sessile and have a porous body and choanocytes
Concept 33.1: Sponges are sessile and have a porous body and choanocytes

  • Sponges, phylum Porifera, live in both fresh and marine waters

  • Sponges lack true tissues and organs



Le 33 4

LE 33-4 suspended in the water that passes through their body

Food particles

in mucus

Flagellum

Choanocyte

Choanocytes

Collar

Osculum

Azure vase sponge

(Callyspongia plicifera)

Spongocoel

Phagocytosis of

food particles

Amoebocyte

Porocytes

Spicules

Epidermis

Water

flow

Amoebocyte

Mesohyl



Concept 33 2 cnidarians have radial symmetry a gastrovascular cavity and cnidocytes
Concept 33.2: Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes

  • All animals except sponges belong to the clade Eumetazoa, animals with true tissues

  • Phylum Cnidaria is one of the oldest groups in this clade



Video: Hydra Eating Daphnia (time lapse)


Le 33 5

LE 33-5 digestive compartment, the gastrovascular cavity

Mouth/anus

Tentacle

Polyp

Medusa

Gastrovascular

cavity

Gastrodermis

Mesoglea

Body

stalk

Epidermis

Tentacle

Mouth/anus


Video: Hydra Budding

Video: Hydra Releasing Sperm


Le 33 6

LE 33-6 digestive compartment, the gastrovascular cavity

Prey

Tentacle

“Trigger”

Discharge

of thread

Nematocyst

Coiled thread

Cnidocyte



Le 33 7

LE 33-7 digestive compartment, the gastrovascular cavity

Scyphozoans (jellies)

Cubozoan (sea wasp)

Anthozoan

(sea anemone)

Hydrozoans


Video: Jelly Swimming digestive compartment, the gastrovascular cavity

Video: Thimble Jellies

Video: Clownfish and Anemone

Video: Coral Reef


Hydrozoans
Hydrozoans digestive compartment, the gastrovascular cavity

  • Most hydrozoans alternate between polyp and medusa forms


Le 33 8 3

LE 33-8–3 digestive compartment, the gastrovascular cavity

Reproductive

polyp

Feeding

polyp

Medusa

bud

MEIOSIS

Gonad

Medusa

Sperm

Egg

SEXUAL

REPRODUCTION

ASEXUAL

REPRODUCTION

(BUDDING)

Portion of

a colony

of polyps

FERTILIZATION

Zygote

Developing

polyp

Mature

polyp

Planula

(larva)

Key

Haploid (n)

1 mm

Diploid (2n)


Scyphozoans
Scyphozoans digestive compartment, the gastrovascular cavity

  • In the class Scyphozoa, jellies (medusae) are the prevalent form of the life cycle


Cubozoans
Cubozoans digestive compartment, the gastrovascular cavity

  • In the class Cubozoa, which includes box jellies and sea wasps, the medusa is box-shaped and has complex eyes


Anthozoans
Anthozoans digestive compartment, the gastrovascular cavity

  • Class Anthozoa includes the corals and sea anemones, which occur only as polyps


Concept 33 3 most animals have bilateral symmetry
Concept 33.3: Most animals have bilateral symmetry digestive compartment, the gastrovascular cavity

  • The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development


Flatworms
Flatworms digestive compartment, the gastrovascular cavity

  • Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats

  • They are flattened dorsoventrally and have a gastrovascular cavity

  • Although flatworms undergo triploblastic development, they are acoelomates


  • Flatworms are divided into four classes: digestive compartment, the gastrovascular cavity

    • Turbellaria (mostly free-living flatworms)

    • Monogenea (monogeneans)

    • Trematoda (trematodes, or flukes)

    • Cestoda (tapworms)


Turbellarians
Turbellarians digestive compartment, the gastrovascular cavity

  • Turbellarians are nearly all free-living and mostly marine

  • The best-known turbellarians are commonly called planarians



Le 33 10

LE 33-10 nerve nets

Pharynx

Gastrovascular

cavity

Eyespots

Ganglia

Ventral nerve cords


Monogeneans and trematodes
Monogeneans and Trematodes nerve nets

  • Monogeneans and trematodes live as parasites in or on other animals

  • They parasitize a wide range of hosts

  • Trematodes that parasitize humans spend part of their lives in snail hosts


Le 33 11

LE 33-11 nerve nets

Mature flukes live in the blood vessels

of the human intestine

Male

Female

1 mm

Larvae penetrate skin and blood vessels of humans.

Blood flukes reproduce sexually in the human host. Fertilized eggs exit host in feces.

Eggs develop in water into ciliated larvae. Larvae infect snails.

Asexual reproduction within snail results in another type of motile larva.

Snail host



Tapeworms
Tapeworms nerve nets

  • Tapeworms are also parasitic and lack a digestive system


Le 33 12

LE 33-12 nerve nets

Proglottids with

reproductive structures

200 µm

Scolex

Hooks

Sucker


Rotifers
Rotifers nerve nets

  • Rotifers, phylum Rotifera, are tiny animals that inhabit fresh water, the ocean, and damp soil

  • Rotifers are smaller than many protists but are truly multicellular and have specialized organ systems

Video: Rotifer


Le 33 13

LE 33-13 nerve nets

0.1 mm



Lophophorates ectoprocts phoronids and brachiopods
Lophophorates: Ectoprocts, Phoronids, and Brachiopods separate mouth and anus that lies within a fluid-filled pseudocoelom

  • Lophophorates have a lophophore, a horseshoe-shaped, suspension-feeding organ with ciliated tentacles



Le 33 14

LE 33-14 plants

Lophophore

Lophophore

Ectoprocts

Phoronids

Brachiopods




Nemerteans
Nemerteans hinge-shelled molluscs, but the two halves of the shell are dorsal and ventral rather than lateral, as in clams

  • Members of phylum Nemertea are commonly called proboscis worms or ribbon worms



Concept 33 4 molluscs have a muscular foot a visceral mass and a mantle
Concept 33.4: Molluscs have a muscular foot, a visceral mass, and a mantle

  • Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids

  • Most molluscs are marine, though some inhabit fresh water and some are terrestrial

  • Molluscs are soft-bodied animals, but most are protected by a hard shell



Le 33 16

LE 33-16 mass, and a mantle

Visceral mass

Coelom

Heart

Intestine

Nephridium

Gonads

Mantle

Stomach

Radula

Mantle

cavity

Shell

Mouth

Radula

Anus

Gill

Mouth

Nerve

cords

Foot

Esophagus



  • There are four major classes of molluscs: visceral mass

    • Polyplacophora (chitons)

    • Gastropoda (snails and slugs)

    • Bivalvia (clams, oysters, and other bivalves)

    • Cephalopoda (squids, octopuses, cuttlefish, and chambered nautiluses)


Chitons
Chitons visceral mass

  • Class Polyplacophora consists of the chitons, oval-shaped marine animals encased in an armor of eight dorsal plates


Gastropods
Gastropods visceral mass

  • About three-quarters of all living species of molluscs belong to class Gastropoda

Video: Nudibranchs


Le 33 18

LE 33-18 visceral mass

A land snail

A sea slug. Nudibranchs, or sea slugs,

lost their shell during their evolution.


  • Most gastropods are marine, but many are freshwater and terrestrial species

  • Most have a single, spiraled shell

  • Slugs lack a shell or have a reduced shell

  • The most distinctive characteristic of gastropods is torsion, which causes the animal’s anus and mantle to end up above its head


Le 33 19

LE 33-19 terrestrial species

Stomach

Intestine

Mantle

cavity

Anus

Mouth


Bivalves
Bivalves terrestrial species

  • Molluscs of class Bivalvia include many species of clams, oysters, mussels, and scallops

  • They have a shell divided into two halves



Le 33 21

LE 33-21 for feeding as well as gas exchange

Coelom

Hinge area

Mantle

Gut

Heart

Adductor

muscle

Shell

Anus

Mouth

Excurrent

siphon

Palp

Water

flow

Foot

Incurrent

siphon

Mantle

cavity

Gill


Cephalopods
Cephalopods for feeding as well as gas exchange

  • Class Cephalopoda includes squids and octopuses, carnivores with beak-like jaws surrounded by tentacles of their modified foot



Le 33 22a

LE 33-22a for feeding as well as gas exchange

Octopuses are considered among the most intelligent invertebrates.



Le 33 22b

LE 33-22b them to swim very quickly

Squids are speedy carnivores with beaklike jaws and well-developed eyes.



Le 33 22c

LE 33-22c survives today

Chambered nautiluses are the only living cephalopods with an external shell.


Concept 33 5 annelids are segmented worms
Concept 33.5: Annelids are segmented worms survives today

  • Annelids have bodies composed of a series of fused rings

  • The phylum Annelida is divided into three classes:

    • Oligochaeta (earthworms and their relatives)

    • Polychaeta (polychaetes)

    • Hirudinea (leeches)


Oligochaetes
Oligochaetes survives today

  • Oligochaetes (class Oligochaeta) are named for relatively sparse chaetae, bristles made of chitin

  • They include the earthworms and a variety of aquatic species

  • Earthworms eat through soil, extracting nutrients as the soil moves through the alimentary canal

Video: Earthworm Locomotion


Le 33 23

LE 33-23 survives today

Cuticle

Coelom

Epidermis

Septum (partition

between segments)

Circular muscle

Longitudinal muscle

Metanephridium

Anus

Dorsal vessel

Chaetae

Intestine

Ventral vessel

Nerve

cords

Nephrostome

Metanephridium

Clitellum

Esophagus

Crop

Pharynx

Intestine

Giant Australian earthworm

Cerebral ganglia

Gizzard

Mouth

Ventral nerve cords

with segmental

ganglia

Subpharyngeal

ganglion

Circulatory

system


Polychaetes
Polychaetes survives today

  • Members of class Polychaetes have paddlelike parapodia that work as gills and aid in locomotion

Video: Tubeworms


Le 33 24

LE 33-24 survives today

Parapodia


Leeches
Leeches survives today

  • Members of class Hirudinea are blood-sucking parasites, such as leeches


Concept 33 6 nematodes are nonsegmented pseudocoelomates covered by a tough cuticle
Concept 33.6: Nematodes are nonsegmented pseudocoelomates covered by a tough cuticle

  • Nematodes, or roundworms, are found in most aquatic habitats, in the soil, in moist tissues of plants, and in body fluids and tissues of animals

  • The cylindrical bodies of nematodes (phylum Nematoda) are covered by a tough coat called a cuticle

Video: C. elegans Crawling

Video: C. elegans Embryo Development (time lapse)


Le 33 26

LE 33-26 covered by a tough cuticle

25 µm



Le 33 27

LE 33-27 and animals

Muscle tissue

Encysted juveniles

50 µm


Concept 33 7 arthropods are segmented coelomates that have an exoskeleton and jointed appendages
Concept 33.7: Arthropods are segmented coelomates that have an exoskeleton and jointed appendages

  • Two out of every three known species of animals are arthropods

  • Members of the phylum Arthropoda are found in nearly all habitats of the biosphere


General characteristics of arthropods
General Characteristics of Arthropods an exoskeleton and jointed appendages

  • The diversity and success of arthropods are largely related to their segmentation, hard exoskeleton, and jointed appendages



Video: Lobster Mouth Parts


Le 33 29

LE 33-29 appendages became more specialized

Cephalothorax

Abdomen

Thorax

Head

Antennae

(sensory

reception)

Swimming

appendages

(two sets

located under

abdomen

Walking legs

Pincer (defense)

Mouthparts (feeding)




  • Molecular evidence suggests that living arthropods consist of four major lineages that diverged early in the phylum’s evolution:

    • Cheliceriforms (sea spiders, horseshoe crabs, scorpions, ticks, mites, and spiders)

    • Myriapods (centipedes and millipedes)

    • Hexapods (insects and relatives)

    • Crustaceans (crabs, lobsters, shrimps, barnacles, and many others)


Cheliceriforms
Cheliceriforms of four major lineages that diverged early in the phylum’s evolution:

  • Cheliceriforms, subphylum Cheliceriformes, are named for clawlike feeding appendages called chelicerae

  • Most marine cheliceriforms are extinct, but some species survive today, including horseshoe crabs



Le 33 31a

LE 33-31a spiders, scorpions, ticks, and mites

Scorpions have pedipalps that are pincers specialized for defense and the capture of food. The tip of the tail bears a poisonous stinger.


Le 33 31b

LE 33-31b spiders, scorpions, ticks, and mites

50 µm

Dust mites are ubiquitous scavengers in human dwellings but are harmless except to those people who are allergic to them (colorized SEM).


Le 33 31c

LE 33-31c spiders, scorpions, ticks, and mites

Web-building spiders are generally most active during the daytime.



Le 33 32

LE 33-32 pairs of appendages, the most anterior of which are the chelicerae

Stomach

Intestine

Brain

Heart

Digestive

gland

Eyes

Ovary

Poison

gland

Anus

Book lung

Gonopore

(exit for eggs)

Spinnerets

Pedipalp

Chelicera

Sperm

receptacle

Silk gland


Myriapods
Myriapods pairs of appendages, the most anterior of which are the chelicerae

  • Subphylum Myriapoda includes millipedes and centipedes

  • Millipedes, class Diplopoda, have many legs

  • Each trunk segment has two pairs of legs



Insects
Insects mandibles

  • Subphylum Hexapoda, insects and relatives, has more species than all other forms of life combined

  • They live in almost every terrestrial habitat and in fresh water

  • The internal anatomy of an insect includes several complex organ systems


Le 33 35

LE 33-35 mandibles

Thorax

Abdomen

Head

Compound eye

Antennae

Dorsal

artery

Crop

Cerebral ganglion

Heart

Anus

Vagina

Malpighian tubules

Ovary

Tracheal tubes

Mouthparts

Nerve cords




Video: Butterfly Emerging


Le 33 36

LE 33-36 by such names as maggot, grub, or caterpillar

Larva (caterpillar)

Pupa

Pupa

Emerging adult

Adult


Video: Bee Pollinating


Le 33 37aa

LE 33-37aA by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Blattodea

4,000

Cockroaches have a dorsoventrally flattened body, with legs modified for rapid running. Forewings, when present, are leathery, whereas hind wings are fanlike. Fewer than 40 cockroach species live in houses; the rest exploit habitats ranging from tropical forest floors to caves and deserts.

German

cockroach


Le 33 37ba

LE 33-37bA by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Beetles comprise the most species-rich order of insects. They have two pairs of wings, one of which is thick and leathery, the other membranous. They have an armored exoskeleton and mouthparts adapted for biting and chewing. Beetles undergo complete metamorphosis.

Coleoptera

350,000

Japanese

beetle


Le 33 37ca

LE 33-37cA by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Earwigs are generally nocturnal scavengers. While some species are wingless, others have two pairs of wings, one of which is thick and leathery, the other membranous. Earwigs have biting mouthparts and large posterior pincers. They undergo incomplete metamorphosis.

Dermaptera

1,200

Earwig


Le 33 37da

LE 33-37dA by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Dipterans have one pair of wings; the second pair has become modified into balancing organs called halteres. Their head is large and mobile; their mouthparts are adapted for sucking, piercing, or lapping. Dipterans undergo complete metamorphosis. Flies and mosquitoes are among the best-known dipterans, which live as scavengers, predetors, and parasites.

Diptera

151,000

Horsefly


Le 33 37ea

LE 33-37eA by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Hemipterans are so-called “true bugs,” including bed bugs, assassin bugs, and chinch bugs. (Insects in other orders are sometimes erroneously called bugs.) Hemipterans have two pairs of wings, one pair partly leathery, the other membranous. They have piercing or sucking mouthparts and undergo incomplete metamorphosis.

Hemiptera

85,000

Leaf-

footed

bug


Le 33 37fa

LE 33-37fA by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Hymenoptera

Ants, bees, and wasps are generally highly social insects. They have two pairs of membranous wings, a mobile head, and chewing or sucking mouthparts. The females of many species have a posterior stinging organ. Hymenopterans undergo complete metamorphosis.

125,000

Cicada-killer wasp


Le 33 37ga

LE 33-37gA by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

2,000

Termites are widespread social insects that produce enormous colonies. It had been estimated that there are 700 kg of termites for every person on Earth! Some termites have two pairs of membranous wings, while others are wingless. They feed on wood with the aid of microbial symbionts carried in specialized chambers in their hindgut.

Isoptera

Termite


Le 33 37ab

LE 33-37aB by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Butterflies and moths are among the best-known insects. They have two pairs of wings covered with tiny scales. To feed, they uncoil a long proboscis. Most feed on nectar, but some species feed on other substances, including animal blood or tears.

Lepidoptera

120,000

Swallowtail

butterfly


Le 33 37bb

LE 33-37bB by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Dragonflies and damselflies have two pairs of large, membranous wings. They have an elongated abdomen, large, compound eyes, and chewing mouthparts. They undergo incomplete metamorphosis and are active predators.

Odonata

5,000

Dragonfly


Le 33 37cb

LE 33-37cB by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

ORDER

EXAMPLES

Grasshoppers, crickets, and their relatives are mostly herbivorous. They have large hind legs adapted for jumping, two pairs of wings (one leathery, one membranous), and biting or chewing mouthparts. Males commonly make courtship sounds by rubbing together body parts, such as a ridge of their hind leg. Orthopterans undergo incomplete metamorphosis.

Orthoptera

13,000

Katydid


Le 33 37db

LE 33-37dB by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Stick insects and leaf insects are exquisite mimics of plants. The eggs of some species even mimic seeds of the plants on which the insects live. Their body is cylindrical or flattened dorsoventrally. They lack forewings but have fanlike hind wings. Their mouthparts are adapted for biting or chewing.

Phasmida

2,600

Stick insect


Le 33 37eb

LE 33-37eB by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Phthiraptera

Commonly called sucking lice, these insects spend their entire life as an ectoparasite feeding on the hair or feathers of a single host. Their legs, equipped with clawlike tarsi, are adapted for clinging to their hosts. They lack wings and have reduced eyes. Sucking lice undergo incomplete metamorphosis.

2,400

Human

body

louse


Le 33 37fb

LE 33-37fB by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Fleas are bloodsucking ectoparasites on birds and mammals. Their body is wingless and laterally compressed. Their legs are modified for clinging to their hosts and for long-distance jumping. They undergo complete metamorphosis.

Siphonaptera

2,400

Flea


Le 33 37gb

LE 33-37gB by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Silverfish are small, wingless insects with a flattened body and reduced eyes. They live in leaf litter or under bark. They can also infest buildings, where they can become pests.

Thysanura

450

Silverfish


Le 33 37hb

LE 33-37hB by such names as maggot, grub, or caterpillar

APPROXIMATE

NUMBER OF

SPECIES

MAIN CHARACTERISTICS

EXAMPLES

ORDER

Trichoptera

The larvae of caddisflies live in streams, where they make houses from sand grains, wood fragments, or other material held together by silk, Adults have two pairs of hairy wings and chewing or lapping mouthparts. They undergo complete metamorphosis.

7,100

Caddisfly


Crustaceans
Crustaceans by such names as maggot, grub, or caterpillar

  • While arachnids and insects thrive on land, crustaceans, for the most part, have remained in marine and freshwater environments

  • Crustaceans, subphylum Crustacea, typically have branched appendages that are extensively specialized for feeding and locomotion





Concept 33 8 echinoderms and chordates are deuterostomes
Concept 33.8: Echinoderms and chordates are deuterostomes which are among the most numerous of all animals

  • Sea stars and other echinoderms, phylum Echinodermata, may seem to have little in common with phylum Chordata, which includes the vertebrates



Echinoderms
Echinoderms deuterostomes:

  • Sea stars and most other echinoderms are slow-moving or sessile marine animals

  • A thin, bumpy or spiny skin covers an endoskeleton of hard calcareous plates

  • Unique to echinoderms is a water vascular system, a network of hydraulic canals branching into tube feet that function in locomotion, feeding, and gas exchange

Video: Echinoderm Tube Feet


Le 33 39

LE 33-39 deuterostomes:

Spine

Stomach

Anus

Gills

Central disk

Madreporite

Digestive glands

Radial

nerve

Ring

canal

Gonads

Ampulla

Podium

Radial canal

Tube

feet


  • Radial anatomy of many echinoderms evolved from the bilateral symmetry of ancestors

  • Living echinoderms are divided into six classes:

    • Asteroidia (sea stars)

    • Ophiuroidea (brittle stars)

    • Echinoidea (sea urchins and sand dollars)

    • Crinoidea (sea lilies and feather stars)

    • Holothuroidea (sea cucumbers)

    • Concentricycloidea (sea daisies)


Sea stars
Sea Stars bilateral symmetry of ancestors

  • Sea stars, class Asteroidea, have multiple arms radiating from a central disk

  • The undersurfaces of the arms bear tube feet, each of which can act like a suction disk


Le 33 40a

LE 33-40a bilateral symmetry of ancestors

A sea star (class Asteroidea)


Brittle stars
Brittle Stars bilateral symmetry of ancestors

  • Brittle stars have a distinct central disk and long, flexible arms


Le 33 40b

LE 33-40b bilateral symmetry of ancestors

A brittle star (class Ophiuroidea)


Sea urchins and sand dollars
Sea Urchins and Sand Dollars bilateral symmetry of ancestors

  • Sea urchins and sand dollars have no arms but have five rows of tube feet


Le 33 40c

LE 33-40c bilateral symmetry of ancestors

A sea urchin (class Echinoidea)


Sea lilies and feather stars
Sea Lilies and Feather Stars bilateral symmetry of ancestors

  • Sea lilies live attached to the substrate by a stalk

  • Feather stars crawl using long, flexible arms


Le 33 40d

LE 33-40d bilateral symmetry of ancestors

A feather star (class Crinoidea)


Sea cucumbers
Sea Cucumbers bilateral symmetry of ancestors

  • Sea cucumbers do not look much like other echinoderms

  • They lack spines, and their endoskeleton is much reduced


Le 33 40e

LE 33-40e bilateral symmetry of ancestors

A sea cucumber (class Holothuroidea)


Sea daisies
Sea Daisies bilateral symmetry of ancestors

  • Sea daisies were discovered in 1986, and only two species are known


Le 33 40f

LE 33-40f bilateral symmetry of ancestors

A sea daisy (class Concentricycloidea)


Chordates
Chordates bilateral symmetry of ancestors

  • Phylum Chordata consists of two subphyla of invertebrates as well as hagfishes and vertebrates

  • Chordates share many features of embryonic development with echinoderms


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