Animal Phyla -The Invertebrates
25.1 Animal Traits and Body Plans
 Animals
• Multicelled heterotrophs that move about during
part or all of the life cycle
• Body cells do not have a wall and are typically
diploid
• The overwhelming majority are invertebrates
Animal Body Plans: Organization
 Tissues
• Cells of a particular type and function, organized
in a specific pattern
 Tissue formation begins in an embryo
• Ectoderm and endoderm
• Mesoderm
Tissue Formation
 Formation of a three-layer animal embryo
Animal Body Plans: Body Symmetry
 Body Symmetry
• Simplest animals are asymmetrical (sponges)
• Jellyfish and hydras have radial symmetry
• Most animals have bilateral symmetry
 Cephalization
• In most bilateral animals, nerve cells are
concentrated at the head end
Body Symmetry
Animal Body Plans: Gut and Body Cavity
 Gut
• Digestive sac (incomplete digestive system) or
tube (complete) that opens at the body surface
 Typically, a body cavity surrounds the gut
• Coelom: Cavity lined by mesodermal tissue
• Pseudocoel: Cavity is partially lined
 Acoelomates have no body cavity
Body Cavities
Introducing the Animals
 Animals are multicelled heterotrophs that
actively move about during all or part of the life
cycle
 Early animals were small and structurally simple
 Their descendants evolved a more complex
structure and greater integration among
specialized parts
The Sponges
 Sponges are simple but successful; they have
survived in seas since Precambrian times
 Sponges (phylum Porifera)
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Attach to seafloor or other surfaces
No symmetry, tissues, or organs
Pores with flagellated collar cells filter water
Sexual or asexual reproduction
Sponge Body Plan
Sponge Reproduction and Dispersal
 Hermaphrodite
• Individual that produces both eggs and sperm
• Sperm are released into water; eggs are retained
• Zygote develops into ciliated larva
 Larva
• Free-living, sexually immature stage in life cycle
• Settles and develops into adult
Cnidarians—True Tissues
 Cnidarians (phylum Cnidaria)
• Radial animals with two tissue layers
• Medusae (jellyfishes) are bell shaped and drift
• Polyps (sea anemones) are tubular with one end
usually attached to a surface
 Four classes: hydrozoans, anthozoans,
cubozoans, and scyphozoans
Two Cnidarian Body Plans
General Cnidarian Features
 Nematocysts
• Stinging organelles in tentacle cells, triggered by
contact, used in feeding or defense
 Nerve net
• Simple nervous system of interconnecting nerve
cells extending through the tissues
 Hydrostatic skeleton
• Fluid-filled structure moved by contractile cells
25.3-25.5 Key Concepts
The Structurally Simple Invertebrates
 Placozoans and sponges have no body
symmetry or tissues
 The radially symmetrical cnidarians such as
jellyfish have two tissue layers and unique
stinging cells used in feeding and in defense
Flatworms—Simple Organ Systems
 Flatworms (phylum Platyhelminthes) have a
three-layer embryo that develops into an adult
with many organ systems but no coelom
 Three main classes: turbellarians, flukes
(trematodes), and tapeworms (cestodes)
Turbellarians:
Structure of a Free-Living Flatworm
 Pharynx
• Muscular tube connecting the mouth with the gut
 Nerve cords
• Two lines of communication along length of body
 Ganglia
• Cluster of nerve cell bodies (simple brain)
Flatworm Organ Systems
Parasites: Flukes and Tapeworms
 In blood flukes (Schistosoma), reproduction
takes place in mammals – immature stages live
in intermediate hosts (snails)
 A tapeworm body consists of proglottids –
repeating hermaphroditic body units that bud
from a region behind the scolex
25.7 Annelids—Segmented Worms
 Annelids (phylum Annelida) are bilateral worms
with a coelom and a segmented body; typically
with chaetae (chitin reinforced bristles)
 Three main groups: marine worms (polychaetes),
oligochaetes (including earthworms), and leeches
Leeches – Bloodsuckers and Others
 Leeches lack chaetae and have a sucker at
either end
Oligochaetes
 Example: earthworms
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Exchange gases across body surfaces
Have five hearts and a closed circulatory system
Nephridia regulate coelomic fluid
Nervous system of ganglia and nerve cords
Hydrostatic skeleton
Hermaphroditic
Earthworm Body Plan
Mollusks—Animals With a Mantle
 Mollusks (phylum Mollusca)
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Bilaterally symmetrical with a reduced coelom
Mantle covers internal organs, secretes a shell
Feed using a hard radula
Have a complete digestive tract
Gills for respiration in aquatic species
Mollusk Diversity
 Chitons
• Eight overlapping plates
 Gastropods (snails, slugs)
• Undergo torsion during development
 Bivalves (mussels, clams, oysters)
• Hinged, two-part shell
 Cephalopods (squids, octopuses)
• Large, fast and smart; closed circulatory system
Mollusk Groups
Bivalve Body Plan: Clam
Cephalopods—Fast and Brainy
 Cephalopod (“head foot”)
• Tentacles attached to the head are evolutionary
modifications of the foot; they surround the
mouth, which has a hard, horny beak
 Include the fastest (squids), biggest (giant
squid), and smartest (octopuses) invertebrates
• Jet propulsion, complex eyes, closed circulatory
system, complex behavior
Fig. 25-26d, p. 418
25.10 Rotifers and Tardigrades—
Tiny and Tough
 Rotifers (phylum Rotifera) and tardigrades
(phylum Tardigrada) are tiny bilateral animals
 Rotifers have a pseudocoelom, but are
genetically closest to annelids and mollusks
 Tardigrades have a coelom and molt, and are
probably relatives of roundworms and insects
Rotifer Body Plan
25.11 Roundworms—
Unsegmented Worms That Molt
 Roundworms (phylum Nematoda) are
unsegmented, pseudocoelomate worms with a
secreted cuticle that is molted
 Most are decomposers, some are parasites
• Parasitic roundworms include Trichinella, Ascaris,
hookworms, Wuchereria, and pinworms
Roundworm Body Plan
Roundworms
25.12 Arthropods—
Animals With Jointed Legs
 Arthropods (phylum Arthropoda) are the most
diverse animal phylum – with more than a million
species
• Trilobites are an extinct group
• Modern arthropods include horseshoe crabs,
spiders, ticks, crabs, lobsters, centipedes, and
insects
Living Arthropod Subgroups
Key Arthropod Adaptations
 A hardened exoskeleton (cuticle)
• Hormones control molting
 Jointed appendages for movement
• Some legs are modified for special tasks
 Highly modified segments
• Example: wings
Key Arthropod Adaptations
 Sensory specializations
• Compound eyes
• Antennae that detect touch and chemicals
 Specialized developmental stages
• Body plans may change by metamorphosis
• Example: Caterpillar and butterfly
Key Arthropod Adaptations
25.13 Chelicerates—
Spiders and Their Relatives
 Chelicerates are arthropods without antennae
• Marine chelicerates include the oldest living
arthropod lineage (horseshoe crabs)
• All land chelicerates are arachnids, including
spiders, scorpions, ticks, and mites
Chelicerates
Fig. 25-32d, p. 422
Chelicerates: The Spiders
 Spiders bite with fanglike chelicerae that deliver
venom from poison glands
 Paired spinners in the abdomen eject silk
 Open circulatory system mingles blood with
tissue fluids; Malpighian tubules move excess
water and wastes to gut for disposal
Body Plan of a Spider
25.14 The Mostly Marine Crustaceans
 Crustaceans are mostly marine arthropods with
two pairs of antennae
• Small crustaceans include krill, copepods, and
barnacles
• Decapod crustaceans include lobsters, crayfish,
crabs and shrimps
Crustaceans
Body Plan of a Lobster
25.15 Myriapods—Lots of Legs
 Myriapods (“many feet”) are arthropods with
two antennae and many body segments
 Centipedes are predators
 Millipedes are scavengers
Myriapods
 Centipede and millipede
25.16 The Insects
 Insects have a three-part body plan
• The head has compound eyes, a pair of
antennae, and specialized mouthparts
• The thorax has three pairs of legs; some lineages
have wings
• Malpighian tubules in the abdomen eliminate
wastes and save water
Insect Body Plan: Bedbug
Specialized Mouthparts
Insects
 Arthropods are the most successful animals, and
insects are the most successful arthropods
 Insects are adapted to life on land; a system of
tracheal tubes delivers air to their tissues
 Development may be direct, or through
incomplete or complete metamorphosis
Insect Development
25.17 Insect Diversity and Importance
 It would be hard to overestimate the importance
of insects, for either good or ill
 Insects help provide us with food crops, are food
for animals, and help dispose of wastes
• The four most diverse groups of insects all
include pollinators of flowering plants
 A few insects eat our crops or carry pathogens
Insect Diversity
25.6-25.17 Key Concepts
The Major Invertebrate Lineages
 One major lineage of animals with tissues
includes the flatworms, annelids, mollusks,
nematodes, and arthropods
 All are bilaterally symmetrical
 The arthropods, which include the insects, are
the most diverse of all animal groups
Echinoderm Body Plan: Sea Star
Echinoderm Diversity
 Echinoderms include about 6,000 marine
invertebrates such as sea stars, brittle stars, sea
urchins, and sea cucumbers
 Echinoderms can regenerate lost body parts;
any portion of a sea star with some of the central
disc can regrow missing parts
25.18 Key Concepts
On the Road to Vertebrates
 Echinoderms are on the same branch of the
animal family tree as the vertebrates
 They are invertebrates with bilateral ancestors,
but adults now have a decidedly radial body plan