Eumetazoa are the "true animals" that possess true epithelia with basal laminae, definite body axes, and specialized tissues. They have several key characteristics including epithelial tissues that form protective barriers and regulate compartments, a hydrostatic skeleton that uses fluid pressure for support and movement, and muscles, neurons, and senses that allow for complex responses to the environment. Eumetazoans undergo gastrulation during development to form the three primary germ layers and have diverse growth forms ranging from solitary to modular colonial organisms.

1. Introduction to Eumetazoa
The ‘true animals’
True epithelium, definite body axes, specialized tissues
EUMETAZOA
Chapter 6

2. True Epithelia
EPITHELIUM – cell layer that covers surfaces, lines
internal cavities and spaces
With basal lamina, apical-basal polarity, cell-cell
junctions, immobile
Allows for establishment of regulated compartments
(connective tissue, gut lumen, coelom)
Epithelial cells control passage of materials
Environment under the epithelium can be chemically
controlled
Chapter 6

3. True Epithelia
APICAL-BASAL POLARITY
BASAL LAMINA
HEMIDESMOSOMES
Epithelial Junctions
•AHDERING
•SEALING
• GAP
Chapter 6

4. Epidermis, Gastrodermis & Gut
EPIDERMIS – the outer skin; covers the surface of the
eumetazoan body
Secretory
Absorptive
Impermeable
GASTRODERMIS – stomach skin; epithelial layer that
lines the gut and joins the epidermis at the mouth
GUT – internal, epithelia-lined cavity; it is a regulated
compartment
• GUT LUMEN (CAVITY) – also regulated
compartment
• Allows EXTRACELLULAR DIGESTION
• Secretes digestive enzymes and absorbs products
Chapter 6

5. Connective Tissue
CONNECTIVE-TISSUE COMPARTMENT – lies between
the epi- and gastrodermis
May just be the basal lamina of the two epithelia
May be very thick
CONNECTIVE TISSUE – consists of ECM and cells; no
collective term for cells of the ECM
Sponges – mesohyl
Metazoans – blastocoel
Cnidarians – mesoglea
Bilateria – simply called ‘connective tissue’
Chapter 6

6. Eumetazoan Skeletons
Skeletons can:
Maintain body shape
Support or protect the body
Transmit force of muscle
contraction
Be internal or external
Be mineral, organic or both
Be fluid or solid
Chapter 6

7. Fluid Skeleton
HYDROSTATIC SKELETON – water-filled skeleton,
a.k.a. a hydrstat; cavity filled with water-based fluid
**This is a eumetazoan innovation!!**
• Surrounded by muscles (circular and longitudinal)
• Muscles contract and pressurize the fluid
• Water is incompressible
•CIRCULAR CONTRACTION – lengthens hydrostat
and stretches longitudinal muscles
• LONGITUDINAL CONTRACTION – shortens
hydrostat and stretches circular muscles
Utility  inflate body, extend parts, locomotion, burrowing
Chapter 6

8. Fluid Skeleton
• Body wall
strengthened by
inelastic fibers (like
collagen)
• Resists aneurisms
Orthogonal Crossed-helical
Chapter 6

9. Solid Skeleton
PLIANT SKELETON – made of materials that
are rubbery and elastic
• Deform when stretched, spring back to original
shape
• Proteins, polysaccharides, water
• Use as CT in comb jellies, mesoglea in jellyfish,
hinges of bivalves
RIGID SKELETON – made of materials
that resist a change of shape (bone/shell)
Insect framework, terrestrial and flying invertebrates, and
fast moving crustaceans of the ocean
Chapter 6

10. Movement and Body Size
Cilia/Flagella VS Muscles
Only in water Water, air, land
Associated with surface Associated with volume
area (effective when S/V is Locomotion, can move
large) body parts with respect to
Small, lilliputian other body parts (ALL
invertebrates ANIMALS DO THIS)
Prone to surface friction Effective when S/V is
and viscous drag from small
environement
Momentum stops when
ciliary/flagellar beat stops
Chapter 6

11. Musculature
EPITHELIOMUSCULAR CELLS:
Arose from epithelium.
Actin-myosin microfilaments
allowed some to become
contractile.
MYOEPITHELIAL CELLS:
Contractile fibers isolated in
independent cells but are still part
of the epithelium.
Outside the basal lamina
MYOCYTES:
True muscle, contractile cells
below basal lamina.
No longer epithelial.
Chapter 6

12. Musculature
SMOOTH MUSCLE:
Contracts slowly.
Tension over large range of
stretch lengths.
Contractile tentacles.
CROSS-STRIATED MUSCLE:
Contracts rapidly.
Tension over limited range.
Snapping jaws or claws.
OBLIQUELY-STRIATED:
Intermediate of the other two.
Extensible animals that can
make quick movements.
ANTAGONISTIC
Chapter 6

13. Neurons and Nervous System
Eumetazoans use nerve cells to
detect, evaluate, and respond to
environmental stimuli.
NEURONS - respond to stimuli
by transmitting information as a
wave of depolarization to the
target cells
Sensory receptors and neurons
arose from epithelia (like
muscle cells).
They became specialized and
migrated to a connective-tissue
compartment
Chapter 6

14. Neurons and Nervous System
Transmission of a nerve impulse from neuron to neuron
or from neuron to effector (muscle) occurs via:
Electrical junctions – gap junctions that join
cells so that membrane depolarization (action
potential) spreads uninterrupted; no
organismal control
SYNAPSE – gap between two cells (synaptic
cleft) through which the action potential is
carried by chemical neurotransmitters
Chapter 6

15. Sense Cells & Organs
Eumetazoans need to obtain a dynamic picture of their
environment so they can respond appropriately.
Response to different stimuli like:
Electromagnetic Mechanical
Chemical Magn etic
Direction and distance to the source of the
stimuli are so important to each eumtezoan
Simple: sensory neuron-effector
Complex: multiple stimuli, whole-body
response, accessory structures, CNS
Chapter 6

16. Gravity Receptors
STATOCYSTS – hollow capsules
lined with MECHANORECEPTORS
and with a dense, heavy STATOLITH
at the center.
Gravity pulls the statolith against
the bed of mechanoreceptors
which then causes alterations of
the depolarization pattern.
Chapter 6

17. Photoreceptors
CILIARY and RHABDOMERIC – can be dispersed as
individual cells or, can be grouped together as EYES.
OCELLUS:PIGMENT SPOT
PIGMENT CUP – EVERTED or INVERTED
Chapter 6

18. Development
Zygote Cleavage Blastula *Gastrulation* Larva
Gastrulation types:
INVAGINATION
EPIBOLY
INGRESSION
Chapter 6

19. Development
DIPLOBLASTIC –
adults have 2
epithelial layers
(epidermis &
gastrodermis)
TRIPLOBLASTIC – adults have 3
epithelial layers (addition of
mesothelium)
MESODERM – located between
ecto- and endoderm and can arise
from either
Chapter 6

20. Growth
Solitary VS Modular
• Juvenile grows via Add new, small,
general increase in size identical units (module –
• ALLOMETRIC – different ZOOID)
body parts grow at different Clonal budding
rates All zooids form a colony
• Surface > Volume Favorable SA:V
• Gills, lungs, gastrodermis ISOMETRIC – similar
• Associated with growth
locomotion and increasing Colony is considered
complexity on one body one individual
axis Self repair, higher
• Self repair, often killed survival if preyed upon
Chapter 6

21. Growth
Chapter 6

22. Growth
POLYMORPHISM – modular growth that results in
specialization of zooids with different forms and
functions (very common in Cnidarians)
Exemplifies the
replication-
specialization-
integration pattern
of metazoans
Chapter 6