Tropical
Marine
Invertebrates
CAS BI 569
Major Animal Characters
Part 2 — Adult Bodyplan Features
by J. R. Finnerty
Metazoan Characters
Part II. Adult Body Plan Features
CHARACTER
EPITHELIUM:
BODY LAYERS:
BODY CAVITIES:
GUT:
SYMMETRY:
SKELETON:
states
present; absent;
diploblastic; triploblastic
precoelomate;
acoelomate;
pseudocoelomate; eucoelomate;
absent; blind sac; through-gut;
asymmetrical; radial; bi-radial; bilateral;
pentaradial
“spicules;” “bones;” hydrostat; exoskeleton
EPITHELIUM
Sheet of cells that lines body cavities
or covers outer body surfaces.
E.g., skin, gut lining
Creates extracellular compartments
four key characteristics:
1.continuous — uninterrupted layer
2. intercellular junctions cell
3. polarity (apical vs. basal)
4. basal lamina (extracellular matrix
on which basal cell surface rests;
collagen secreted by cells)
Ruppert et al., Figure 6.1
3
Body Layers (Germ Layers)
Germ layers form during gastrulation
ectoderm
blastocoel
blastocoel
endoderm
gut
blastoderm
BLASTULA
blastopore
4
Diploblastic Condition
Two germ layers, endoderm & ectoderm
blastocoel
endoderm
blastocoel
gut
gut
ectoderm
ectoderm
5
Triploblastic Condition
Three germ layers, endoderm, ectoderm, & mesoderm.
blastocoel
gut
ectoderm
Body Cavities
I. Blastocoel
the central cavity in the hollow blastula
the 1st body cavity
II. Archenteron
“primitive gut”
opens to the outside via the blastopore
lined by endoderm
III. Coelom
cavity entirely lined by mesoderm
A pseudocoelom is only partially lined by
mesoderm. It may represent a persistent
blastocoel.
Character — Presence of COELOM
Character States
precoelomate
for diploblastic animals that diverged from the
metazoan stem prior to the invention of
mesoderm
acoelomate
absence of a cavity within the mesoderm
pseudocoelomate
cavity partially lined with mesoderm
eucoelomate (true coelom)
cavity partially lined with mesoderm
Body Cavities
COELOM
ACOELOMATE
EUCOELOMATE
PSEUDOCOELOM
PSEUDOCOELOMATE
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Character — COELOM FORMATION
Character States
Schizocoely
a cavity opens up in a solid clump of cells
occurs in the spiral cleavers in cells derived from the 4d
mesentoblast
Enterocoely
coelom forms via outpocketings of the gut
typical of radial cleavers
(enteron means “gut”)
Endomesoderm & Coelom Formation
Schizocoely from 4d cell
Ectoderm
Blastocoel
Mesoderm
Endoderm
Archenteron
The Gut
“internal, epithelium-lined
cavity for the digestion and
absorption of food
sponges lack a gut
simplest gut = blind sac
(Cnidaria)
blastopore gives rise to dualfunction mouth/anus
through-guts evolve later
Protostome = blastopore
contributes to the mouth
Deuterostome = blastopore
becomes the anus; mouth is a
second opening
Enterocoely
Protostome = blastopore contributes to the mouth
blastopore
mouth
anus
Deuterostome = blastopore becomes anus
blastopore
anus
mouth
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Bodyplan Feature — SYMMETRY
DEFINITION—Arrangement of the body about some axis.
Animals that can be bisected along at least one plane to
produce two (approximate) mirror images are said to exhibit
symmetry.
Character States
Asymmetrical—lacking a plane of mirror symmetry
Spherical symmetry—having an infinite number of planes of
mirror symmetry, all of which pass through a central point.
Radial symmetry—having an infinite number of planes of
mirror symmetry, all of which pass through a central axis.
Biradial symmetry—having two planes of mirror symmetry,
which both pass through a central axis.
Bilateral symmetry—having a single plane of mirror
symmetry.
Asymmetrical and spherically symmetrical animals do not
exhibit polarity or differentiation along an axis.
Spherical Symmetry
No polarity.
radiolarian protozoan
Radial Symmetry
Polarity along the
primary body axis.
Confronts the
environment equally
well from many
directions.
Usually sessile or
drifting organisms.
e.g., Hydra
Bilateral Symmetry
Transverse plane
Anterior
Dorsal
Posterior
Lateral
Ventral
Frontal plane
MidSagittal
plane
Polarity along two axes (primary A-P and secondary D-V).
A-P axis is the “locomotory axis.”
D-V axis is commonly the “environmental axis.”
Common in animals with directed locomotion.
But also in sessile sea anemones and corals. WHY
Spherical Symmetry
radiolarian protozoan
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Skeletons
“any structure that maintains
body shape, supports or
protects a body, and transmits
contractile forces.
SPONGES have small rigid
elements called SPICULES
linked into a network
vertebrate BONE
arthropod EXOSKELETON
HYDROSTATIC SKELETON
Hydrostatic skeleton
fiber reinforced water filled membrane
shape is under the control of muscle contraction
water is incompressible, so volume is constant
long and thin, or short and squat
fiber reinforcement transmits forces and distributes pressure to
eliminate local distensions
90° fiber angles resist bending or shape change while oblique
fiber angles permit bending without kinking