Download Bilateral symmetry - Cal State LA

Last common ancestor of all animals was likely:
- multicellular, heterotrophic
- asymmetry (= no symmetry)
- cellular level of organization (no tissues)
- intracellular digestion (eats bacteria)
- similar to choanoflagellate protists
Last common ancestor of all Metazoans
- radial symmetry
- 2 embryonic tissue layers:
ectoderm + endoderm
- extracellular digestion, but incomplete gut
this is a true group,
Acoela – the flatworms
whose ancestor did not
have a coelom
Last common ancestor of Bilaterians
- bilateral symmetry
- cephalized: had a head
- 3 embryonic tissue layers (mesoderm)
- still had an incomplete gut
- resembled a planula larva?
Features of Bilateral Symmetry
Anterior (= head end)
left
right
Dorsal
(top side, or back)
Ventral
(belly, or
bottom side)
Posterior (= tail end)
Organization of Body Plans: Symmetry
Asymmetrical - sponges
- without tissues
Radial
Symmetry
Radial symmetry - Cnidarians
- 2 embryonic tissues layers
(diploblastic)
Bilateral symmetry
- 3 embryonic tissue layers
(triploblastic)
- with organs
Bilateral Symmetry
Early developmental stages
micromeres
fertilized
ova (egg)
Blastula = embryonic stage
before the germ layers form
macromeres
or
or
Different kinds of blastulae
From Blastula to Gastrula
ectoderm
blastocoel
endoderm
archenteron
blastopore
Invagination
Gastrulation = stage when embryonic germ layers form
- this process separates cells that are destined to interact
with the environment (movement, sensory, protection)
from those that will process food (= the gut)
Gastrulation is critical: it lays out the plan for adult body
Acoelomates
- only internal cavity is the gut
- space between gut and body wall is tissue-filled
- no fluid-filled space (coelom) to provide rigidity, hence no
hydroskeleton for muscles to attach to
Coelomates
A true coelom is a fluid-filled cavity enclosed in tissue that
develops from mesoderm layer in embryo
- acts as hydrostatic skeleton: point of muscle attachment;
gives solidity and strength (so you aren’t just a wet noodle)
True acoels – the first worms?
- no permanent gut, gonads or excretory organs
- space between temporary gut and body wall is tissue-filled
- “interstitial” – often live between grains of sand
common in marine habitats, but small
True acoels – the first worms?
- gut forms temporarily after eating
- some prey on small crustaceans
- some harbor photosynthetic algal symbionts
cell mass, derived
from endoderm
simple pharynx
- particles swept into pharynx by cillia
Platyhelminthes
(flatworms)
Annelid
worms
Molluscs
Nematodes
Arthropods
Last common ancestor of
coelomate Bilaterians
- complete gut
lost in some groups
- coelom
- nephridia
Deuterostomes (starfish, us)
Platyhelminthes
(flatworms)
Annelid
worms
Molluscs
Nematodes
Arthropods
We now recognize there are 2
distinct kinds of flatworms:
- basal acoels (never had a coelom)
- advanced Platyhelminthes
(lost the ancestral coelom + complete gut)
Deuterostomes (starfish, us)
“false” acoelomates
Platyhelminthes
Nemerteans
(ribbon worms)
Although lacking any functional coelom, molecular phylogenetic
studies indicate both groups evolved from an ancestor that
did have a coelom (because all their relatives have one)
- indicates secondary loss of an ancestral trait, likely an
adaptation to their environment and mode of hunting
Phylum Platyhelminthes
~20,000
species
- Bilaterally symmetric and triploblastic: mesoderm gives rise
to a thick, muscular mesenchyme but does not form a coelom
- Incomplete digestive system; complex branching gut
- Cephalized: nerves concentrated into anterior ganglion
- sensory receptors clustered on head, 1st to enter a new area
- longitudinal nerve cords, connected like a ladder
- Protonephridia, simple osmoregulatory/excretory glands
- complex reproductive systems
- weird Müller’s larva, or direct development (no larval stage)
Classes of Platyhelminthes
(1) Turbellaria – free-living flatworms
freshwater planarians,
marine polyclads
(2) Trematoda – parasites w/ 2-3 hosts
complex life cycles
with multiple larval stages
Fasciola,liver fluke
(3) Cestoda – tapeworms
Taenia
Orders of Class Turbellaria
(1) Tricladida
3-branched gut
- freshwater planaria
(2) Polycladida
multi-branched gut
- marine species
Dugesia
Class Turbellaria
Freshwater planaria have a simple,
3-branched gut
Dugesia
Many colorful marine species have
multi-branched guts
Branched digestive systems also acts as a circulatory system,
helping to distribute nutrients + O2 and remove wastes
Class Turbelaria: Anatomy of a Planarian
Pharynx (muscular eating tube)
emerges from middle of posterior
side of body
branched
GVC
feeding in Polycladida
intestine
mesenchyme
muscles
Ruffled,
plicate pharynx
pharynx
entended
Protonephridia
Cilliated flame bulbs
push fluid through
nephridioducts
Water, some wastes exit
through excretory pore
to the outside
Crucial to osmoregulation
in freshwater flatworms
Digenean
fluke
Note: they are arranged segmentally,
to some extent...
Müller’s larva
8 ventrally-directed, ciliated lobes used
for swimming
Class Trematoda: Complex life cycles
2nd, infects fish
or crab
1.
2.
3.
4.
5.
miracidum
redia
cercaria
metacercaria
adult
1st infects
a snail
Redia, packed full of
developing cercaria
Complex lifecycles of trematodes
cercaria
eyes
Swimming
cercaria
Redia stage, in snail -packed full of developing cercaria
Metacercaria of Himasthla
encysted on shore crab
Innovations of Bilaterians, seen in Platyhelmithes
- bilateral symmetry (dorsal - ventral, anterior – posterior axes)
- triploblastic: mesoderm  complex organs, muscle tissue
- cephalization: sensory structures concentrated on head,
the 1st region to encounter new environments
Limitations of flatworm body plan:
- rely on diffusion for respiration: must stay wet and thin
for O2 to randomly wander in through tissues, and CO2 out
- no coelom = can only move by gliding over surfaces
despite having muscle, there’s nothing for muscles to
attach to and work off of (no skeleton)
Phylum Nemertea
~ 900
species
- large but floppy bodies (>2 m long), lacking obvious coelom
- closed circulatory system, some w/ hemoglobin
- relieves many of the complications that otherwise limit
the acoelomate body plan
- complete digestive system.. now with anus!
- proboscis, long venomous “tongue” used to capture prey;
stored in the rhynchocoel, a hydrostatic cavity that may
actually be their coelom..?
- can reproduce asexually by fragmentation; may develop
through a pilidium larva stage
Phylum Nemertea
know the evolutionary innovations
that allowed these worms to
achieve large body size, and
function as effective predators
4 m long
Proboscis –
how to hunt when you’re slow
rhynchodeum
mouth
everted proboscis (= shot out)
Classification: Proboscis Placement
2 classes are distinguished by whether the rhynchodeum
(opening of rhynchocoel) is connected to the mouth
Rhynchocoel
Rhynchodeum
Rhynchocoel
Circulation + Excretion
Blood vessels in the closed
circulatory system are closely
associated with excretory
protonephridia
Lateral
blood
vessel
Flame bulb
Protonephridia efficiently
remove wastes from blood
Allows large body size,
without segmentation or
a big coelom
Pilidium larva
Produced by species
in the order
Heteronemertea
Free-swimming,
planktotrophic
larva with
incomplete gut
- adult anus
forms later,
independent of
larval digestive system
Pilidium larva
Juvenile worm develops inside larval pilidium (helmet shape)
Instead of a metamorphosis, a complete juvenile just punches
its way out of
the larval skin
The “Pseudocoelomates”
PHYLA:
ROTIFERA
ACANTHOCEPHALA
- group with coelomate
worms, molluscs
NEMATODA (roundworms)
GNATHOSTOMULIDA
GASTROTRICHA
KINORYNCHA
- molting phyla;
all group with
arthropods
NEMATOMORPHA
PRIAPULA
...we now know “pseudocoelomates” are a polyphyletic group