Transcript Lecture 12, Intro to Bilateria - Cal State LA

Last common ancestor of all animals
- multicellular, heterotrophic
- asymmetry (= no symmetry)
- cellular level of organization (no tissues)
- intracellular digestion (ate bacteria)
- similar to choanoflagellate protists
Last common ancestor of the Metazoa
- radial symmetry
- 2 embryonic tissue layers
(ectoderm + endoderm)
- extracellular digestion, but with an
incomplete gut
Last common ancestor
of the Bilateria
- bilateral symmetry
- cephalized (had a head)
- embryos had mesoderm
- still had an incomplete gut
Organization of Body Plans: Symmetry
Asymmetrical - sponges
- without tissues
Radial symmetry - Cnidarians
- 2 embryonic tissues layers
(diploblastic)
Bilateral symmetry
- 3 embryonic tissue layers
(triploblastic)
- with organs
Features of Bilateral Symmetry
Anterior (= head end)
left
right
Dorsal
(top side, or back)
Ventral
(belly, or
bottom side)
Posterior (= tail end)
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
Acoels – the first hunters?
- no permanent gut, gonads or excretory organs;
stomach form temporarily after eating, then cells disband!
- space between temporary gut and body wall is solid
(tissue-filled), no coelom
- often live between grains of sand
tiny, common marine worms
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)
coelomate animals are divided
into 2 major lineages that are
distinguishable by features of
their embryos:
Protostomes
Deuterostomes
Platyhelminthes
(flatworms)
Annelid
worms
Molluscs
Nematodes
Arthropods
Last common ancestor of
coelomates
- coelom
- complete gut
- nephridia (kidneys)
Deuterostomes (starfish, us)
Platyhelminthes
(flatworms)
coelomate
ancestor
Protostomes
Annelid
worms
Molluscs
Nematodes
Arthropods
Deuterostomes
Platyhelminthes
(flatworms)
coelomate
ancestor
Annelid
worms
Molluscs
Nematodes
Arthropods
Platyhelminthes
(flatworms)
coelomate
ancestor
Annelid
worms
Molluscs
Nematodes
Arthropods
Platyhelminthes
(flatworms)
Annelid
worms
Molluscs
Nematodes
Arthropods
We now recognize two distinct
kinds of flatworms:
- primitive acoels (never had a coelom)
- advanced (but simple-looking) 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
- Bilateral symmetry (forward movement) but no coelom;
glide on sheet of mucus
- Cephalized: nerves concentrated in cerebral ganglion
at front of head, near sense organs
- Triploblastic: mesoderm
gives rise to a muscular
tissue layer in adult body
- Incomplete digestive system
with complex, branched gut
- NO coelom (but ancestor had one)
3 Classes of Platyhelminthes
(1) Turbellaria – free-living flatworms
- rely on diffusion of gas across their
thin body wall for respiration
(2) Trematoda – parasites with
2 or more hosts
many larval stages
in life cycle
(3) Cestoda – tapeworms
Fasciola, liver fluke
Taenia
Class Turbellaria
Freshwater planaria have a simple,
3-branched gut
Dugesia
Many colorful marine species have
multi-branched guts
Branched digestive systems also used as a circulatory system
Class Turbelaria: Anatomy of a Planarian
Pharynx (muscular eating tube)
emerges from middle of posterior
side of body
branched
GVC
Cerebral ganglion (cluster of nerves) is fed
by sensory neurons from eyespots, nerve
chords running along either side of body
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
Class Cestoda: Tapeworms
Intestinal parasites of vertebrates; up to 20 m long
Head, w/ hooks
for attaching
to intestine wall
chain of sex organs
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)
Movement – 3 kinds of worms
Compare movement in worm phyla with different body plans:
1. flatworms – Planaria
- watch them glide on cilia and a sheet of secreted mucus
2. nematodes (roundworms) – “vinegar worms”
- longitudinal muscles run along the body,
but no circular muscles around the middle
- look for distinctive, twitchy movements in lab
3. annelids: earthworms and marine polychaetes
- coelom acts as an internal skeleton: solid bodies
- circular muscles: make one end fat or skinny
- parapodia used as paddle-feet in marine worms