Last common ancestor of all animals - multicellular, heterotrophic Sponges 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 Sponges Cnidarians Last common ancestor of the Metazoa - radial symmetry - 2 embryonic tissue layers (ectoderm + endoderm) - extracellular digestion, but with an incomplete gut

- still had an incomplete gut Sponges Cnidarians Acoel flatworms Last common ancestor of the Bilateria - bilateral symmetry - cephalized (had a head) - embryos had mesoderm - still had an incomplete gut coelomates

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)

Protostomes Deuterostomes Sponges coelomate animals are divided into 2 major lineages that are distinguishable by features of their embryos: Cnidarians acoel flatworms Protostomes Deuterostomes

Last common ancestor of coelomates Platyhelminthes (flatworms) Annelid worms Molluscs Nematodes Arthropods Sponges Cnidarians acoel flatworms Last common ancestor of coelomates - coelom - complete gut - nephridia (kidneys) Deuterostomes (starfish, us)

Platyhelminthes (flatworms) Protostomes Annelid worms Molluscs Nematodes Arthropods Protostomes coelomate ancestor Echinoderms Chordates Deuterostomes

Platyhelminthes (flatworms) Annelid worms Molluscs Nematodes Arthropods coelomate ancestor Echinoderms Chordates

Platyhelminthes (flatworms) Annelid worms Molluscs Nematodes Arthropods coelomate ancestor Echinoderms Chordates

Deuterostomes (starfish, us) Platyhelminthes (flatworms) Annelid worms Molluscs Nematodes Arthropods Sponges Cnidarians acoel flatworms 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 Nemerteans (ribbon worms) Platyhelminthes 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 (3) Cestoda – tapeworms many larval stages in life cycle Fasciola, liver fluke Taenia

Class Turbellaria Dugesia Freshwater planaria have a simple, 3-branched gut Many colorful marine species have multi-branched guts Dugesia 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 1. miracidum 2. redia 3. cercaria 4. metacercaria 5. adult 2nd, infects fish or crab 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