Last common ancestor of all animals was likely: Sponges 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 Sponges Cnidarians Last common ancestor of all Metazoans - radial symmetry - 2 embryonic tissue layers: ectoderm + endoderm - extracellular digestion, but incomplete gut

Last common ancestor of Bilaterians - bilateral symmetry Sponges Cnidarians this is a true group, Acoela – the flatworms whose ancestor did not have a coelom Acoel flatworms 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 - Cnidarians - 2 embryonic tissues layers (diploblastic) Bilateral symmetry - 3 embryonic tissue layers (triploblastic) - with organs Radial Symmetry Bilateral Symmetry

Early developmental stages micromeres fertilized ova (egg) macromeres Blastula = embryonic stage before the germ layers form or or Different kinds of blastulae

From Blastula to Gastrula ectoderm blasto- coel Invagination endoderm archenteron blastopore 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

Last common ancestor of coelomate Bilaterians Platyhelminthes (flatworms) Annelid worms Molluscs Nematodes Arthropods Sponges Cnidarians acoel flatworms Last common ancestor of coelomate Bilaterians - complete gut - coelom - nephridia lost in some groups Deuterostomes (starfish, us)

Deuterostomes (starfish, us) Platyhelminthes (flatworms) Annelid worms Molluscs Nematodes Arthropods Sponges Cnidarians acoel flatworms 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 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 - 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 (2) Trematoda – parasites w/ 2-3 hosts (3) Cestoda – tapeworms freshwater planarians, marine polyclads complex life cycles with multiple larval stages Fasciola,liver fluke Taenia

Orders of Class Turbellaria (1) Tricladida 3-branched gut - freshwater planaria (2) Polycladida multi-branched gut - marine species Dugesia

Class Turbellaria Dugesia Freshwater planaria have a simple, 3-branched gut Many colorful marine species have multi-branched guts Dugesia 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 Note: they are arranged segmentally, to some extent... Digenean fluke

Müller’s larva 8 ventrally-directed, ciliated lobes used for swimming

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

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 Lateral blood vessel Blood vessels in the closed circulatory system are closely associated with excretory protonephridia Protonephridia efficiently remove wastes from blood Allows large body size, without segmentation or a big coelom Flame bulb

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 NEMATODA (roundworms) GNATHOSTOMULIDA GASTROTRICHA KINORYNCHA NEMATOMORPHA PRIAPULA ...we now know “pseudocoelomates” are a polyphyletic group - group with coelomate worms, molluscs - molting phyla; all group with arthropods