1. Triploblastic – have three primary germ layers 2. Acoelomate – without a coelom 3. Classified into three phyla – › Phylum Platyhelminthes (flatworms) › Phylum Nemertea (unsegmented) › Phylum Gastrotricha (bottom dwellers)

 aka – Flatworms  Most common = planarian  Contains over 34,000 species  Currently, no uniquely defining characters (synapomorphies) in this phylum  Adult size from 1 mm or less to 25 m  Live in marine, freshwater, and damp terrestrial habitats

 Bilateral Symmetry  Cephalization (have a so called head)  Mesoderm tissue includes a loose tissue (parenchyma) that fills spaces between specialized tissues, organs, and body wall. › May provide skeletal support, nutrient storage, motility, transport of materials, oxygen storage, etc.

 Most cells are close to external environment ∴ materials can pass easily into and out of their bodies (via diffusion)  Rely on diffusion for respiration, excretion, and circulation  Other functions vary among species (free-living or parasitic)

 Free-living flatworms: › Class Turbellaria  Parasitic Species: › Class Monogenea › Class Trematoda › Class Cestoidea

 Mostly free-living bottom dwellers in freshwater and marine environments  Crawl on stones, sand, or vegetation  Named for the turbulence that their beating cilia create in the water

 Over 3,000 species  Few terrestrial species live in humid tropics and subtropics  Less than 1 cm long (rare terrestrial/tropic = up to 60 cm long)  First group of bilaterally symmetrical animals to appear

 Carnivores (small invert.), scavengers (dead), some herbivores (algae)  Sensory cells (chemoreceptors) on their heads help detect food far away  Digestive cavity has a single opening (or mouth) through which food and waste pass

 Pharynx – muscular tube that extends out of the mouth and pumps food into the digestive cavity or gut › highly branched gut transports food to all parts of the body (this is lacking in many parasitic species)

 Cilia on epidermal cells help to glide through the water and over the bottom of a stream or pond  A layer of mucus is laid down to aid in adhesion and help cilia gain traction

 Muscle cells controlled by the nervous system allow them to twist and turn so that they are able to react rapidly to environmental stimuli › Dorsoventral muscles essential for maintaining flatness (ie-for diffusion)

 Do not have respiratory organs ∴ done via diffusion › Respiratory gases (CO 2 and O 2 ) › Metabolic wastes (ammonia)  Depends on environment › Marine = osmotic equilibrium › Freshwater = hypertonic

 Protonephridia is a network of fine tubules for excretion of metabolic wastes.  Flame cells are ciliated and induce currents to push fluids through tubules.

 Tubules eventually merge and open to the outside of the body wall through a minute opening called a nephridiopore.

 Subepidermal nerve plexus (resembles cnidarians but depends on species) › Mechanoreceptor (excited by pressure) at anterior end detects body position(due to gravity) › Cerebral ganglia – more centralized nerve net (“brain”) › Longitudinal nerve cords – ladderlike appearance (evolutionary advancement to a nervous system)

 Auricles – sensory lobes on side of head aid in food location (chemoreceptor)  Ocelli – eye spots; orient in direction of light (photoreceptor)  See how it works… See how it works…

 Asexual reproduction  Budding or fission  Regeneration

 Can reproduce either asexually or sexually › Asexual = planarian detaches its tail end and each half regrows the lost parts (referred to as zooids). › Sexual = each Planaria gives and receives sperm. › Benefits to sexual over asexual?  Have both testes and ovaries (hermaphroditic)  Eggs develop inside the body and are shed in capsules called cocoons › Weeks later the eggs hatch and grow into adults.  Can also reproduce by regeneration (fragmentation) › If it is cut into two halves, both halves may become two new Planaria.

 What type of symmetry?  How do they feed?  What type of sensory organs?  How do they reproduce?

› Class Monogenea › Class Trematoda › Class Cestoidea

 External parasites (ectoparasitic)  Commonly found on the gills, skin or fins of fishes and lower aquatic invertebrates  Have a series of hooks that enable them to attach while feeding  Have one generation in their life cycle from egg to adult

 8,000 parasitic species (aka – flukes)  Internal parasites (endoparasitic)  Complex life cycles specialized in parasitism in animal or human tissues  One or more suckers around anterior end (oral sucker)

 Life cycle involves two - four hosts and a number of developmental stages, including two types of free-living larvae (most complex in the animal kingdom)  The definitive (final) host of adult is always a vertebrate  Snails are common intermediate hosts

1. Eggs reach freshwater and Miracidium (ciliated larva) swims out 2. Finds host (snail) 3. Penetrates snail, loses cilia, develops into a sporocyst (contain embryonic cells) 4. Develops into daughter sporocysts (hundreds can form from one miracidium) 5. Hundreds of next larval stage are produced (cercariae) 6. Cercariae leave the snail and find 2 nd or final host 7. Penetrates host and becomes a Metacercaria 8. When the definitive host eats the 2 nd intermediate host, it becomes an adult

Dicrocoelium dendriticum and Opisthorchis sp. liver flukes of mammals blood flukes, Schistosoma spp. are among most widespread and serious parasites of humans

 aka – tapeworms  Most highly specialized class of flatworms  Intestinal parasites  No digestive tract ∴ reside in digestive system of vertebrates  Absorb nutrients across body wall  Adults range from 1mm to 25 m in length

 The body consists of an anterior scolex solely for attachment to the host's gut and a string of proglottids, each of which possesses both male and female organs

 aka – ribbon worms or proboscis worms  ~ 900 species  More closely related to other acoelomates than to any flatworm  Longest of any invertebrate (30 m but 2x as long when fully extended)

 Unsegmented  Proboscis tipped with barbs to capture prey (see it in action!) (see it in action!)  Complete digestive tract with an anus  Closed circulatory system (w/o a heart)

 ~ 500 species  Bottom dwellers in both marine & freshwater  Range from 0.1 mm to 4 mm  Dorsal contains scales, bristles, or spines  Have a forked tail  Development is direct (there is no larval stage)