1. Triploblastic – have three primary germ layers

2. Acoelomate – without a coelom3. Classified into three phyla –

› Phylum Platyhelminthes (flatworms)› Phylum Nemertea (unsegmented)› Phylum Gastrotricha (bottom dwellers)

aka – Flatworms Most common =

planarian Contains over 34,000

species 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

Free-living flatworms:› Class Turbellaria

Parasitic Species:› Class Trematoda› Class Cestoidea

Mostly free-living bottom dwellers in freshwater and marine environments

Crawl over 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 (CO2 and O2)

› Metabolic wastes (ammonia) Depends on environment

› Marine = osmotic equilibrium› Freshwater = hypertonic solutions

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…

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 theyfeed?

What type ofsensory organs?

How do theyreproduce?

› Class Trematoda› Class Cestoidea

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)

Unsegmented

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 sporocyte (contain embryonic cells)

4. Develops into daughter sporocyte (hundreds can form from one miracidium)

5. Hundreds of next larval stage are produced (cercariae)

6. Cercariae leave the snail and find 2nd or final host

7. Penetrates host and becomes a Metacercaria

8. When the definitive host eats the 2nd 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