1. The Triploblasitc, Acoelomate Body Plan
Zoology Chapter 10

2. Acoelmates, Pseudocoelomates, and Coelomates

3. All animals in this chapter are:
Triploblastic – have three primary germ layers
Acoelomate – without a coelom
Classified into three phyla:
Phylum Platyhelminthes (flatworms)
Phylum Nemertea (unsegmented)
Phylum Gastrotricha (bottom dwellers)

4. Phylum Platyhelminthes:
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

5. Flatworm Characteristics:
Bilateral Symmetry
Cephalization
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

6. Three Classes of Platyhelminthes:
Free-living (not parasites) flatworms:
Class Turbellaria
Parasitic Species:
Class Trematoda
Class Cestoidea

8. Class Turbellaria
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

9. Class Turbellaria cont.
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 evolve

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

12. Feeding cont.
Pharynx – muscular tube pumps food from the mouth into the digestive cavity or gut
Highly branched gut transports food to all parts of the body
(this is lacking in many parasitic species)

13. Locomotion
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

14. Locomotion cont. Muscle cells are controlled by the nervous system
Allows animal to move & react rapidly to environmental stimuli
Dorsoventral muscles maintain flatness (allows for diffusion)

16. Exchanges with Environment:
Do not have respiratory organs ∴ done via diffusion
Respiratory gases (CO2 and O2)
Metabolic wastes (ammonia)
Depends on environment
Marine = osmotic equilibrium
Freshwater = hypertonic

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

18. Nephridiopore
An opening on the outside of the body from which tubules eventually merge

19. Nervous System & Sense Organs:
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 – ladder-like appearance (evolutionary advancement
to a nervous system)

21. 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…

22. Asexual Reproduction
Budding or fission
Regeneration

23. Sexual Reproduction
Monoecious: 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.
Practice “penis fencing” to determine which flatworm carries the fertilized eggs
See it here

25. Turbellaria review: What type of symmetry? How do they feed?
sensory organs?
reproduce?

26. Parasitic worms:
Class Trematoda
Class Cestoidea

27. Class Trematoda 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)

29. The species that infect humans: Schistosomes & non-Schistosomes
Dicrocoelium dendriticum and Opisthorchis sp. liver flukes of mammals
blood flukes, Schistosoma spp. are among most widespread and serious parasites of humans

30. Schistosomiasomes (blood flukes)

31. Class Cestoidea 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

32. Anterior scolex solely for attachment to the host's gut
a string of proglottids, each of which possesses both male and female organs