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:
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

5. Flatworm Characteristics:
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.

7. Form and Function in Flatworms
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

8. 3 Main Classes of Platyhelminthes:
Free-living flatworms:
Class Turbellaria
Parasitic Species:
Class Trematoda
Class Cestoidea

10. Class Turbellaria
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

11. 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 appear

13. Feeding 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

14. Feeding cont. 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)

15. 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

17. Locomotion cont.
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)

19. 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 solutions

20. 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.

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

22. 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 –
ladderlike appearance
(evolutionary
advancement
to a nervous system)

23. 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…

24. Reproduction
Asexual reproduction
Budding or fission
Regeneration

25. Reproduction cont. 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.

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

28. Parasitic worms:
Class Trematoda
Class Cestoidea

29. 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)
Unsegmented

30. Subclass Digeneans
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

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

32. The species that infect humans can be divided into groups: 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

33. Schistosomiasomes (blood flukes)

34. 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

35. 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