1. The Triploblastic, Acoelomate Body Plan

2. Phyla Involved
Platyhelminthes
Nemertea
Gastrotricha

3. Characteristics of All 3
1st to exhibit bilateral symmetry
Triploblastic
Acoelomate

4. Platyhelminthes
Turbellarians
Free-swimming
Flukes
Parasitic
Tapeworms

5. Platyhelminthes (cont.)
(plat”e-hel-min’thez)
34,000 species
Range in size from less than 1mm to 25m
Their mesodermally derived tissues include a loose tissue called the parenchyma.

6. Parenchyma Depending on the species, the parenchyma may provide:
Skeletal support
Nutrient storage
Motility
Reserves of regenerative cells
Transport of materials
Structural interactions with other tissues
Modifiable tissue for morphogenesis
Oxygen storage
Other functions

7. General Charateristics of the Phylum Platyhelminthes
Usually flattened dorsoventrally, triploblastic, acoelomate, bilaterally symmetrical
Unsegmented worms
Incomplete gut usually present
Somewhat cephalized, with an anterior cerebral ganglion and usually longitudinal nerve cords

8. General Charateristics of the Phylum Platyhelminthes (cont.)
Protonephridia as excretory/osmoregulatory structures
Most forms are monoecious; complex reproductive systems
Nervous system consists of a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves and located in the mesenchyme

9. Class Turbellaria: The Free-Living Flatworms
(tur’’bel-lar’e-ah)
Free-living bottom dwellers in freshwater or marine environments, a few terrestrial in humid and tropical climates
3000 species
Predators and scavengers
Usually less than 1cm, some can be up to 60cm in length
Normally colored black, brown or gray, occasionally brightly colored

10. Movement The 1st group of bilaterally symmetrical animals to appear
Bilateral symmetry is usually a characteristic of an active lifestyle
Turbellarians glide over the substrate
They use cilia and muscular undulations to move
As they move, they lay down a sheet of mucus that aids in adhesion and helps the cilia to gain traction

11. Digestion & Nutrition
Some feed on small, live invertebrates or scavenge on larger, dead animals
Some are herbivores and feed on algae that they scrape from rocks
Sensory receptors (chemoreceptors) found on their head help them to detect food at considerable distances
Digestion is primarily extracellular
Pharyngeal glands secrete enzymes that help break down food

12. Exchanges with the Environment
Do not have respiratory organs thus they “breathe” through their body walls by diffusion
Wastes are also removed by diffusion
In marine environments
Inverts are in osmotic equilibrium
In freshwater environments
Inverts are hypertonic

13. Coping with Freshwater
In order to move into bodies of freshwater turbellarians had to develop something that would regulate their osmotic concentration
They developed protonephridia
Protos = 1st
Nephros = kidney
Networks of fine tubules that run the length of the turbellarian

14. Reproduction and Development
Many produce asexually by transverse fission
Fission usually begins with a constriction point behind the pharynx
The two (or more) animals that result from fission are called zooids and they regenerate the missing parts.
Turbellarians are monoecious
Reciprocal sperm exchange

15. Class Monogenea
Named because they only have one generation in their life cycle
One adult develops from one egg
Mostly external parasites (ectoparasites)
Attaches to the gill filaments and feed on epithelial cells, mucus or blood of freshwater and marine fishes
Attachment is possible due to the presence of an opisthaptor, a posterior attachment organ

16. Class Trematoda (trem’’ah-todah) 8000 species; parasitic Called flukes
Almost all adult flukes can be found in vertebrates, whereas their immature forms can be found in inverts and verts
Flat, oval and elongated
1mm to 6cm in length
Digestive tract includes a mouth and a muscular pumping pharynx

17. Subclass Aspidogastrea
Mostly endoparasites of molluscs
Possess large opisthaptor
Most lack an oral sucker
Aspidogaster, Cotylaspis, Multicotyl

18. Subclass Digenea Adults are endoparasites in verts
At least two or more life cycle forms in two or more hosts
Have oral sucker and acetabulum
Schistosoma, Fasciola, Clonorchis

19. Trematode Parasites of Humans
The Chinese Liver Fluke, Clonorchis sinensis, is a common human parasite in Asia, where over 30 million people are infected.
They live in the bile ducts of the liver where it feeds on epithelial tissue and blood.
Eggs are eliminated in the feces.
People are infected by eating infected raw or undercooked fish (sushi, sashimi, ceviche).

20. Trematode Parasites of Humans (cont.)
Fasciola hepatica is called the sheep liver fluke because it is common in sheep raising areas and uses sheep or humans as its definitive host
Adults live in the bile ducts of the liver
Snail – Plant – Sheep
Snail – Plant - Human

21. Class Cestoidea: The Tapeworms
(ses-toid’e-ah)
Most highly specialized of the flatworms
3500 species
Endoparasites, usually in vertebrate digestive system
Range from 1mm to 25m in length

22. Adaptations for the Parasitic Lifestyle
Tapeworms lack a mouth and a digestive tract in all of their life-cycle stages; absorb nutrients directly across the body wall
Most adult tapeworms consist of a long series of repeating units called proglottids; each proglottid contains one or two complete sets of reproductive structures

23. Home Sweet Home
Live in a very stable environment because the vertebral digestive system has very few environmental variations that would require the development of great anatomical or physiological complexity in any single tapeworm system
It is thought that tapeworms may have lost some characteristics that were present in the ancestral species
Tapeworms are a good example of evolution not always resulting in greater complexity

24. Subclass Cestodaria
Endoparasites in the digestive tracts of primitive fishes
15 species

25. Subclass Eucestoda True tapeworms Holdfast structure called the scolex
The scolex narrows to form a neck, which gives rise to the strobila
The strobila consists of a series of linearly arranged proglottids, which function as reproductive units
New proglottids are added in the neck region pushing older proglottids posteriorly
Neck = immature
Strobila = mature
End = gravid

26. Reproduction of Tapeworms
Monoecious
Made to breed
Multiple testes
Single pair of ovaries
Usually breed with other mature proglottids on the same organism or a mature proglottid on a different tapeworm in the same organism
Cross-breeding leads to “Hybrid Vigor”

27. Tapeworm parasites of humans
Taeniarhychus saginatus, the beef tapeworm
25m in length
About 80,000 eggs per proglottid
Taenia solium, the pork tapeworm
Usually 2-3m, can be up to 10m

28. Nemertea
A small group of elongate, segmented, soft bodied worms that are mostly marine and free-living
900 species
AKA proboscis worms
From a few mm to several cm in length

29. Characteristics of the Phylum Nemertea
Triploblastic, acoelomate, bilaterally symmetrical, unsegmented worms possessing a ciliated epidermis containing mucous glands
Complete digestive tract with an anus
Protonephridia
Cerebral ganglion, longitudinal nerve cords, and transvere commisures
Closed circulatory system
Body musculature organized into 2 or 3 layers

30. Stuff
The most distinctive feature of nemerteans is a long proboscis held in a sheath called a rhyncocoel
The proboscis may be tipped with a barb called a stylet
Also, nemerteans are diecious

31. Breakthroughs
They have a mouth for ingesting food and an anus for eliminating wastes
This allows for the mechanical breakdown of food, digestion, absorption, and feces formation to proceed sequentially in an anterior to posterior direction
Also, they have a circulatory system consisting of two lateral blood vessels
No heart present

32. Nemertean Fact
Include the longest invertebrate animal, Lineus longissimus, which can be 30m long

33. Gastrotricha
Includes members that inhabit the space between bottom sediments
500 free-living freshwater and marine species
From 0.1 to 4mm in length
Contains a single class divided into 2 orders

34. Reproduction
Most marine species reproduce sexually and are hermaphroditic
Most freshwater species reproduce asexually by parthenogenesis; the females can lay 2 kinds of unfertilized eggs
Under favorable conditions they lay thin-shelled eggs that hatch into females
Under non-favorable conditions they lay thick shelled eggs that hatch into females