1. Acoelomorpha, Platyzoa and Mesozoa
Chapter 8
Acoelomorpha, Platyzoa and Mesozoa

2. Acoelomate Bilateral Animals
Characteristics:
bilateral symmetry
Marine, freshwater, terrestrial
Free-living and parasitic
cephalization
Triploblastic
Ectoderm
Mesoderm
Endoderm
organ level of development
no coelom cavity

4. Pseudocoelomate Bilateral Animals
Characteristics:
has an internal cavity that surrounds gut
But, cavity is not completely lined with mesoderm tissue as in a true coelomate
Cavity may be filled with fluid or a type of matrix material
Digestive, excretory, reproductive, waste storage, hydrostatic support

5. Phylum Acoelomorpha “without” “hollow” “form” (350 species)
New “contentious” Phylum (used to be in Platyhelminthes as a part of Turbellaria)
Characteristics:
Entirely Marine living in sediment
Small, flat, under 2 mm
No gut: mouth, pharynx, vacuole
No excretory system
Monoecious, but no gonads: gametes formed from mesenchyme cells in the body

6. Acoelomorpha

7. Protostome subgroups:
Ecdysozoa (“to strip off”) (8 phyla)
includes all animals that molt
Lophotrochozoa (“crest or tuft”, “wheel”) (17 phyla)
Horseshoe-shaped whorl of tentacles
Band of cilia found on larvae (trochophore)

8. Lophotrochozoa Clade Platyzoa Include the following phyla:
Platyhelminthes
Gastrotricha
Gnathostomulida
Micrognathozoa
Rotifera
Acanthocephala

9. Phylum Platyhelminthes
– Flat worms (20,000 species)
1. Class Turbellaria
free living – fresh/marine, terrestrial
scavengers
glandular cells
rhabdites (mucous sheath)
adhesive (attachment)
releaser (dissolver)

10. Organ Systems Digestion branched tract running down body pharynx
chemoreceptors
carnivores/herbivores
Respiration: no organs – diffusion through body wall
Excretion--protonephridia – tubules down each side of body
flame cells – enlarged cells with cilia
waste released through a nephridiopore

11. Nervous
nerve net – sub-epidermal nerve plexus
statocyst – equilibrium
cerebral ganglia or nerve cords and neurons
Sense organs
auricles – sensory lobes on side of head chemoreceptors for finding food
ocelli – eye spots or cups orients animal to light (anti-light) through these photoreceptors

12. Reproduction
asexually –
transverse fission
regeneration of body parts
sexually – mostly monoecious
male parts: testes run along back side
vas deferens – sperm ducts
seminal vesicle – store sperm
Penis

13. female parts:
ovaries 1- ? pairs
oviduct
genital chamber
genital pore
copulatory sac

14. Copulation usually between two different animals.
Sperm deposited in copulatory sac
moves to genital chamber
to oviducts
to ovaries
fertilization
zygote formed
yolk glands cover egg
Eggs will be laid outside of body
Some will have cocoon.
If summer: takes 2-3 weeks to “hatch”.
If autumn: wait until spring.

15. Planaria cs

17. Planaria

18. Undergoing transverse fission

19. Other Turbellarians

22. Turbellarians

23. 2. Class Monogenea – single host
Ectoparasites of fish – attach to gills and feed on epithelial cells
Don’t seem to do much harm
Opisthaptor – large, posterior attaching device (suction/hooks)
Egg—larva--host

24. Monogenea

25. Monogenea

26. 3. Class Trematoda – flukes (8,000)
All are parasites of vertebrates
Tegument over outer body –cells fuse to form a continuous layer of cells known as syncytium
Able to form cysts
Complicated life cycle

27. Fluke body plan

28. Subclass Digenea – two different forms
Largest group
most medically important group of flukes
adhesive organs
oral sucker
Ventral sucker - acetabulum
operculum – lid on egg
miracidium – ciliated larvae

29. Life cycle
Egg containing miracidium swims to intermediate host (snail)
loses cilia becomes a sporocyst
undergoes asexual reproduction producing 100’s of cercaria (“polyembryony”)
leaves snail and finds a 2nd I.H. or final host
becomes a metacercariae (cyst)
eaten by definitive host
You are in BIG TROUBLE!

31. Egg with Operculum

32. Egg with miracidium

33. Cercaria

34. Adult worm

35. Examples:
Chinese liver flukes
Blocks bile ducts
50 million people affected
raw fish
sheep liver fluke
Schistosomiasis (blood)
200 million affected
Paragonimus (lung)
Swimmers itch (skin)

38. Chinese Liver Fluke

39. Schistosomiasis

40. Male and Female worm

42. Swimmers Itch

43. 4. Class Cestoda – tape worms (3,500 species)
long, flat bodies
endoparasites
no mouth, no digestive system
tegument
proglottids (strobila)
1 mm to 15 meters in length
two hosts required

44. Body structure:
scolex
neck
strobila – proglottids
gravid (full of mature eggs)
life cycle of beef tapeworm Taeniarhynchus saginatus
proglottid (holding up to 80,000 eggs!) breaks off and is expelled from the small intestine
each egg will become a larva having 6 hooks known as a onchosphere

46. Scolex

48. Tapeworm Larvae

49. Beef Tapeworm

50. cattle grazing on grass eat the larva
the onchosphere uses hooks to bore into the intestinal wall to the blood stream then to muscles
encysts in the muscle becoming a cysticercus (bladder worm) causing what is known as “measly beef”
human eats beef that is improperly cooked, the cysticercus then is released from the meat and starts to live in the intestine
the cycle starts over again

51. Figure 14.19

52. Other important tapeworms:
Taenia solium Pork (can get into the brain)
Diphyllobothrium latum fish (Great Lakes)

54. Moral to the Story:
COOK YOUR MEAT!!!!!!!!

55. Phylum Gastrotricha “stomach” “hair” (460 species)
Benthic – found in fresh and marine
um long
bottom sediments
bristles and spines
forked tail with adhesive glands
Monoecious –male system nonfunctional in freshwater

56. Gastrotrich

60. Glade or Group Gnathifera
Groups of “jaw bearing” tiny, free-living aquatic animals
Coeloms: “a-” or “pseudo-”
Some protostome and deuterostome characteristics
Four phyla
Eutely – constancy in the number of nuclei present (epidermis)

61. Phylum Gnathostomulida “jaw” “mouth” “diminutive” (80 species)
Jaw worms (80 species)
Characteristics:
usually less than 1 mm
marine coastlines (sand and silt)
live in very low oxygen - Respiration through diffusion
Acoelomate
pharynx has a pair of lateral jaws to gather food
No anus
One cilia in each epidermal cell
no circulatory system
Monoecious - simultaneous

62. Jaw Worm

64. Phylum Micrognathozoa
Only one species found - Greenland
Tiny – less than 142 micro-meters
Interstitial in sand
Two-part head, thorax, an abdomen and short tail.
Dorsal plates
Cilia
Very complicated jaw with plates and teeth, mouth leads to a simple gut and an anus
Only females found so far

65. Micrognathozoan

66. Phylum Rotifera “wheel” “to bear” (2000 species)
corona – ciliated organ at head
freshwater
foot with toes
pedal glands secrete adhesive
external cuticle
mastax – pharynx where food is ground

67. Reproduction – a little strange!
If there are no males – females produce amictic (diploid) eggs which develop into females
If there are males and females – females will produce amictic and mictic (haploid) eggs. Amictic turn into females. Mictic if left unfertilized turn into males, if fertilized become females
So diploids are females, haploids are males
Males have an eversible penis that can inject sperm into the pseudocoelom -- known as hypodermic impregnation

68. Rotifera

72. Phylum Acanthocephala “spine” “head” (1100 species)
Spiny-headed worms
2 mm – 1 m
pseudocoelom
endoparasites in the intestional tract of vertebrates (mainly fish)
two hosts are needed for life cycle
juveniles – crustaceans and insects
adults – vertebrates
proboscis covered with retractable spines
glycocalyx covers tegument—protects against host immune defenses
no digestive system – absorbs host food like tapeworms

73. dioecious – fertilization internal life cycle:
eggs will be in a shelled larvae which leaves the host
must be eaten by an insect or arthropod
larvae will emerge called an acanthor
the acanthor will burrow through gut of host where it will develop into an acanthella and then into a cystacanth
first host will be eaten by a fish, bird (or other vertebrate) the cystacanth will excyst and attach to the intestinal wall.
Here we go again!

74. Spiny Headed Worms

76. Phylum Mesozoa “middle” “animal”
Characteristics :
Has no organs or tissues
Body possesses no digestive tract
Body only has two cell layers in most places
Only cells
Has no nervous system
Has some cells develop inside other cells
Reproduction quite complex involving both sexual and asexual aspects
All are endoparasites on other marine invertebrates