1. PHYLUM PLATYHELMINTHES The Acoelomates
ZLY 103
Animal Diversity

2. Introduction
The first triploblastic animals with a third tissue layer, mesoderm; between the endoderm and endoderm.
Due to absence of a coelom or pseudocoel, they are termed acoelomate
They are flat in shape and possess a blind gut (mouth without anus).
Size varies from a millimeter to several meters.
Some are free-living (Turbellaria) and parasitic.
Majority are bottom-dwellers in aquatic & moist lands.

3. Introduction
Eumetazoa - animals with tissue
Bilateria - have bilateral symmetry and are triploblastic.
Protostomia - a group of animals whose mouth develops from the blastopore, and the mesoderm forms from an area near the blastopore.
Acoelomata - They have a true mesoderm which fills the original blastocoel between the outer epidermis and digestive tract.

4. Characteristics
They are triploblastic, acoelomate, non metamerically segmented metazoans.
They are at organ system level of organization.
They exhibit bilateral symmetry,
Body contains no internal cavity and absence of blood system.
Protonephridial excretory organ (Flame cells) instead of anus.
Nervous system made up of longitudinal fibers instead of a nerve net.
Dorsoventrally flattened.

5. Characteristics They carry out hermaphroditic reproduction.
Free-living forms feed on small animals and other life forms.
Parasitic forms feed on nutrients derived from the host tissues.
Some species especially the free-living occur in major biomes; marine, freshwater & moist lands.
Central nervous system consists of a brain and two or more ganglionated nerve cord.
They include tape worms, flatworms, flukes and planarians etc.
Highly successful, with about 25,000 species.

6. Characteristics Shows a degree of cephalization
Epidermis soft and ciliated or covered by cuticle and with external suckers or hooks or both
Simple sense organs with eyespot in some species.
Parasitic species normally move between different habitats during their developmental stages.
A few species are commensals deriving harmonious existence with another larger organism.
Absence of fossil records due to their soft bodies but they are thought to have evolved around 550my ago.

7. Triploblastic Condition
The embryo divides into three instead of two as it is in cnidarians (diploblastic).
With the presence of a (parenchyma) mesoderm, the platyhelminthes:
Have an increase in size,
A separation of the alimentary canal from the body wall,
A variety of organs are now formed,
There is a movement towards the organ-system level of organization,
Presence of developing muscles, Cilia and flagella are becoming too irrelevant for the larger sizes evolving.

8. The Acoelomate Condition
The large size of platyhelminthes with the emergence of the mesoderm (next slide) created the problems of efficient transport system, hence the acoelomate condition.
The animals in this group do not have a choice but to be dorsoventrally flattened for it to present a large surface area for diffusion to satisfy the metabolic requirements (flatworms).

9. Internal section through the gut
Plate 1: Transverse section through a generalized Acoelomate

10. Body Plan Lining of gut =ENDODERM Epidermis/ ECTODERM gut cavity
no body cavity; region between gut and body wall packed with organs within MESODERM tissue
Acoelomate organism
(flatworm)
Body Plan

12. Body plan

13. Body design in Platyhelminthes

14. Taxonomy/Classification
Class Turbellaria
Dugesia
Class Monogenea
Gyrodactylus
Class Trematoda
Clonorchis, Fasciola, Schistosoma
Class Cestoda
Taenia, Dipylidium, Echinococcus

15. Distinct Features of the Classes in Flatworms
Proglotids and scolex in Cestoda
Rabdites in Turbellaria
Combination of characteristics- protonephridia for excretion and acoelomates in all except Cestodes
Larva of each class are unique
Turbellaria - Planaria
Cestodes - Oncosphere/Cysticerci (bladder worm)
Trematodes - Metacercaria/Cerceria

16. Distinct Features of the Classes in Flatworms
Integuments- Rhabdites and one cell layer epidermis in Turbellaria and usually ciliated; syncytial tegument in other classes.
Skeletal - hydrostatic
Muscle - longitudinal, transverse, and circular muscles are present.
Digestive - incomplete with intracellular and extracellular digestion; no system in Cestoda.

17. Distinct Features of the Classes in Flatworms
Excretory - flame cells, or excretory tubes in Cestodes.
Respiratory - no system, through diffusion
Circulatory - none, diffusion.
Nervous - anterior ganglia, ventral ladder-like system (two lateral cords with transverse cords).

18. Distinct Features of the Classes in Flatworms
Endocrine - hormones produced by nervous system
Reproductive - monoecious in most
Well developed reproductive organs, mostly internal fertilization.
Two of the parasitic classes have complex life cycles
Trematoda - miracidium, sporocyst, redia, cercaria, metacercaria (usually snail is secondary host.
Cestoda - oncosphere, cysticercus

19. Distinguishing Features among the Classes
Turbellaria
Trematoda
Monogenea
Cestoda
Free living; aquatic
Endoparasitic
 Ectoparasitic, larva is free-living
or ectoparasitic
Delicate, soft body
Leaf-like shape
 leaf-like/cylindrical in shape
Elongated, proglottides (able to detarch)
Suckers rarely present
Ventral sucker plus sucker on ‘head’ for attachment to host
 posterior attachment with hooks, suckers or clamps
Suckers and hooks on ‘head’ plus sucker on ‘scolex for attachment
Enteron present
 Enteron absent
No enteron

20. Turbellaria
Trematoda
Monogenea
Cestoda
Cuticle absent
(outer surface covered with cilia)
Thick cuticle with spines (not parasitic)
 Syncytial teguments, no cilia
Thick cuticle (protection); no cilia in adult
Hermaphroditic/asexual fusion
Indirect development
 Direct development
Sense organs in adult
Sense organs only in free-living larval stages
e.g. Planaria, Dugesia, Planocera, Microstomum
e.g. Fasciola (liver fluke) Schistosoma (blood fluke)
 e.g. Dactylogyrus, Polystoma, Gyrodactylus
e.g. Taenia (tapeworm)

21. Reproduction in Platyhelminthes
Asexual by budding or transverse fission
•Regeneration
Sexual
Hermaphroditic; complex reproductive system
Internal fertilization
Direct development, but some polyclads have free-swimming larvae

22. Reproductive system of Platyhelminthes

23. • Free-living flatworms, mainly carnivorous.
Class Turbellaria
• Free-living flatworms, mainly carnivorous.
Mostly aquatic, a few terrestrial.
Benthic (bottom dwellers).
Variable in shape, size and features.
Best example of cephalization.
Surprising level of internal complexity

24. Class Turbellaria
They are characterized by a soft epidermis ciliated at on the ventral surface.
The movement of the cilia propels the smaller forms.
Larger species glide along by muscular waves, usually over mucous beds secreted by special cells.

25. Turbellarians
Generally divided into five groups based primarily on differences in the form of the digestive cavity.
The most primitive are the acoels, (no digestive cavity).
The ventral mouth, and sometimes a simple pharynx, lead to an inner mass of nutritive cells.
Most species measure less than 1/8 in. (3 mm) in length.

26. Planaria: Tubellaria

27. Turbellarians
The rhabdocoels have straight, unbranched digestive cavities.
Studies show that the rhabdocoels gave rise to both the trematodes and cestodes,
Several rhabdocoel species exhibit commensal relationships, presumably led to parasitism.
The allocoels were formerly classified together with the rhabdocoels; the gut can be either saclike or branched.

28. Diversity in Planarians
The triclads, also known as planarians, are relatively large flatworms named for their three-branched gut (NEXT SLIDE).
Most species range from 1/8 in. (3 mm) to about 1 in. (2.5 cm) in length.
Planarians have more sense organs and a more complex brain than the other turbellarians (Polyclads).

29. Diversity in intestinal pattern of two orders of Tubellarians
Tricladida
Polycladida

30. Biology of Turbellaria
The freshwater species Dugesia tigrina has primitive eyes and tactile lobes, or auricles, on the sides of the head.
Muscular pharynx can be extruded for food capture. 
Dugesia and many other planarians can regenerate entirely new individuals from small pieces cut from the body.

31. Biology of Turbellaria
The group of turbellarians known as polyclads tend to be larger (1–2 in./2.5–5 cm) and more oval-shaped than the triclads.
Their bodies are extremely flat and leaf-like, and the gut is subdivided into numerous branches.
Many are brightly colored and some have ruffled edges. Some species have numerous eyes scattered over the front end of the body.

32. Class Trematoda
Flukes - oral and ventral suckers, no hooks, parasites, body with a syncytial tegument without cilia. Larva stages unique.
Clonorchis
Fasciola
Schistosoma

33. • Most important is subclass Digenea
Class Trematoda
E.g. Flukes
• Most important is subclass Digenea
11,000 species; second most diverse group of parasites (Nematoda#1)
Endoparasites of vertebrates
Some cause debilitating diseases to livestock and humans

34. Types of Hosts
Often have complex life cycles that alternate between sexual and asexual stages.
Most require at least 2 different kinds of hosts to complete their life cycle:
1. Definitive host (primary host)
Host in which the parasite matures and reproduces (sexually)
Host in which eggs are released
Intermediate host
Hosts in which larval stages develop and undergo asexual reproduction
Results in increase in the number of the individuals

35. Clonorchis
Human liver fluke - has two intermediate or secondary host snail and fish.
Cerceria
Adult human liver fluke

36. 8.8

37. Adult sheep liver fluke
Fasciola
Adult sheep liver fluke
metacercaria on grass.

38. Fasciola Body Plan

39. Life cycle of Fasciola

40. Schistosoma Blood Flukes
Cerceria usually infect by burrowing through skin.

41. Schistosomiasis
Schistosomiasis (bilharzia) is an infection with blood flukes and is a major infectious diseases.
More then 200 million people are infected worldwide with these flukes.
They acquire infection by swimming or walking in water domiciled by the intermediate snail host.
Schistosome eggs enter the water when infected people urinate or defecate in or near water.

42. Schistosomiasis
Eggs hatch and the miracidium seeks out a snail.
Within the snail, it develops into a sporocyst and asexual reproduction takes place. Cercaria are eventually released into the water.
Cercaria swims and avoids UV light which can damage it, but is very sensitive to the scent of humans.
Certain molecules from human skin makes cercaria jerks and releases chemicals that soften the skin and burrows in shedding its tail at the same time (Schistosomula).

43. 8.9a and b

44. Class Monogenea
They were previously classified as an order of the Trematoda, but recent work suggests they are more closely related to cestodes (tapeworms).
They are small (usually < 2cm) typically external parasites of fish that clamp onto the gills using a hooked organ (often with suckers) called an opisthaptor.
Some also parasitize the urinary bladder or rectum of frogs and turtles; there is a species that parasitizes squid and one that attaches to the eyeball of hippopotomuses.

45. Class Monogenea
Unlike the trematodes, Monogeneans have only a single host (hence “Mono” in the name). Most feed on the host’s epidermis using their protrusible pharynx, but some are blood feeders.
Monogeneans are hermaphrodites (male organs develop first) and can move around a host in search of a mate (they will also self-fertilize).
The egg hatches into a ciliated larva (an oncomiracidium) which seeks out its host in the water.

46. Monogenean Fluke on gill of longjaw
mudsucker fish
8.11

47. Class Cestoda (Tapeworms)
Have distinct head structure (Scolex) and reproductive units called proglottids (Strobila).
Absence of a digestive system.
Larva are oncospheres and cysticerci (bladder worms)
Taenia
Dipylidium
Echinococcus

48. Class Cestoda (tapeworms)
They are parasites of the vertebrate digestive tract and about 4000 species are known.
Almost all tapeworms require at least two hosts with the definitive host being a vertebrate, although intermediate hosts can be invertebrates.
They are quite different in appearance from the other members of the Platyhelminthes.

49. Cestodes Proglottids (Body Plan) & Head
Defining Characteristics

50. Class Cestoda
New proglottids form behind the scolex and the strobila may become extremely long.
Tapeworms live in the intestines where it is immersed in digested food therefore lack a digestive system. Instead they simply absorb food across their tegument.
To facilitate the absorption of food, the tegument has huge numbers of tiny projections called microtriches.
The surface area of the tegument for absorption is greatly increased.

51. 8.12

52. Class Cestoda
Tapeworms are usually monoecious (have both male and female reproductive organs).
A proglottid is fertilized by another proglottid in the same or a different strobila.
Shell-encased embryos form in the uterus and exit the proglottid via a uterine pore or the entire proglottid may detach and pass out of the host.

53. 8.15

54. Juveniles in flea and louse
Dipylidium (dog tape)
Juveniles in flea and louse
Note the two gential (red arrows) pores and reproductive systems in each proglottid.
Proglottid

55. Echinococcus
Hydatid cyst (cysticercus)- many scolices and enlarges to the size of a basketball.
Often called sand.
Only three proglottides

56. Constraints Associated with Parasitic Living
Reproduce within the definitive host
Get fertilized eggs out of the host
Contact a new and appropriate host
Obtain entrance into the host
Locate the appropriate environment within host
Maintain position within the host
Withstand an often anaerobic environment
Avoid digestion or attack by the hosts immune system
Avoid killing the host, at least until reproduction is completed

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