A body Plan: All have bilateral symmetry. Exhibit Cephalization




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Unit 5 Notes #4 - Phylum Platyhelminthes: Flatworms

 

A) Body Plan: - All have bilateral symmetry.



-Exhibit Cephalization: Concentration of sensory organs and nervous tissue at the anterior end of the body (head).

-Well-developed third tissue layer called the mesoderm between the ectoderm and endoderm.

-Mesoderm includes: reproductive, excretory, and muscular organs.

-Possess an incomplete (blind) gut: single opening (mouth/anus) to the gastrovascular cavity; use mouth to ingest food and use mouth to expel undigested wastes.

 

B) Diversity:

1. Class Turbellaria: -Free living

-Incomplete gut -No suckers or hooks

-Ciliated epidermis Example: Planaria

 

2.Class Trematoda: -Parasitic



-Incomplete gut -Suckers

-Outer Cuticle (tegument) Example: Flukes

 

3. Class Cestoda: -Parasitic



-No gut Example: Tapeworms

-Suckers and hooks together form a scolex for attachment to host.

-Body consists of repeating sections called proglottids.


Example:
Tapeworm

 
C)Adaptations Common To Most Parasitic Flatworms:

1) Parasitic flatworms often have suckers and hooks for attachments, form a structure called a scolex.

2) Outer cuticle (tegument) for protection so as not to be digested or destroyed by the host.

3) Loss of digestive system in some (tapeworms) – these will absorb nutrients through ectoderm.



4) Complicated life cycle with the production of many eggs and/or offspring and use of many hosts to ensure transfer.


D)Characteristics Of Common Platyhelminthes Members:
1. PLANARIA

a. Digestion: Feed on small animals and the remains of larger dead animals.

-Mouth/pharynx is located mid-way along the ventral surface (not near the head) it acts to push food into the gastrovascular cavity.

-Digestive glands are present in the pharynx and gastrovascular cavity (they secrete enzymes to digest food)



-Gastrovascular cavity will also circulate nutrients and oxygen to various parts of the body.

-Indigestible wastes are eliminated out of the mouth.


b. Nervous : -Most primitive animal with a real

nervous system.

-A pair of ventral (along belly) nerve cords run the length of a nervous system.

-Connect to a pair of small brains or ganglia (a group of nerve cells) at the anterior end.

-Show cephalization with a variety of sensory cells.

-Sensory cells are sensitive to touch, light, and various chemicals.

 

c. Muscles: -Three layers of true muscle are located below the ectoderm.



-The outer layer of circular muscle that constrict the worm, making it longer and thinner.

-Next layer uses longitudinal muscles to shorten the worm.

-There is also a third layer that runs dorso-ventral (back to belly) and makes the worm a little flatter.

d. Locomotion: -Body moves by co-ordinating its muscles and gliding over a mucous trail that it secretes from glands in the epidermis.

-Cilia on the ventral epidermis assist in gliding.
2. FLUKES





a. Digestion: -They are parasitic, usually in the blood, liver, or intestine of a variety of animals.

-Mouth present in the middle of the anterior (oral) sucker.

-The pharynx swallows the host’s tissues and bodily fluids (blood) into its gastrovascular cavity.
b. Circulation and Respiration

-Gastrovascular cavity for circulation and respiration

-They live in tissues that are well supplied with nutrients and oxygen by the host’s blood.
c. Nervous: - Similar to Planaria but they lack most of the special sensory cells found in Planaria.
d. Muscle: -Similar to planaria, but lack cilia on the ventral epidermis.

3. TAPEWORMS

 

a. Digestion: -Have no mouth or gastrovascular cavity.



-Attach themselves to the lining of the host’s intestine by suckers and hooks (Scolex)



-Worm feeds by absorbing digested food.

- Has a modified epidermis called a Tegument, this thicker skin prevents them from being digested.
b) Circulation: -Receive oxygen from blood vessels in the host’s intestine.

-When oxygen is not available, they can live without it.
E) Reproduction In These Three Common Members of Platyhelminthes:

1. Planaria: -Reproduce asexually by fission (splitting)

-Missing parts will regenerate.

-Can also reproduce sexually.

-Hermaphrodites (contains both sexes in one individual), but requires reciprocal exchange of sperm and eggs between two individuals.

 

2. Flukes: -Also hermaphrodites.

-Complex life cycle with numerous larval stages that infect a number of hosts.



 

3. Tapeworms: - Constantly budding new body sections (proglottids) posterior to its scolex.

-Each proglottid contains both male and female reproductive organs.

-Mature proglottids with fertilized eggs detach and pass out of the host with the feces.

-Often have larval stages that infect a number of different hosts

 




Advances of the Platyhelminthes Over the Cnidaria

1. Platyhelminthes have their tissues organized into organs and even have some simple systems.


 

2) Unlike the previous two Phyla (Porifera and Cnidaria), the Platyhelmithes have 3 embryonic layers with the development of the middle layer (the mesoderm). The mesoderm has also provided for better muscle development and thus has resulted in an animal that moves around more efficiently.




3) As a result of moving around, platyhelminthes have further developed cephalization.

 

4) Cnidarians had a nerve net for conducting impulses, but the platyhelminthes have centralized the nervous system into two longitudinal nerve cords. (faster transmission of nerve impulses).


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