Platyhelminthes: General Characters and classification B Sc.F.Y. Zoology (First Semester)

Platyhelminthes: General Characters and classification

B Sc.F.Y. Zoology (First Semester)

Morphological Advances:

Platyhelminthes shows an advancement over the previous metazoan phyla (coelenterata and ctenophora) in the following respects

1. Tissue-organ level of organization.

2. Body is Triploblastic . Three layer

mesoderm instead of mesogloea.

3. Definitely bilateral Symmetry of body at the lowest level. 

4. Head and tail are definite in shape.

5. Mesodermal muscles in bundles and layers. 

6. More complex and efficient gastrovascular cavity.

7. Nervous system are large ganglionated , anteriorly situated ganglia and nerve cords running along the body.

8. Better developed sensory organs. oliw

9. Internally situated gonads with permanent gonoducts and copulatory structures.

10. Behaviour organization through learning.

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General Characters of Platyhelminthes:

1. Free-living, commensal or parasitic forms. 2. Body is Tissue organ of organization and that is the body cells aggregate into definite tissues and make up tissues organs.

3.Body derived from three embryonic germ layers as a Triploblastic such as ectoderm, mesoderm and endoderm. 

4.Body is Bilaterally symmetrical with definite anterior head and posterior tail.

5. Body is Dorso-ventrally flattened. Usually with a well-defined ventral surface bearing mouth and gonopore.

6. Body unsegmented (except in class Cestoda). 

7. Body is Acoelomate that is without any body cavity. 

8. In parasitic form Adhesive hooks, spines and suckers and adhesive secretions are in a common present.

9. Epidermis cellular or syncytial, frequently ciliated. Absent in some.. 

10. Muscular system of mesodermal origin. 

11. Digestive system branched and incomplete without anus. Altogether absent in Acoela and Cestoda.

12. Internally bodys Skeletal, respiratory and circulatory systems are wanting.

13. Excretory system includes lateral canals and protonephridia (flame cells). Absent in some primitive forms.

14. Primitive type and ladder Nervous system . 

15. Nervous system Contains a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves.

16. Simple Sense organs develop . 

17. Mostly amimals are monoecious such as hermaphrodite with complex reproductive system and Well developed gonads, gonoducts and accessory organs. Eggs mostly devoid of yolk. Yolk produced separately in vitelline glands.

18. Fertilization internal somtime may be cross or self fertilization.

19. Development is direct or indirect. 

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Classification of Platyhelminthes:

Class 1. Turbellaria

(L., turbella, a stirring)

1. Usually animals are non-parasitic in nature. It is free-living worms are called planarians. 

2. Terrestrial marine or freshwater.

3. Body unsegmented, flattened and covered wit ciliated cellular syncytial epidermis or containing mucus secreting cells and rod shaped bodies called rhabdites.

4. Mouth ventral. Intestine preceded by muscula pharynx.

5. Suckers absent. 

6. Mostly hermaphroditic. Some reprodu asexually. Development usually direct. 

Class 2. Trematoda

(Gr., tremta, hole + eidos, form)

1. Ecto or endoparasitic flatworms called flukes. 

2. Body unsegmented dorso-ventrally flattened, leaf-like. Thick Tegument without cilia and rhabdites.

3. Suckers and sometimes hooks present.

4. Alimentary canal contains with anterior mouth,pharynx and two main branches including it. 

5. Three pairs of longitudinal nerve cords.

6. Mostly monoecious. Development is direct or indirect

Class 3. Cestoda ( Cestoidea)

(Gr., kestos, girdle + eidos, form)

1. Endoparasitic flatworms called tapeworms. 

2. Body segmented, elongate, flat, ribbon like. Tegument with microvilli.

3. Head or Scolex with suckers or hooks.

4. No alimentary canal. No sense organs.

5. In male and female are monocuous proglottis. It is mature segment .

6. Life cycle is completed into two intermediate host and life cycle is also a complicated system .Embryos with hooks.

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