platyhelminthes, aschelminthes, annelida - topperlearning

BIOLOGY ANIMAL KINGDOM

Go to Top

www.topperlearning.com 2

Platyhelminthes, Aschelminthes, Annelida

Contents Phylum Platyhelminthes ...................................................................................................................................................... 3

Phylum Aschelminthes ........................................................................................................................................................ 6

Phylum Annelida ................................................................................................................................................................... 9


Go to Top


Phylum Platyhelminthes

Phylum Platyhelminthes includes about 13,000 species.

They are also called flatworms because of their dorsoventrally flattened body.

1. Habitat Majority of the flatworms are endoparasites.

Some flatworms are free-living which are either terrestrial, freshwater or

marine animals.

2. Shape Dorsoventrally flat

3. Symmetry Flatworms are bilaterally symmetrical.

4. Germ Layers Triploblastic. The outer ectoderm and inner endoderm enclose the

middle layer of mesoderm.

5. Level of

Organisation

Organ level of organisation

6. Body Wall In free-living flatworms, the outer epidermis is syncytial, ciliated.

Rhabdites are present in the epidermis of free-living flatworms.

In parasites, the body does not have an epidermis, but it is covered with

a cuticle.

7. Coelom Acoelomate

The space between the body wall and organs is filled with connective

tissue called parenchyma.

8. Respiration Free-living forms respire through the body surface.

Parasitic flatworms are anaerobes.

9. Circulation The circulatory system is absent.

10. Digestive Tract The digestive tract is incomplete, i.e. without an anus. However, a

muscular suctorial pharynx and highly branched intestine are present.

11. Excretion They are ammonotelic.

They possess protonephridia which contain flame cells.

12. Skeleton They do not have any skeleton.

Some flatworms have a cuticle which protects their body from the host’s

digestive juices.

Gegenbaur separated and placed all the flatworms under the phylum

Platyhelminthes.

He also coined the term ‘Platyhelminthes’.


Go to Top


13. Nervous System It consists of the brains and two longitudinal nerves.

Both nerves are connected at intervals by transverse commissures.

Hence, the nervous system appears ladder-like.

14. Sense Organs In free-living flatworms, a pair of eye spots is present at the anterior end

which is sensitive to light.

Absent in parasitic forms.

15. Organs of

Adhesion

In parasitic forms, there are organs of adhesion such as hooks, suckers

and spines.

16. Reproduction Asexual reproduction is by transverse binary fission.

Possess highly developed reproductive organs.

All flatworms are hermaphrodites, except Planaria which is a unisexual

animal.

Fertilisation is internal.

Taenia exhibits self-fertilisation.

17. Development Life cycle is complicated with one or more larval stages.

In liver fluke, different larval stages are miracidium, sporocyst, redia,

cercaria and metacercaria.

In tapeworms, the larval stages are onchosphere, hexacanth and

cysticerus.

18. Regeneration Planarians possess a great ability of regeneration.

The flame cell is a peculiar feature of Phylum Platyhelminthes.

It is a hollow cell whose broad base encloses a tuft of cilia or flagella.

Cilia or flagella beat incessantly which resembles the flickering

of a flame.


Go to Top


Classification of Phylum Platyhelminthes

There are three classes which fall under Phylum Platyhelminthes.

Turbellaria Trematoda Cestoda

o Free-living. o Either ectoparasites or

endoparasites.

o Endoparasites in

vertebrates.

o Body is unsegmented. o Body is unsegmented. o Body is made of

segments called

proglottids.

o Cuticle is present. o Thick tough tegument is

present as the outer cover.

o Cuticle is present.

o Organs of adhesion are

absent.

o Oral sucker is present. o Hooks, spines and

suckers are present.

o Example:

Planaria

o Example:

Fasciola (Liver fluke)

o Example:

Taenia (Tapeworm)

o Dugesia

o Polystoma

o Echinococcus (Dog

tapeworm)

Economic Importance of Flatworms:

Most of the flatworms are parasites and cause diseases in humans, sheep, goat and other

domesticated animals.


Go to Top


Phylum Aschelminthes

Phylum Aschelminthes is also known as Phylum Nematoda or Nemathelminthes.

The phylum includes about 15,000 species.

1. Habitat Either free-living or parasitic.

They may be terrestrial or aquatic.

2. Shape Cylindrical, unsegmented, thread-like with tapering ends.

3. Symmetry Round worms are bilaterally symmetrical.

4. Germ Layers Round worms are triploblastic.

5. Level of

Organisation


6. Body Wall Body is covered by a cuticle.

After the cuticle is the syncytial epidermis, i.e. all epidermal cells have

a continuous cytoplasm with many nuclei.

Muscle fibres are arranged in four longitudinal quadrants.

7. Coelom Pseudocoelomate

Pseudocoel is filled with pseudocoelomic fluid.

8. Respiration A respiratory system is absent.

9. Circulation A circulatory system is absent.

10. Digestive Tract The digestive tract is complete.

Shows the presence of the muscular pharynx.

11. Nutrition Parasitic and found on plants and in animals.

12. Excretion Excretion is carried out by a large H-shaped renette cell.

The group was proposed by Karl Grobben.


Go to Top


13. Skeleton Skeleton is absent.

14. Nervous System A circumpharyngeal ring is present from which nerves arise forward

and backward.

15. Sense Organs Sense organs such as papillae, amphids and phasmids are present.

16. Reproduction Sexes are separate.

Show sexual dimorphism. The male is smaller than the female, and

its posterior end is curved.

Fertilisation is internal.

Asexual reproduction is absent.

17. Development Development may be direct or indirect.

During indirect development, the larval stage is present.

Example: Filariform larva in Ancylostoma (hookworm)


Go to Top


Examples:

1. Ascaris (Roundworm)

2. Wuchereria (Filarial worm)

3. Ancylostoma

Some common parasitic nematodes:

Parasite Site of Parasite Mode of Infection Diseases Caused

1. Ascaris Intestine Contaminated food and

water

Ascariasis (anaemia,

diarrhoea)

2. Ancylostoma Intestine Larvae bore the feet and

enter the blood stream

Itching and inflammation

of the skin

3. Filarial worm Lymph Glands Bite of female Culex

mosquito

Elephantiasis


Go to Top


Phylum Annelida

Phylum Annelida includes about 9,000 species.

Animals of this phylum have a body made of ring-like segments, and hence, the phylum is named

Annelida. ‘Annulus’ is a Latin word which means ‘little ring’.

1. Habitat They may be terrestrial or aquatic. If terrestrial, they are found in

moist soil.

Some are parasitic too.

2. Shape and Size Body is soft, elongated and cylindrical.

3. Symmetry Bilaterally symmetrical

4. Germ Layers Triploblastic

5. Level of

Organisation


6. Body Wall Body is covered by a thin cuticle.

Below the cuticle is a dermo-muscular layer.

The dermis layer consists of circular and longitudinal muscles.

7. Coelom First true coelomate.

The coelom is filled with coelomic fluid consisting of coelomic cells.

8. Locomotory

Organs

Chitinous setae are present in all forms, except leeches.

In Nereis, parapodia are the locomotory organs.

9. Respiration Respiration is by moist skin, gills or modified parapodia.

10. Circulation Closed type of circulatory system is present.

11. Digestive Tract The digestive system is complex.

The digestive tract is made of muscular pharynx, gizzard and

intestine.

12. Excretion Nephridia act as excretory organs which are segmentally arranged.

13. Skeleton Coelomic fluid serves as the hydroskeleton and helps organisms in

locomotion.

14. Nervous System Paired ganglia connected to a double ventral nerve cord by lateral

nerves.

15. Sense Organs Various sense organs are present in annelids:

The term ‘Annelida’ was coined by Lamarck in 1801.


Go to Top


Statocyst is also present.

16. Reproduction Some animals are hermaphrodites and some are unisexual, i.e.

sexes are separate.

Asexual reproduction is absent.

17. Development Development may be direct (Example: Earthworm) or indirect.

During indirect development, the larval stage is the trochophore

larva.

Example: Nereis

Tactile Receptors

(Sensitive to touch)

Chemoreceptors (Sensitive to

Chemical Signals

Gustatory Receptors

(Receptors of Taste)

Photoeceptors (Sensitive to

Light)


Go to Top


Classification of Phylum Annelida

Polychaeta Oligochaeta Hirudinea

o Exclusively marine animals. o Freshwater or terrestrial

animals.

o Marine, freshwater or

terrestrial animals.

o Parapodia and setae are

present.

o Parapodia are absent and

setae are embedded in the

skin.

o Parapodia and setae are

absent.

o Suckers are absent. o Suckers are absent. o Suckers are present.

o Presence of trochophore

larva.

o Direct development without

any larval stage.

o Direct development without

any larval stage.

o Examples: Nereis

o Examples: Earthworm

o Examples: Leech

o Sabella (Peacock worm)

o Tubifex

o Pontobdella


Go to Top


Economic Importance of Annelids:

Earthworms are called farmer's friends or nature's ploughmen. They aerate the soil and make it fertile because of their mode of feeding.

Tubifex feeds on the organic matter present in sewage. Hence, their presence indicates sewage contamination.

Hirudo was earlier used to remove dirty blood from bruises and clots.

Leeches are ectoparasites on cattle and human beings.

platyhelminthes, aschelminthes, annelida - topperlearning

Documents