Chapter 3 Animal Kingdom Part 3 (Phylum Platyhelminthes, Nematoda , Annelida and Arthropoda) by TEACHING CARE online tuition and coaching classes

Chapter 3 Animal Kingdom Part 3 (Phylum Platyhelminthes, Nematoda , Annelida and Arthropoda) by TEACHING CARE online tuition and coaching classes


Animal Kingdom Part 3

(Phylum Platyhelminthes, Nematoda, Annelida and Arthropoda)

Phylum Platyhelminthes

The phylum Platyhelminthes includes flatworms (Gr., platys =flat;

helmins = worms); They have a dorsoventrally flattened body like a leaf. About10,000 species known.


General characters


  • They show organ level of body
  • They are acoelomate animals.The cavity in platyhelminthes is filled with mesenchyme or parenchyma
  • First time in animal kingdom, triploblastic condition is established having three germ layers namely ectoderm, mesoderm and endoderm


  • First time, bilaterally symmetrical
  • Some members have segmented body but the segmentation in platyhelminthes is called as pseudometamerism
  • Some of the parenchyma cells give rise to muscle fibres. The muscle fibres are arranged in circular, longitudinal and vertical


  • The digestive system is incomplete e. it does not have anus. Alimentary canal is absent in absent in Cestoda and Acoela. The alimentary canal is branched in Turbellarians.
  • The respiratory organs are absent. In parasites, respiration is anaerobic
  • There is no circulatory system


  • The excretory system consists of protonephridia (flame cells)



  • The nervous system is formed of longitudinal nerve cords with A pair of anterior ganglia form the brain. The longitudinal nerve cords are connected together by transverse connectives.
  • They are monoecious / hermaphrodites, e., both the male and female sex organs are present in the same organism


  • Fertilization is Self or cross fertilization takes place, cross fertilization is more common, development is direct or indirect. Endoparasites normally show indirect development with many larval stages. Their life cycle is completed involving one or two hosts.


  • They are free living or The parasitic worms have adhesive organs like hooks, spines, suckers and adhesive secretions.



(a) Class 1. Turbellaria


  • Most turbellarians are free living but some of them are ecoto- commensal or parasitic
  • The epidermis is either cellular or syncytial and covered with

Epidermis contains rhabdites

  • No Segmentation
  • Alimentary canal is present except in a few
  • Suckers are absent
  • Life cycle is simple Example : Dugesia (= Planaria)



Class (2) – Trematoda

  • They are ecto or endoparasites of vertebrates; commonly called


  • Body without cilia, but covered by a thick, resistant
  • Suckers, hooks and spines, present for attachment to the host

Examples : Polystomum,Fasciola, Schistosoma (blood fluke of man and other mammals).

Life Cycle of Fasciola hepatica



Class (3) – Cestoda

  • All cestodes are Mostly in alimentary canal of vertebrates; commonly called tapeworms.
  • Body long, slender, tape-like, usually divided into small segments called proglottids).
  • Body non-ciliated but with a thick



  • Anterior end with


  • Mouth and digestive system absent ; digested food is absorbed from host tissues by diffusion through body
  • Sense organs
  • Each proglottid contains one or two complete sets of hermaphrodite (bisexual) reproductive
  • Life-cycle usually complicated with alternation of Embryo hooked. Examples – Taenia





PhylumNematoda /Nemathelminthes

The most important characteristics of phylum nematoda is that the space between body wall and alimentary canal is not lined by peritonium / mesoderm. Such a body cavity is known as a false body cavity, or pseudocoel. The pseudocoel represents the persistant embryonic blastocoel. The term “Nematoda” literally means “threadworms” (Gr., nema = thread + eidos = form) or“roundworms” About 12,000species are known.

General characters:


  • The nematodes have a soft, cylindrical, unsegmented and elongated body usually tapering at both


  • Body wall is composed of a cuticle, asyncytial hypodermis beneath cuticle, and an innermost layer of large peculiar longitudinally extended muscle cells arranged in four


  • The pseudocoel is filled with a fluid without free cells
  • The alimentary canal is complete with a terminal mouth and anus and has a triradiate pharynx
  • The Circulatorysystem and respiratory organs are
  • A simple excretory system in the form of a Rennete cell is


  • The nervous system is well–
  • Reproductive system well–developed. Usually unisexual with sexual dimorphism.
  • Many Nematodes areparasites of many useful plants and domestic Some of them are pathogenic causing serious diseases. Even man is a host for more than 50species,out of which Ascaris lumbricoides and enterobius vermicularis (pinworm) are quite common. Other common human nematodes are Wuchereria which causesFilaria, Trichinella causing trichinosis, and Ancylostoma causing hookwormdisease.



T.S. Body of a roundworm







On the basis of the presence or absence of some specialized sense organs and caudalglands, nematodes are classified intotwo classes:



Class (1) Phasmidia


  • Mostly


  • They have a pair of unicellular, pouch-like caudal sense organs, called
  • A pair of reduced, pore like sense organs, called amphids are presentnear anterior
  • Excretory system has paired lateral


  • Caudalglands are


Examples – Ascaris, Enterobius, Ancylostoma, Wuchereria, etc.


Class(2) Aphasmidia:


  • Mostly small, free-



  • Amphids spiral, cord like or disc like, rarely pore


  • No lateralexcretory


  • Caudal glands


Examples – Tichinella,



Phylum Annelida

The annelids have a long, narrow, worm like body which is divided into ring-like, true (metameric) segments (Gr., annelus = little ring.

Therefore, they are also known as “segmented worms”. About 9,000 species are known.


General Characters

  • They are bilaterally symmetrical animals having organ-system level of body organization
  • They are triploblastic, coelomate First time true coelom

appears in annelida in the animal kingdom

  • The body wall has thick muscle layers hence called as dermo- muscular
  • The body is divided into a of segments called the metameres.

The segmentation is known as metamerism

  • Locomotory takes place by unjointed chitinous appendages called


  • Digestive system complete
  • Blood vascular system is a closed type
  • Excretory system consists of segmentally arranged nephridia.


  • Nervous system consists of a pair of cerebral ganglia (brain) and a double ventral nerve cord
  • The annelids are mostly The gonoducts are derived from coelom (coelomoducts). The coelomoducts are connected with nephridia The development is direct or indirect




Class 1. Polychaeta

  • They are marine and
  • Head region consists of prostomium and peristomium and bear eyes, tentacles, cirri and palps,
  • Setae are numerous and are born up on lateral appendages of the body wall known as parapodia
  • Clitellum absent
  • Alimentary canal has an eversible buccal region and protrusible pharynx with
  • Excretory organs are segmentally paired nephridia
  • Saxes are separate
  • Fertilization is external; free swimming larval stage is trochophore
  • Asexual reproduction occurs by budding. Examples : Nereis, Aphrodite, Polynae,

Trochophore larva present in polychaeta (Neries). Aphrodite is commonly known as Sea mouse.


T.S. of a polychaeta showing the presence of a pair of parapodia.



Class 2. Oligochaeta

  • They are mostly terrestrial, some fresh water
  • Their body has conspicuous external and internal
  • Dsitinct head, eyes and tentacles are
  • Parapodia
  • Setae usually arranged
  • Clitellum is mostly
  • Pharynx is not eversible and without
  • They are hermaphrodites, development direct and takes place within cocoons secreted by clitellum.
  • No free larval stage Examples : Tubifer, Pheretima, (Earthworms).


Class 3. Hirudinea

  • The members of this class are mostly ecto-parasitic and fresh water forms, a few
  • Body consists of definite number of segments, each segments further breaks up into 2 to 4 rings or annuli
  • Parapodia and setae absent


  • Body has an anterior and a posterior sucker, both located ventrally
  • They are hermaphrodite e., sexes united. Reproduction sexual. Eggs are usually laid in cocoons. Development is direct without free swimming larval stage

Examples : Acantaobdella, Hirudinaria (Leeches)


Class 4 Archiannelida

  • They are small, marine exclusively marine,worm-like annelids
  • Setae and parapodia are usually absent
  • Prostomium bears two or three tentacles
  • Larva is typical trochopore Examples : Polygordius



Phylum Arthropoda

The arthropods are    metamerically segmented animals with each body segment having a pair of jointed locomotory appendages. “Arthopoda” literally means jointed feet (Gr., arthron = jointed + podos = foot) animals. They also have a protective chitinous exoskeleton.


General characters

  • The pair of lateral jointed appendages in each segment is adapted for food ingestion, locomotion, respiration, copulation, etc.
  • Muscular system well-developed whose muscle fibres are always
  • Digestive tract complete. Most head appendages form mouth parts with lateral jaws for chewing or
  • Coelom restricted to small cavities in excretory and reproductive organs; It is replaced elsewhere by blood sinuses which fuse together to form a large perivisceral cavity known as haemocoel. The Sinuses form an “open blood vascular system” and are filled with haemolymph which


may contain haemocyanin. Haemocoel communicates with a long tubular mid–dorsal heart.

  • Respiration takes by gills, book gills ( in aquatic forms), or tracheae or book lungs (in terrestrial forms) and by diffusion through general body surface in some forms
  • Excretion takes place by specialized green glands or coxal glands, or

malpighian tubules

  • Nervous system annelidan type; head with a brain-ring which is connected to a double ventral nerve cord, having paired segmental
  • Sexes mostly separate with sexual dimorphism
  • Fertilization internal, in female’s body. Eggs megalecithal. Oviparous or viviparous
  • Life–cycle includes one or more larval stages that metamorphose into


Anterior surface of the head of an insect


Parts of an Arthropod leg


Classification :

Subphylum Trilobita

  • Most primitive, extinct, marine arthropods of Cambrian to Permian Body covered by a hard segmented shell; They have a distinct head of four fused somites with a pair of antennae, four pairs of appendages and a pair of eyes.
  • Trunk is divided into 3 lobes by two longitudinal
  • Each segment, except the last one, bears a pair of biramous jointed

Example – Triarthrus.

Subphylum Chelicerata

  • They are mostly terrestrial, free living and small sized.
  • Body distinguished into head, thorax and abdomen (= opisthosoma). Head and thorax are fused to form a cephalothorax or prosoma.
  • Cephalothorax bears eyes and six pairs of appendages – One pair of clawed and jointed chelicerae in place of mandibles, one pair of pedipalps, and four pairs of walking legs. Antennae absent. Abdomen may or may not have appendages, but is distinguished into a large and broader mesosoma, a small metasoma and a long and narrow, tail–like
  • Respiration by gills, book lungs or
  • Excretion by malpighian tubules or coxal glands, or

(5) Sexes mostly separate; females oviparous; development direct or through a larval stage.

This subphylum is divided into three classes-

Class (1) – Merostoma

  • They are
  • Respiration by
  • Cephalothroax with lateral compound eyes and six pairs of
  • Abdomen with 5 to 6 pairs of gill bearing
  • Hind end forms a long like

Example– Limulus (The king-crab). Limulus is a living fossil.


Class (2) – Arachnida

  • Mostly
  • Respiration by book lungs or
  • Abdomen without
  • Some have poison glands and poison fangs, jaws of stings.
  • Gills
  • Life cyle without

Examples – spiders, scorpions, mites, ticks, Palamnaeus (scorpion), Lycosa (the common web–spinning spider; web–spinning glands are located in posterior part of abdomen)

Class (3) – Pycnogonida or Pentapoda

  • Small marine sea
  • Cephalothorax is 3 segmented; forms major part of body; abdomen
  • Suctorial mouth on top of a long
  • No special respiratory and excretory
  • Unisexual; females oviparous. Eggs carried by males. Example –

Subphylum Mandibulata or Antennata

  • Body divided into head and trunk, or head, thorax and
  • 1 or 2 pairs of antennae, 1 pair of mandibles in place of chelicerae, one or more pairs of maxillae and 3 or more pairs of walking legs.
  • Respiration by gills or
  • Excretion by malpighian tubules or antennal
  • Unisexual

It is divided into six classes

Class (1) – Crustacea

  • Mostly
  • Body divided into cephalothroax and
  • On dorsal side, cephalothorax covered by a thick exoskeletal called
  • Respiration through body surface or by
  • Excretion by special coxal glands in antennae or


Examples –Palaemon (prawn), Cancer(Crab) , Cyclops (Water-flea),


Class (2) – Insecta

  • Aquatic, terrestrial or
  • Body divided into head, thorax and
  • Segments 3 in thorax and 10 in
  • Legs typically 3 pairs (Hexapoda); The aerial forms have one or two pairs of wings.
  • Head with 1 pair of large, compound eyes, 1 pair of antennae and variously modified mouth–parts.
  • Respiration by tracheal sysem.
  • Excretion by specialized malpighian tubules.
  • Unisexual; females

Examples – Periplaneta (Cockroach), Musca (house-fly), Mosquitoes, locusts butterflies, bees, wasps, termites, silverfish (Lepisma), beetles, etc. Silverfish is not a fish.



Parts of an insect body



Class (3) – Diplopoda (Millipedes)

  • Terrestrial, long body, cylindrical, worm
  • Head with 1 pair each of antennae, mandibles and maxillae; 2 pairs


of simple eyes.

  • Thorax of has 4 segments, each except the first bears a pair of jointed
  • Abdomen with each segment apparently formed by fusion of two and, hence, bears 2 pairs of legs.
  • Respiration by
  • Excretion by malpighian
  • Unisexual; gonad single; females oviparous. Example – Julus (millipede)


Class (4) – Chilopods (Centipedes)

  • Terrestrial with long, worm like body divided into head and
  • Boy segments not fused in pairs; each segment with a single pair of legs; first pair of legs is claw like and contains a poison
  • Respiration by
  • Unisexual; females oviparous or
  • Excretion by malpighian tubules. Example – Scolopendra (centipede).



Practice Test Paper


  1. Cestodes are distinguished from other flatworms by the absence of

(a) Nervous System (b) Digestive system (c) Excretory system (d) Reproductive system

  1. Playhelminthes represents an example of

(a) Cellular grade of organisation (b) Tissue grade of organisation (c) Organ-system grade of organisation (d) Tissue-organ grade of organisation


  1. The excretory organs of flatworms are


(a) Flame cells (b) Nephridia (c) Malpighian tubules (d) Rennate glands

  1. Body cavity of Ascaris is

(a) Pseudocoel (b) Enterocoel (c) Coelom (d) Acoelom

  1. Metamerically segmented body is present in

(a) Earthworm (b) Leech (c) Nereis (d) All of the above


  1. Earhworm is placed in the group

(a) Oligochaeta (b) Polychaeta (c) Hirudinea (d) Crustacea 7.Peripatus is a connecting link between

(a) Reptiles and birds (b) Annelida and arthropoda (c) Mollusca and annelida (d) Mollusca and arthropoda

8.The connecting link between annelida and mollusca is

(a) Neopilina (b) Nautilus (c) Glochidium larva (d) Valiger larva

‘9.Hexapoda’ is another name of

(a) Crustacea (b) Arachnida (c) Insecta (d) Archiannelida

  1. In cockroach, the excretory organs are

(a) Malpighian tubules (b) Nephridia (c) Malpighian corpuscles (d) Solenocytes (flame cells)

  1. The Nematodes have simple excretory system in the form of a ……
  2. The excretory organs in platyhelminthes include special cell known as……..
  3. The true coelom in the animal kingdom appears first time in the phylum…..


  1. The locomotion in Annelids takes place by unjointed chitinous appendages called as …..


  1. What is clitellum
  2. Enlist respiratory organs found in Arthropods
  3. Mention atleast three excretory organs found in Arthropoda
  4. Write down the parts of an arthropod leg in a sequence
  5. Write down the respiratory and excretory organs in insects
  6. What is pseudocoel?

Answers and solutions

  1. b
  2. c
  3. a
  4. a
  5. d
  6. a
  7. b
  8. a
  9. c
  1. a
  2. Rennete cell
  3. Flame cells


  1. Annelida
  2. Setae
  3. The clitellum is a glandular tissue which secrets cocoon during development
  4. gills, book gills, tracheae and book lungs
  5. green glands, coxal glands and malpighian tubules
  6. Coxa, basidium, Ischium, merus, carpus, propodus, and dactylus
  7. Respiration by tracheal sysem and excretion by specialized malpighian tubules
  8. The most important characteristics of phylum nematoda is that the space between body wall and alimentary canal is not lined by peritonium / mesoderm. Such a body cavity is known as a false body cavity, or The pseudocoel represents the persistant embryonic blastocoel

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