[Biology] Non-Chordates

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INTRODUCTION

  • Non-chordates are animals lacking a Notochord.
  • Notochord is a rod-like elastic structure which gives rise to the vertebral column.
  • Therefore, non-chordates do not possess a vertebral column and are termed as invertebrates.
  • All organisms from phylum porifera to phylum hemichordata are referred to as non-chordates.

PHYLA OF NON-CHORDATES

All the organisms included in following phylums of animal kingdom are non-chordates:

  • Phylum Porifera
  • Phylum Coelenterata (Cnidaria)
  • Phylum Platyhelminthes
  • Phylum Nematoda
  • Phylum Annelida
  • Phylum Arthropoda
  • Phylum Mollusca
  • Phylum Echinodermata
  • Phylum Hemichordata

PHYLUM PORIFERA

  • Members of phylum porifera are pore-bearing animals, commonly known as the sponges.
  • Among all 5000 species of Poriferans, 150 species live in freshwater while the remaining are marine organisms.
  • Poriferans show cellular aggregate level of organization- the cells are grouped into an aggregation. They lack tissue organization and have no organs.
  • Sponges lack symmetry and therefore are termed as asymmetrical.
  • They possess a vase or cylinder-like body.
  • Their body wall is formed of an outer layer and inner layer and enclose a hollow body cavity known as spongocoel.
  • The outer layer is the pinacoderm, which consists of cells called pinacocytes.
  • The inner layer is choanoderm made of flagellated collar cells called choanocytes.
  • The body surface is perforated due to the presence of numerous pores. These pores are termed as Ostia, through which water enters the body of sponges.
  • Sponges consist of one or more large openings, the oscula by which the water exists.
  • All members of Porifera range in size from a few millimeters wide to more than one meter tall.
  • They are macroscopic and can be seen with a naked eye.
  • The adult sponges are sessile and spend their lives attached to the rocks at the bottom or other solid objects. However, larvae of Poriferans are able to move (swim).
  • There is no definite nervous system in Poriferans.
  • Sponges reproduce both by asexual and sexual means.

FOR EXAMPLE

  • Sycon: It is a typical marine sponge.
  • Leucosolenia: Sponge consisting of assembly of erected tubes.

PHYLUM COELENTERATA

  • Coelenterata consists of aquatic animals found in marine as well as in freshwater habitats.
  • Members of this phyla display a tissue level of organization.
  • Body of Coelenterata shows radial symmetry.
  • Coelenterates are diploblastic-composed of two germ layers i.e. the ectoderm and the endoderm.
  • They reproduce predominantly by asexual means (budding) however sexual reproduction is also observed in some coelenterates.
  • Coelenterates are also known as Cnidarians due to the presence of special cells called cnidocytes. These cells give rise to another type of cell-nematocysts, which are the stinging structures.
  • Body cavities are present in coelenterates. The gastro-vascular cavity in coelenterates acts as digestive as well as the body cavity.
  • They lack a nervous and a circulatory system. Excretion and respiration occurs through simple diffusion.
  • Coelenterates are found in two basic forms:
    • Polyps
    • Medusae
  • They have alternation of generation in their life cycle.
  • Coelenterates exhibit polymorphism( occurrence of structurally and functionally more than two different types of individuals from the same organism)
  • Specialized polyps in the coelenterates form coral reefs.

FOR EXAMPLE

  • Hydra: A freshwater – coelenterate only exists in polyp form.
  • Obelia: A marine colonial coelenterate that exhibits alternation of generations.

Cnidaria - Wikipedia

PHYLUM PLATYHELMINTHES

  • Platyhelminthes are soft and dorsoventrally compressed organisms
  • They are also known as flatworms.
  • They are triploblastic-consisting of three germ layers i.e. the ectoderm, mesoderm and endoderm.
  • Platyhelminthes are acoelomates (lacking a coelom/body cavity).
  • The body of these organisms is unsegmented, however they exhibit bilateral symmetry.
  • They have an organ level of organization.
  • They have a poorly developed, branching sac-like digestive system.
  • The excretory system of platyhelminthes consists of branching tubes having bulb-like cells at the ends called flame cells.
  • Flatworms have a well-developed nervous system however; respiratory and circulatory systems are absent. Respiration occurs by simple diffusion through the body surface.
  • The free living forms of Platyhelminthes are motile and movement occurs by the help of cilia.
  • Reproduction occurs by both sexual and asexual means.
  • Sexual species are termed as hermaphrodite i.e. both male and female reproductive organs are present in the same individual.
  • They possess the ability to regenerate.

FOR EXAMPLE

  • Dugesia (planaria): a free-living flatworm with a ciliated outer surface.
  • Fasciola (Liver Fluke): an endo-parasite of sheep and occasionally in human beings.

PHYLUM NEMATODA

  • The name nematode means “pointed ends”
  • Nematoda is also known as Aschelminthes.
  • The animals included in this phylum have an elongated, cylindrical, worm like body with pointed ends.
  • They possess a tissue level of organization.
  • They are triploblastic pseudocoelomates.
  • They possess an unsegmented body and display bilateral symmetry.
  • They occur either as free living or parasitic.
  • The digestive system has an alimentary canal with two openings i.e. mouth and anus.
  • They have sensory structures in the form of sensory papillae which are present on the mouth end.
  • Locomotion in nematodes occurs through undulating waves of contraction and relaxation.
  • The circulatory and respiratory systems are absent.
  • The members of this phylum are parasites and cause diseases in humans and plants.

FOR EXAMPLE

  • Ascaris lumbricoides: An intestinal parasite of man
  • Enterobius Vermicularis (pinworm): Parasites of the human large intestine.

The Phylum Nematoda consists of the species commonly know...

PHYLUM ANNELIDA

  • Phylum Annelida mainly consists of segmented worms
  • The animals are triploblastic coelomates
  • They have a segmented body and exhibit bilateral symmetry.
  • They possess a well-developed circulatory and digestive system.
  • Digestive system is in the form of an alimentary canal which has two openings, mouth and anus.
  • Excretion occurs with the help of specialized structures called nephridia.
  • A well-developed central nervous system is present consisting of a simple brain and a solid double, longitudinal, ventral nerve cord.
  • Annelids are the first group of invertebrates which occupy a closed circulatory system.
  • They lack a respiratory system; exchange of gasses occurs through the body surface.
  • They possess the ability of regeneration.
  • Circular and longitudinal muscles in the body wall help in the locomotion.

FOR EXAMPLE

  • Nereis: A marine water annelid.
  • Earthworms: Annelids that are found in damp soil.

PHYLUM ARTHROPODA

  • Arthropods are triploblastic animals.
  • They possess a segmented body and display bilateral symmetry.
  • They show an organ level of organization.
  • The word “arthron” means joint whereas “pods” means feet. They are named so due to the presence of jointed appendages.
  • These appendages are modified to perform locomotion.
  • Their body is divided into a head, thorax and abdomen.
  • They have an open circulatory system and haemocoel serves as a circulatory fluid.
  • The digestive system is in the form of an alimentary canal with two openings, the mouth and anus.
  • A highly developed excretory system consisting of Malpighian tubules is present in arthropods.
  • The nervous system of arthropods consists of paired ganglia (simple brain) connected to a ventral double nerve cord
  • A pair of compound eyes and antennae forms the sensory organs of arthropods.
  • They undergo metamorphosis which is characterized by abrupt change of form and structure during the life cycle of organisms.

FOR EXAMPLE

  • Insects such as ants, dragonflies, and bees.
  • Arachnids such as spiders and scorpions.

PHYLUM MOLLUSCA

  • This phylum includes over 50,000 living species and is the second largest phylum of invertebrates. Animals such as snails and clams are included in this phylum.
  • Mollusks are triploblastic coelomates with a bilaterally symmetrical body.
  • Their body is unsegmented and soft.
  • An organ system level of organization is found in these organisms.
  • The boy of mollusks is divided into a head, visceral mass, muscular foot and mantle.
  • Mantle is the glandular epithelial envelope covering the body of mollusks which secretes a calcareous shell.
  • They have highly organized and complex digestive, respiratory, circulatory, excretory, nervous and reproductive systems.
  • Their digestive system consists of a gut with two openings, the mouth and the anus.
  • Respiration occurs through the body surface, gills or pulmonary sac.
  • Excretion occurs with the help of paired nephridia.
  • Their organ of locomotion is a muscular foot; however in many species the movement is slow.

FOR EXAMPLE

  • Helix aspersa: Also known as garden snail.
  • Anodonta: A freshwater mussel.

PHYLUM ECHINODERMATA

  • There are over 5,000 known species of echinoderms.
  • They are exclusively marine organisms with a star shaped appearance and are spherical or elongated.
  • Echinoderms are triploblastic coelomates and possess radial symmetry.
  • They display an organ level of organization.
  • They possess an open circulatory system.
  • Respiration occurs through cloacal respiratory trees or gills.
  • They have an unsegmented body with a radial nervous system and an excretory system is absent.
  • They possess the ability of regeneration.
  • The motile species of this phylum move with the help of tube feet

FOR EXAMPLE

  • Asterias (starfish)
  • Sea urchin
  • Sea cucumber
  • Cake urchin
  • Brittle star

PHYLUM HEMICHORDATA

  • The members of this phylum show a combination of both invertebrate (Echinoderm) and chordate characteristics.
  • They have a close relationship to chordates and that’s why they are called pre-chordates.
  • They are mostly marine animals and may live solitary or in the form of colonies.
  • They possess a cylindrical and unsegmented body.
  • Their body is divided into an anterior proboscis, collar and trunk.
  • Their body wall is made of unicellular epidermis with mucus-secreting cells.
  • Complete digestive system is present in these organisms.
  • They have a closed circulatory system with a heart and two vessels.
  • Respiration occurs by ill-slits forming a dorsal row behind the collar.
  • The proboscis glands constitute the excretory system in these organisms.

FOR EXAMPLE

  • Balanoglossus
  • Saccoglossus.

REFERENCES

  • Arihant’s handbook of biology. Animal Kingdom. Page no: 43-73.
  • NCERT biology; textbook for class 11. Animal Kingdom. Page no: 46-62.
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