- 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
- 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.
- Sycon: It is a typical marine sponge.
- Leucosolenia: Sponge consisting of assembly of erected tubes.
- 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:
- 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.
- Hydra: A freshwater – coelenterate only exists in polyp form.
- Obelia: A marine colonial coelenterate that exhibits alternation of generations.
- 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.
- Dugesia (planaria): a free-living flatworm with a ciliated outer surface.
- Fasciola (Liver Fluke): an endo-parasite of sheep and occasionally in human beings.
- 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.
- Ascaris lumbricoides: An intestinal parasite of man
- Enterobius Vermicularis (pinworm): Parasites of the human large intestine.
- 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.
- Nereis: A marine water annelid.
- Earthworms: Annelids that are found in damp soil.
- 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.
- Insects such as ants, dragonflies, and bees.
- Arachnids such as spiders and scorpions.
- 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.
- Helix aspersa: Also known as garden snail.
- Anodonta: A freshwater mussel.
- 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
- Asterias (starfish)
- Sea urchin
- Sea cucumber
- Cake urchin
- Brittle star
- 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.
- Arihant’s handbook of biology. Animal Kingdom. Page no: 43-73.
- NCERT biology; textbook for class 11. Animal Kingdom. Page no: 46-62.