INTRODUCTION

• Non-chordates are animals lacking a Notochord.
• Notochord is a rod-like elastic structure which gives rise to the vertebral column. Hence, non-chordates do not possess a vertebral column and are termed as invertebrates.

PHYLA OF NON-CHORDATES

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

1. Phylum Porifera
2. Phylum Coelenterata (Cnidaria)
3. Phylum Platyhelminthes
4. Phylum Nematoda
5. Phylum Annelida
6. Phylum Arthropoda
7. Phylum Mollusca
8. Phylum Echinodermata
9. Phylum Hemichordata

PHYLUM PORIFERA

• Members of this phylum are pore-bearing animals, commonly known as the sponges.
• 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 encloses a hollow body cavity known as
• 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.
• 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.
• 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 the animals under this phylum shows radial symmetry.
• Coelenterates are diploblastic-composed of two germ layers namely the ectoderm and the endoderm.
• They reproduce predominantly by asexual means (budding) though sexual reproduction is also present.
• Coelenterates are also known as Cnidarians due to the presence of special cells called Cnidocytes.
• They lack a nervous and a circulatory system. Excretion and respiration occurs through simple diffusion.
• Coelenterates are found in two basic forms:

1. Polyps
2. 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.

PHYLUM PLATYHELMINTHES

• Platyhelminthes are soft and dorso-ventrally compressed organisms.
• They are also known as flatworms.
• They are triploblastic-consisting of three germ layers namely, the ectoderm, mesoderm and endoderm.
• Platyhelminthes are acoelomates (lacking a coelom/body cavity).
• The body of these organisms is un-segmented, 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
• 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.

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 body 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, Brittle star etc.

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 un-segmented 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