Introduction

Invertebrates are animals that lack a vertebral column (backbone). They form the majority of the animal kingdom and are classified into different phyla based on their structural characteristics.

1. Phylum Porifera (Sponges)

  1. Commonly known as sponges.
  2. They are asymmetrical and have a cellular level of organization.
  3. Body has numerous pores (Ostia) for water circulation.
  4. Skeleton made of spicules or spongin fibers.
  5. Reproduction is both asexual (budding) and sexual.
  6. Examples: Sycon, Spongilla, Euplectella.

2. Phylum Cnidaria (Coelenterata)

  1. Radially symmetrical, diploblastic animals.
  2. Body forms: Polyp (sessile) and Medusa (free-floating).
  3. Possess specialized stinging cells called cnidocytes for defense and capturing prey.
  4. Exhibit tissue-level organization.
  5. Examples: Hydra, Jellyfish, Coral, Sea Anemone.

3. Phylum Platyhelminthes (Flatworms)

  1. Bilaterally symmetrical, triploblastic, acoelomate animals.
  2. Dorsoventrally flattened body.
  3. Exhibit organ-level organization.
  4. Mostly parasitic (tapeworms, liver flukes).
  5. Reproduce sexually and show hermaphroditism.
  6. Examples: Planaria, Liver Fluke, Tapeworm.

4. Phylum Nematoda (Roundworms)

  1. Bilaterally symmetrical, triploblastic, pseudocoelomate animals.
  2. Body is elongated, cylindrical, and unsegmented.
  3. Complete digestive system with separate mouth and anus.
  4. Mostly parasitic but some are free-living.
  5. Sexes are separate (dioecious).
  6. Examples: Ascaris (Roundworm), Wuchereria (Filarial Worm), Hookworm.

5. Phylum Annelida (Segmented Worms)

  1. Bilaterally symmetrical, triploblastic, coelomate animals.
  2. Body is segmented (metameric segmentation).
  3. Possess a closed circulatory system.
  4. Excretion through nephridia.
  5. Examples: Earthworm, Leech, Nereis.

6. Phylum Arthropoda (Joint-Legged Animals)

  1. Largest phylum in the animal kingdom.
  2. Bilaterally symmetrical, triploblastic, coelomate animals.
  3. Body is divided into head, thorax, and abdomen.
  4. Possess jointed appendages.
  5. Exoskeleton made of chitin.
  6. Examples: Insects (Butterflies, Ants), Crustaceans (Crabs, Lobsters), Arachnids (Spiders, Scorpions).

7. Phylum Mollusca (Soft-Bodied Animals)

  1. Bilaterally symmetrical, triploblastic, coelomate animals.
  2. Body divided into head, muscular foot, and visceral mass.
  3. Exoskeleton made of calcium carbonate (shell).
  4. Possess a radula for feeding.
  5. Examples: Snails, Octopuses, Squids, Clams.

8. Phylum Echinodermata (Spiny-Skinned Animals)

  1. Radially symmetrical (adults), bilaterally symmetrical (larvae), triploblastic, coelomate animals.
  2. Possess a water vascular system for locomotion.
  3. Have an internal skeleton made of calcium carbonate.
  4. Regeneration ability is well developed.
  5. Examples: Starfish, Sea Urchins, Sea Cucumbers.

Additional Key Points

  1. Invertebrates lack a backbone and have diverse body structures.
  2. Porifera have the simplest body plan with no true tissues.
  3. Cnidarians possess stinging cells for defense and capturing prey.
  4. Platyhelminthes are the first group to show bilateral symmetry.
  5. Nematodes have a complete digestive system and separate sexes.
  6. Annelids have true segmentation, improving movement and specialization.
  7. Arthropods are the most successful phylum due to their exoskeleton and jointed appendages.
  8. Mollusks are the second-largest phylum and have a unique radula for feeding.
  9. Echinoderms are exclusively marine and show radial symmetry as adults.
  10. Understanding invertebrate classification helps in studying evolution and biodiversity.

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