PRELIMS

Zoology: Invertebrate Classification

Animals are divided into Unicellular (Protozoa) and Multi-cellular (9 major Phyla).

I. Phylum Protozoa (Unicellular)

• Features: One cell, one or more nuclei; respiration and excretion by diffusion.
• Examples: Amoeba, Euglena, Trypanosoma.

II. Phylum Porifera (Pores)

• Features: Marine; bear numerous pores called Ostia. Skeleton made of calcareous or silicon spicules.
• Examples: Sycon, Sponge.

III. Phylum Coelenterata (Tentacles)

• Features: Aquatic; Diploblastic; radial symmetry; specialized Cnidoblast cells for catching food; tentacles around the mouth.
• Examples: Hydra, Jelly fish, Sea Anemone.

IV. Phylum Platyhelminthes (Flatworms)

• Features: Triploblastic; no body cavity; dorso-ventrally flattened; Flame cells for excretion; Hermaphrodite.
• Examples: Planaria, Tapeworm, Liver fluke.

V. Phylum Aschelminthes (Roundworms)

• Features: Cylindrical; complete alimentary canal (mouth and anus); Protonephridia for excretion; unisexual.
• Examples: Ascaris, Pinworm (causes itching in children), Wuchereia (causes Filarial disease).

VI. Phylum Annelida (Segmented)

• Features: Soft, metamerically segmented body; locomotion by Setae (Chitin); respiration through skin; excretion by Nephridia.
• Examples: Earthworm (has 4 pairs of heart), Leech, Nereis.

VII. Phylum Arthropoda (Jointed Legs)

• Largest Phylum in the animal kingdom.
• Features: Body divided into Head, Thorax, Abdomen; thick Chitinous exoskeleton; jointed legs; open circulatory system (Haemocoel).
• Insects: Have 6 feet and 4 wings (social like Ants/Termites).
• Examples: Cockroach (has 13 chambers in its heart), Prawn, Crab, Mosquito, Bees.

VIII. Phylum Mollusca (Soft Bodies)

• Features: Soft body divided into head and muscular foot; Mantle present (secretes calcareous shell); respiration through gills (Ctenidia).
• Examples: Pila, Octopus (Devil-fish), Sepia (Cuttle-fish), Loligo.

IX. Phylum Echinodermata (Spiny Skin)

• Features: Marine; Water vascular system; Tube feet for locomotion/sensation; high capacity of regeneration.
• Examples: Star fish, Sea urchin (contains Aristotle’s lantern to chew food), Sea cucumber.

UPSC Relevance

The structural diversity of invertebrates is a major contributor to GS-III Science.

• Specialized Cells: Identifying Flame cells (Platyhelminthes) or Cnidoblasts (Coelenterata) is a favorite for Prelims.
• Health Links: Understanding that Filaria is caused by a roundworm (Wuchereia) and Pinworm symptoms in children.
• Factual Records: The 13-chamber heart of the Cockroach and the 4 pairs of heart of the Earthworm.

X. Confused Pairs — Commonly Mislabelled Invertebrates

• Starfish, Jellyfish, Cuttlefish are NOT fish: Starfish (Echinodermata), Jellyfish (Coelenterata), Cuttlefish/Sepia (Mollusca) — none are vertebrate fish (Pisces), despite the name.
• Sponge vs Coral: Sponges (Porifera) have pores (Ostia) and lack true tissues; Corals are Coelenterates with Cnidoblasts and radial symmetry.
• Roundworm (Aschelminthes) vs Flatworm (Platyhelminthes): Roundworms have a complete alimentary canal (mouth + anus) and are unisexual; Flatworms lack a body cavity, are dorso-ventrally flattened, and are hermaphrodite.
• Octopus vs Squid vs Cuttlefish: All three are Mollusca (Cephalopods) — Octopus (8 arms, no shell), Squid/Loligo (10 arms, internal pen), Cuttlefish/Sepia (10 arms, internal cuttlebone).
• Open vs Closed Circulatory System: Arthropods (Cockroach, Prawn) have an open system (Haemocoel); Annelids (Earthworm) have a closed system with true blood vessels.