Major Animal Phyla

The Kingdom Animalia is classified into several phyla based on structural and functional complexities.

Phylum Porifera (Sponges)

• Level of Organization: Cellular level.
• Habitat: Mostly marine, generally asymmetrical.
• Water Canal System: Water enters through minute pores called Ostia into a central cavity (Spongocoel) and exits through the Osculum. This system aids in food gathering, respiratory exchange, and waste removal.
• Specialized Cells: Choanocytes (collar cells) line the spongocoel and canals.
• Skeleton: Made of spicules or spongin fibers.
• Example: Sycon, Euspongia (Bath sponge), Spongilla (Freshwater sponge).

Phylum Coelenterata (Cnidaria)

• Level of Organization: Tissue level; Diploblastic; Radial symmetry.
• Cnidoblasts: Specialized stinging cells (nematocytes) present on tentacles used for anchorage, defense, and prey capture.
• Digestion: Extra-cellular and intra-cellular; central gastro-vascular cavity with a single opening (hypostome).
• Body Forms: Two basic forms—Polyp (sessile and cylindrical like Hydra) and Medusa (umbrella-shaped and free-swimming like Jellyfish).
• Example: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Meandrina (Brain coral).

Phylum Ctenophora (Sea Walnuts/Comb Jellies)

• Habitat: Exclusively marine; Radial symmetry; Diploblastic.
• Comb Plates: Possess eight external rows of ciliated comb plates which help in locomotion.
• Bioluminescence: The property of a living organism to emit light is well-marked in ctenophores.
• Example: Pleurobrachia and Ctenoplana.

Phylum Platyhelminthes (Flatworms)

• Body Plan: Dorso-ventrally flattened body; Bilateral symmetry; Triploblastic; Acoelomate.
• Parasitic Adaptations: Hooks and suckers are present in parasitic forms. Some absorb nutrients directly through their body surface.
• Excretion: Specialized cells called Flame cells help in osmoregulation and excretion.
• Example: Taenia (Tapeworm), Fasciola (Liver fluke), Planaria (shows high regeneration capacity).

Phylum Aschelminthes (Roundworms)

• Body Plan: Circular in cross-section; Bilateral symmetry; Triploblastic; Pseudocoelomate.
• Alimentary Canal: Complete with a well-developed muscular pharynx.
• Sexual Dimorphism: Often females are longer than males.
• Example: Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).

Phylum Annelida (Segmented Worms)

• Segmentation: Body surface is distinctly marked out into segments or metameres (Metamerism).
• Locomotion: Longitudinal and circular muscles; Nereis (aquatic) possesses lateral appendages called parapodia for swimming.
• Circulatory System: Closed circulatory system is present.
• Excretion: Nephridia help in osmoregulation and excretion.
• Example: Pheretima (Earthworm), Hirudinaria (Blood-sucking leech), Nereis.

Phylum Arthropoda (Largest Phylum)

• Diversity: Includes over two-thirds of all named species on earth.
• Exoskeleton: Body is covered by a chitinous exoskeleton.
• Body Divisions: Head, thorax, and abdomen.
• Appendages: Jointed appendages (Arthropoda = jointed feet).
• Respiration: Gills, book gills, book lungs, or tracheal system.
• Excretion: Malpighian tubules.
• Example: Apis (Honey bee), Anopheles (Mosquito), Limulus (King crab – a living fossil).

Phylum Mollusca (Second Largest Phylum)

• Body Plan: Bilateral symmetry; Triploblastic; Coelomate.
• Morphology: Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot, and visceral hump.
• Mantle: A soft and spongy layer of skin forms a mantle over the visceral hump. The space between the hump and mantle is the mantle cavity, containing feather-like gills.
• Feeding Organ: File-like rasping organ called Radula.
• Example: Pila (Apple snail), Octopus (Devil fish), Sepia (Cuttlefish).

Phylum Echinodermata (Spiny Bodied)

• Symmetry: Adult echinoderms are radially symmetrical, but larvae are bilaterally symmetrical.
• Water Vascular System: Most distinctive feature; aids in locomotion, capture and transport of food, and respiration.
• Excretory System: Distinct excretory system is absent.
• Example: Asterias (Starfish), Echinus (Sea urchin), Antedon (Sea lily).

Phylum Hemichordata

• Structure: Worm-like marine animals with organ-system level organization.
• Body Divisions: Cylindrical body composed of an anterior proboscis, a collar, and a long trunk.
• Circulatory System: Open type; Respiration via gills.
• Example: Balanoglossus and Saccoglossus.

Phylum Chordata

• Essential Features: Presence of a Notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits.
• Circulatory System: Closed; Heart is ventral.
• Post-anal tail: Present in most forms.

Comparative Table: Diagnostic Organs of Excretion and Respiration

Trivia for UPSC Prelims

• Living Fossil: Limulus (King Crab) has remained unchanged for millions of years.
• Regeneration King: Planaria possesses an incredible capacity to regenerate a whole body from a small fragment.
• The “Devil Fish”: Octopus belongs to Mollusca, despite lacking a typical external shell.
• Pearl Formation: Occurs in Pinctada (Pearl oyster), which is a Mollusc.