Unit 3: Arthropoda, Mollusca and Echinodermata

1. Phylum Arthropoda

Arthropoda is the largest phylum in the animal kingdom, characterized by jointed appendages and a chitinous exoskeleton.

General Characters

• Symmetry: Bilaterally symmetrical body.
• Organization: Organ-system level of organization.
• Exoskeleton: A tough, chitinous cuticle that is periodically shed through a process called molting or ecdysis.
• Segmentation: Metamerically segmented body usually divided into head, thorax, and abdomen.
• Appendages: Presence of jointed appendages.
• Body Cavity: Hemocoel (coelom reduced and filled with blood).

Classification up to Classes

• Onychophora: Peripatus (connecting link between Annelida and Arthropoda).
• Crustacea: Head and thorax fused into cephalothorax; usually aquatic (e.g., Prawn, Crab).
• Myriapoda: Terrestrial; many legs (e.g., Centipede, Millipede).
• Insecta: Three pairs of legs; body divided into head, thorax, and abdomen (e.g., Cockroach, Honeybee).
• Arachnida: Four pairs of legs; no antennae (e.g., Spider, Scorpion).
• Merostomata: Horseshoe crabs (Limulus).

Vision in Arthropoda

Arthropods possess unique sensory structures for vision, most notably compound eyes.

• Simple Eyes (Ocelli): Detect light intensity but do not form clear images.
• Compound Eyes: Composed of thousands of individual visual units called ommatidia.
• Mosaic Vision: Each ommatidium perceives a small portion of the visual field, which the brain integrates into a mosaic image.

2. Phylum Mollusca

Mollusca is the second largest phylum, consisting of soft-bodied, usually unsegmented animals.

General Characters

• Body Divisions: Usually divided into a head, a ventral muscular foot, and a dorsal visceral mass.
• Mantle: A delicate fold of skin covering the visceral mass, which often secretes a calcareous shell.
• Radula: A rasping organ in the mouth used for feeding (except in bivalves).
• Respiration: Occurs via gills (ctenidia), mantle, or pulmonary sacs.

Classification up to Classes

• Monoplacophora: Single shell (e.g., Neopilina).
• Aplacophora: Worm-like, shell-less molluscs.
• Polyplacophora: Shell with eight plates (e.g., Chiton).
• Gastropoda: Asymmetrical due to torsion; single coiled shell or none (e.g., Pila, Snail).
• Pelecypoda (Bivalvia): Shell with two valves (e.g., Unio, Pinctada).
• Cephalopoda: Shell internal, reduced, or absent; foot modified into arms/tentacles (e.g., Loligo, Octopus).

Torsion in Gastropods

Definition: Torsion is a 180° counter-clockwise rotation of the visceral mass during larval development.

• Process: It brings the mantle cavity and anus to the anterior position, just above the head.
• Significance: It allows the head to be withdrawn into the shell first for protection and places sensory organs (osphradium) at the front to sample water.

3. Phylum Echinodermata

Echinodermata includes "spiny-skinned" marine animals with a water-vascular system.

General Characters

• Symmetry: Adults show pentamerous radial symmetry, while larvae are bilaterally symmetrical.
• Endoskeleton: Calcareous ossicles with spines.
• Water-Vascular System: A unique system used for locomotion, food capture, and respiration.
• Regeneration: High power of regenerating lost body parts.

Classification up to Classes

• Asteroidea: Star-shaped; five arms (e.g., Asterias/Starfish).
• Ophiuroidea: Brittle stars; central disc with slender arms.
• Echinoidea: Sea urchins; globular body, no arms.
• Holothuroidea: Sea cucumbers; elongated body.
• Crinoidea: Sea lilies; stalked and branched arms.

Water-Vascular System in Starfish

This system, also called the ambulacral system, consists of water-filled canals.

• Pathway: Madreporite → Stone canal → Ring canal → Radial canals → Lateral canals → Tube feet (podia).
• Function: The hydraulic pressure generated in the tube feet allows the starfish to move and grip prey.

4. Exam Focus Enhancements

Frequently Asked Questions

1. Define metamerism and explain its presence in Arthropoda.
2. What are ommatidia? How do they function in compound eyes?
3. Explain the biological significance of torsion in Gastropoda.
4. List the pathway of water in the ambulacral system of Asterias.