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
Exam Tip: Always mention the madreporite when describing the water-vascular system; it is the entry point for water.
Common Pitfall: Do not confuse Torsion with Coiling. Coiling is the spiral winding of the shell, whereas torsion is the rotation of the visceral mass itself.
Frequently Asked Questions
- Define metamerism and explain its presence in Arthropoda.
- What are ommatidia? How do they function in compound eyes?
- Explain the biological significance of torsion in Gastropoda.
- List the pathway of water in the ambulacral system of Asterias.