6. Mollusca

Definition and Characteristics:
- The term "Mollusca" is derived from Latin meaning "soft ones".
- It is a large and diverse phylum, with over 100,000 extant species and approximately 35,000 fossil species.
- Comprises 7 higher taxa (classes): clams, snails, slugs, squid, chitons, and others.
Adaptive Radiation:
- Habitats: Molluscs occupy a variety of environments: marine, freshwater, and terrestrial.
- Feeding Modes: They include algae grazers, herbivores, predators, and parasites.
Economic Importance:
- Includes economically significant species such as oysters, mussels, scallops, squid, and contributes to the pearl and shell trade.
- Some species can be agricultural pests and disease vectors.
Ecological Importance:
- Molluscs serve as keystone species in many food chains (e.g., the parasitic relationship between Leucochloridium paradoxum and its snail host).

Basic Body Plan:
- Triploblastic organization with endoderm, mesoderm, and ectoderm; hence they possess differentiated muscles.
- Classified as protostomes where the mouth forms from the blastopore.
- Exhibit bilateral symmetry, unsegmented bodies, and a complete gut with a mouth and anus.
- Have a true coelom, albeit small, and feature specialized organs for respiration and circulation, such as ctenidia (gills) and a heart.
- Divided into regions:
- Head-Foot: Primarily for movement, perception, and feeding.
- Visceral Mass: Contains organs for digestion, circulation, excretion, and reproduction.
- Mantle: A soft covering that often secretes the shell, creating a cavity for the ctenidia and other structures.
Mollusc Shell:
- Shell consists of various layers:
- Periostracum: An organic, often dark or colored outer layer.
- Prismatic Layer: Composed of calcium carbonate (CaCO3).
- Nacreous Layer: Inner layer made up of plate-like CaCO3 crystals, known as mother-of-pearl.

Molluscs generally have an open circulatory system, except for cephalopods which possess a closed circulatory system:
- The heart has two auricles and one ventricle.
- Blood vessels and hemolymph present, with haemocyanin as the oxygen-carrying pigment.
Respiration:
- Gills (ctenidia) located in the mantle cavity, ciliated for effective water flow and gas exchange.
- Uses a counter-current exchange system to maximize oxygen absorption.

Feeding modes in molluscs vary widely:
- Most primitive forms are grazers.
- Radula: A specialized, ever-growing rasping structure used to scrape food.
- Food is moved through the digestive tract with the aid of cilia, with both intracellular and extracellular digestion occurring.

The cephalization seen in molluscs leads to the development of a complex nervous system with ganglia forming a nerve ring around the esophagus.
- Includes paired nerve cords and specialized sense organs, often directly associated with their feeding or sensory mechanisms (e.g., eyes).

Class Gastropoda:
- Largest class consisting of snails and slugs, known for their asymmetric bodies and coiled shells.
- Exhibits mechanisms for torsion and shell coiling, providing advantages and challenges regarding body organization and sanitation.
Class Bivalvia:
- Characterized by a two-valve shell and lack of a radula, mostly filter feeders.
- Includes commercially significant species like clams, oysters, and mussels.
Class Cephalopoda:
- Highly evolved with well-developed head, eyes, and limbs (tentacles/arms).
- Includes squids and octopuses and is recognized for complex behavior and locomotion using jet propulsion.
- Closed circulatory system facilitates high metabolic demands.