Unit 3: Arthropoda, Mollusca and Echinodermata
Table of Contents
1. Phylum Arthropoda
Arthropoda (from Greek: arthron = joint, podos = foot) is the largest phylum in the animal kingdom. Their success is attributed to a chitinous exoskeleton, jointed appendages, and segmentation.
1.1 General Characters and Classification
General Characters
- Exoskeleton: A hard, protective outer covering made of chitin, which must be shed (molting or ecdysis) to allow for growth.
- Jointed Appendages: Paired, jointed limbs that are modified for various functions like walking, feeding, swimming, and sensing.
- Segmentation and Tagmosis: The body is segmented, but segments are often fused into specialized body regions called tagmata (e.g., head, thorax, and abdomen in insects).
- Circulatory System: Open type. Blood (hemolymph) flows through a dorsal heart into open sinuses (the hemocoel).
- Respiratory Organs: Diverse, including gills (aquatic), book lungs (arachnids), and tracheae (insects).
Classification of Arthropoda (Up to Classes)
| Class | Key Characteristics | Examples |
|---|---|---|
| Crustacea | Mostly aquatic; two pairs of antennae; respiration by gills. Tagmata: Cephalothorax and abdomen. | Prawn, Crab, Daphnia |
| Myriapoda | Terrestrial; numerous segments, each with legs. Tagmata: Head and trunk. Respiration by tracheae. | Centipedes, Millipedes |
| Insecta (Hexapoda) | Largest class. Terrestrial; one pair of antennae; three pairs of legs. Tagmata: Head, thorax, abdomen. Respiration by tracheae. | Cockroach, Butterfly, Grasshopper |
| Arachnida | Terrestrial; no antennae; four pairs of legs. Tagmata: Cephalothorax and abdomen. Respiration by book lungs or tracheae. | Spider, Scorpion, Tick, Mite |
1.2 Vision in Arthropoda
Arthropods have two main types of eyes: simple eyes (ocelli) and compound eyes. The compound eye is the most characteristic and advanced visual organ in the phylum, especially in insects and crustaceans.
Compound Eyes
- Structure: The compound eye is made of numerous independent visual units called ommatidia (singular: ommatidium).
- Ommatidium: Each ommatidium has its own lens (cornea), a crystalline cone, and a group of light-sensitive retinular cells. It is a complete, tiny "eye."
- Image Formation: They form a mosaic image, where each ommatidium contributes a small piece of the overall picture. This type of vision is excellent for detecting movement but provides lower resolution (a less detailed image) than a human's lens eye.
- Types of Mosaic Vision:
- Apposition Image (Bright light): Each ommatidium works independently, isolated by pigment cells. This creates a sharp but dim point-by-point image.
- Superposition Image (Dim light): The pigment cells retract, allowing light from several ommatidia to be focused onto a single rhabdome. This creates a brighter but less distinct, overlapping image, which is useful in low-light conditions.
2. Phylum Mollusca
Mollusca (from Latin: mollis = soft) is the second-largest phylum. They are soft-bodied animals, usually protected by a hard calcareous shell.
2.1 General Characters and Classification
General Characters
- Body Plan: Unsegmented, soft body with a distinct head, visceral mass, and a ventral muscular foot.
- Mantle: The visceral mass is covered by a fleshy fold of skin called the mantle, which secretes the shell.
- Shell: An external calcareous shell is usually present (it is internal in squid and absent in octopus and slugs).
- Radula: Most molluscs have a rasping, tongue-like organ called the radula in their mouth, used for scraping food.
- Circulatory System: Open type (except in cephalopods, which have a closed system).
- Respiratory Organs: Gills (ctenidia) in aquatic forms, or a "lung" (modified mantle cavity) in terrestrial forms.
Classification of Mollusca (Up to Classes)
| Class | Key Characteristics | Examples |
|---|---|---|
| Gastropoda | Largest class. Asymmetrical body due to torsion. Usually have a single, coiled shell. Well-developed head. | Pila (apple snail), Helix (garden snail), Slug |
| Bivalvia (Pelecypoda) | Body laterally compressed, enclosed in a shell with two valves. Head is indistinct, radula is absent. Filter feeders. | Unio (freshwater mussel), Oyster |
| Cephalopoda | Most advanced molluscs. Head is large with complex eyes. Foot is modified into arms/tentacles. Shell is internal or absent. Closed circulatory system. | Loligo (squid), Sepia (cuttlefish), Octopus |