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 |
2.2 Torsion in Gastropods
Torsion is the most unique and defining characteristic of the Class Gastropoda. It is a developmental process, not the coiling of the shell.
Torsion is a 180° counter-clockwise rotation of the visceral mass, mantle, and mantle cavity with respect to the head and foot.
Process:
This rotation occurs in the free-swimming larval stage (veliger larva). It is a rapid muscular contraction that twists the entire top portion of the body.
Consequences of Torsion:
- Anterior Mantle Cavity: The mantle cavity, gills, and anus are moved from a posterior position to an anterior position, lying just behind the head.
- U-Shaped Gut: The digestive tract is twisted into a U-shape.
- Twisted Nervous System: The main nerve cords are twisted into a figure-of-eight (a "streptoneurous" condition).
- Sanitation Problem: The anus now discharges waste over the head and gills. This created a major evolutionary problem that different gastropod groups have solved in various ways (e.g., developing holes in the shell for excurrent flow).
Common Mistake: Do not confuse torsion with the coiling of the shell. Torsion is the twisting of the body. Coiling is the spiral winding of the shell. They are two separate evolutionary events.
3. Phylum Echinodermata
Echinodermata (from Greek: echinos = spiny, derma = skin) are an exclusively marine phylum, characterized by their spiny skin and unique water-vascular system.
3.1 General Characters and Classification
General Characters
- Symmetry: Adults exhibit penta-radial symmetry (body parts arranged in 5s). Their larvae are bilaterally symmetrical.
- Endoskeleton: A calcareous endoskeleton made of plates or ossicles, often with external spines.
- Water-vascular System: A unique hydraulic system of canals and tube feet used for locomotion, food capture, and gas exchange.
- Circulatory & Excretory Systems: Circulatory system (hemal system) is poorly developed. Specialized excretory organs are absent.
- Regeneration: Possess remarkable powers of regeneration (e.g., a starfish can regrow a lost arm).
Classification of Echinodermata (Up to Classes)
| Class | Key Characteristics | Examples |
|---|---|---|
| Asteroidea | Star-shaped body with arms not sharply set off from the central disc. Tube feet have suckers. | Asterias (starfish / sea star) |
| Ophiuroidea | Star-shaped body with long, slender, flexible arms sharply set off from the central disc. No suckers on tube feet. | Ophiura (brittle star) |
| Echinoidea | Globular or disc-shaped body, lacking arms. Endoskeleton forms a rigid test. Body covered with movable spines. | Echinus (sea urchin) |
| Holothuroidea | Elongated, cucumber-shaped body, soft and leathery. No arms or spines. | Holothuria (sea cucumber) |
3.2 Water-vascular System in Star Fish (Asteroidea)
The water-vascular system is the most distinctive feature of echinoderms. It is a hydraulic system derived from the coelom, and it powers the tube feet.
Structure and Path of Water
- Madreporite: A sieve-like plate on the aboral (upper) surface where water enters the system.
- Stone Canal: A short, calcareous tube connecting the madreporite to the ring canal.
- Ring Canal: A circular canal located in the central disc, surrounding the mouth.
- Radial Canals: Five canals that radiate from the ring canal, one into each arm, running along the ambulacral groove.
- Lateral Canals: Arise from the radial canals, each leading to a tube foot.
- Tube Feet (Podia): Small, hollow, muscular tubes that project from the body. Each consists of an upper, bulb-like ampulla (inside the body) and a lower, sucker-like podium (the external "foot").
Function in Locomotion
- To extend a tube foot, the ampulla contracts.
- This forces water into the podium, which elongates and extends.
- The podium's sucker attaches to the substrate.
- The longitudinal muscles of the podium then contract, shortening the tube foot and pulling the animal forward. Water is forced back into the ampulla.
- The sucker releases, and the cycle repeats. Thousands of tube feet work in a coordinated fashion to produce a slow, gliding movement.