📘 Unit 3 – Pisces (Fishes)
🔹 Introduction
- Pisces = true fishes, largest group of chordates.
- Aquatic, cold-blooded vertebrates with fins for locomotion & gills for respiration.
- Show great diversity (small seahorses → large sharks).
- Important evolutionary step → first vertebrates with jaws & paired fins.
A. General Characters of Class Pisces
- Habitat: Exclusively aquatic (freshwater & marine).
- Body: Streamlined, covered with scales.
- Respiration: Gills; protected by operculum (bony fishes).
- Circulation: Two-chambered heart (1 auricle + 1 ventricle).
- Skeleton: Cartilaginous (sharks) or bony (rohu, catla).
- Fins: Paired fins (pectoral, pelvic) + unpaired fins (dorsal, caudal, anal).
- Reproduction: Mostly oviparous; external fertilization in bony fishes.
- Cold-blooded: Body temp varies with water.
B. Classification of Pisces
Pisces divided into two subclasses:
1. Chondrichthyes (Cartilaginous Fishes)
- Skeleton made of cartilage.
- Mouth ventral; no operculum.
- Tail heterocercal (unequal lobes).
- Examples: Shark (Scoliodon), Dogfish, Rays.
Mnemonic: "Cartilage = Shark"
2. Osteichthyes (Bony Fishes)
- Skeleton made of bone; mouth terminal.
- Operculum covers gills; tail homocercal (equal lobes).
- Examples: Rohu, Catla, Tuna, Flying fish, Seahorse.
Mnemonic: "Osteo = Bone"
C. Economic Importance of Fishes
- Food: Rich in protein & omega-3 (Rohu, Catla, Hilsa).
- Oil: Cod liver oil (vitamin A & D).
- Ornamental: Goldfish, Angel fish.
- Fisheries: Employment & economy.
- By-products: Glue, manure, leather.
D. Examples to Remember
- Cartilaginous: Shark, Rays, Dogfish.
- Bony: Rohu, Catla, Tuna, Flying fish, Seahorse.
E. Exam-Oriented Points
- 2 marks: Difference between cartilaginous & bony fishes.
- 5 marks: General characters of Pisces.
- 12 marks: Structure & economic importance of Rohu/Shark.
✅ Summary
Pisces are cold-blooded, aquatic vertebrates with gills, fins, and either cartilaginous or bony skeleton, showing great diversity and economic importance.
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