🐟 Unit II — Cyclostomata & Pisces
A. Cyclostomata (Jawless fishes — Agnatha)
1. Definition
Cyclostomes are the most primitive living vertebrates. They lack jaws and paired fins, have an eel-like body, and a circular mouth — hence the name Cyclostomata (cyclic mouth).
2. Examples
- Petromyzon (Lamprey) — parasitic.
- Myxine / Eptatretus (Hagfish) — scavengers, slime eels.
3. General characteristics
- Elongated body, no paired fins.
- Circular suctorial mouth; horny teeth in lamprey.
- Cartilaginous skeleton, smooth skin.
- Multiple gill pouches; 2-chambered heart.
- Dorsal hollow nerve cord; simple brain.
- Mostly external reproduction; lampreys are anadromous.
- Lampreys = ectoparasites; Hagfish = scavengers producing slime.
4. Petromyzon vs Myxine
| Feature | Petromyzon (Lamprey) | Myxine (Hagfish) |
|---|---|---|
| Feeding | Ectoparasite — sucks blood/tissue | Scavenger — burrows in dead fishes |
| Mouth | Circular sucker with horny teeth & rasping tongue | Slit-like, tooth plates, slime glands |
| Gills | 7 pairs | 5–15 pairs |
| Slime | Little | Large, defense |
| Eyes | Well developed | Reduced/degenerate |
| Lifestyle | Some anadromous | Marine, benthic scavengers |
Hagfish can knot its body to escape slime and gain leverage when feeding.
5. Life history (lamprey)
- Adults attach to host, feed, migrate upstream, spawn, and die. Larvae (ammocoete) burrow and filter-feed.
[Lamprey lateral view diagram — show mouth, gills, notochord]
[Hagfish diagram — show slime glands, barbels]
B. Pisces (Fishes) — Overview
I. Chondrichthyes
Examples: Sharks, Rays, Skates.
- Cartilaginous skeleton, jaws present.
- Placoid scales, 5–7 gill slits, no operculum.
- No swim bladder; buoyancy via oily liver + heterocercal tail.
- Paired fins; internal fertilization via claspers.
- Sensory: Lateral line, Ampullae of Lorenzini.
[Shark lateral view — label fins, gills, liver, ampullae, tail]
II. Osteichthyes
Examples: Rohu, Catla, Tuna, Lungfish.
- Ray-finned (Actinopterygii) — Rohu, Tuna
- Lobe-finned (Sarcopterygii) — Lungfish, Coelacanth
- True bone, operculum, cycloid/ctenoid scales, swim bladder, homocercal tail.
- Mostly external fertilization; lateral line sensory system.
[Bony fish diagram — operculum, swim bladder, lateral line]
C. Comparative Table
| Feature | Cyclostomata | Chondrichthyes | Osteichthyes |
|---|---|---|---|
| Jaws | Absent | Present | Present |
| Skeleton | Cartilage | Cartilage | Bone |
| Scales | None | Placoid | Cycloid/ctenoid/ganoid |
| Gill covering | Separate openings | No operculum | Operculum |
| Swim bladder/liver | Absent | Absent (liver) | Swim bladder present |
| Fertilization | External | Internal | Mostly external |
| Examples | Lamprey, Hagfish | Shark, Rays | Rohu, Tuna |
D. Accessory Respiratory Organs
- Lungs — Lungfish (Protopterus, Neoceratodus)
- Suprabranchial organ — Clarias (air-breathing catfish)
- Labyrinth organ — Anabas (Climbing perch)
- Cutaneous respiration — Eels, Mudskippers
- Modified swim bladder — Gouramis, Lungfish relatives
- Pharyngeal respiration — Some species use pharyngeal mucosa
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