Unit 4: Protochordates, Agnatha, Fishes and Amphibia
1. Protochordates
1.1 General Features
Protochordates (or Acraniata) are an informal group of "lower" chordates, lacking a cranium, true brain, and vertebral column. They are crucial for understanding the origin of vertebrates. They include Urochordata and Cephalochordata.
The Four Diagnostic Chordate Features:
All chordates, at some point in their life, possess:
- Notochord (skeletal rod)
- Dorsal Hollow Nerve Cord
- Pharyngeal Slits or Clefts (gills)
- Post-anal Tail
Subphylum Urochordata (Tunicates)
- Adults are sessile (attached) and enclosed in a leathery tunic.
- The adult body is a "filter-feeding bag" and shows only one chordate feature: a large, perforated pharynx with gill slits.
- The free-swimming larva (ascidian tadpole) is crucial, as it clearly displays all four chordate characteristics, which are lost during retrogressive metamorphosis.
Subphylum Cephalochordata (Lancelets)
- Marine, burrowing, fish-like animals (e.g., Amphioxus).
- The notochord, dorsal nerve cord, and pharyngeal slits persist throughout life.
- The notochord extends the entire length of the body, even into the head region.
- They are filter feeders and show segmented muscles (myotomes).
2. Agnatha and Cyclostomes
2.1 General Features
Agnatha is a superclass of jawless vertebrates. They are the most ancient vertebrate lineage. The only living members belong to the Class Cyclostomata.
General Characteristics of Cyclostomes
- No Jaws: This is their defining feature. They have a circular, sucking mouth, often lined with horny teeth.
- No Paired Fins: They lack pectoral and pelvic fins.
- Skeleton: Entirely cartilaginous; no bone.
- Notochord: Persists throughout life (a primitive feature).
- Gills: Gills are in the form of 5-16 pairs of gill pouches.
- Skin: Smooth, scaleless, and contains slime glands.
- Examples: Hagfishes (e.g., Myxine) and Lampreys (e.g., Petromyzon).
3. Fishes (Pisces)
3.1 General Features and Classification
Fishes are aquatic, poikilothermic (cold-blooded), jawed vertebrates (Gnathostomata) that breathe using gills and typically have fins and scales.
General Features
- Habitat: Exclusively aquatic (marine and freshwater).
- Respiration: By gills, which are protected by an operculum in bony fish.
- Locomotion: By paired (pectoral, pelvic) and unpaired (dorsal, caudal) fins.
- Heart: Two-chambered (one atrium, one ventricle), pumping only deoxygenated blood.
- Buoyancy: Many bony fish possess a swim bladder (air sac) to control buoyancy.
Classification (Up to Orders)
Fishes are broadly divided into two main classes:
| Class |
Key Characteristics |
Example Orders |
Chondrichthyes
(Cartilaginous Fishes) |
Cartilaginous skeleton; placoid scales; no swim bladder; no operculum. |
Carcharhiniformes (Sharks), Rajiformes (Skates, Rays) |
Osteichthyes
(Bony Fishes) |
Bony skeleton; cycloid/ctenoid scales; swim bladder present; gills covered by an operculum. |
Cypriniformes (Carp, Labeo), Siluriformes (Catfishes), Perciformes (Perch, Anabas) |
3.2 Osmoregulation in Fishes
Osmoregulation is the active control of the body's water and salt balance. This is a critical challenge for aquatic animals.
Key Problem:
- Freshwater fish are hypertonic to their environment (saltier than the water). They constantly gain water and lose salts.
- Marine fish are hypotonic to their environment (less salty than the water). They constantly lose water and gain salts.
Solution in Freshwater Fish (e.g., Labeo)
- Does NOT drink water.
- Absorbs water passively through gills and skin.
- Produces large amounts of dilute urine to excrete excess water.
- Actively absorbs salts from the water using salt-absorbing cells in the gills.
Solution in Marine Bony Fish (e.g., Tuna)
- Drinks large amounts of seawater to replace lost water.
- Produces very small amounts of concentrated urine.
- Actively excretes excess salts using salt-secreting cells in the gills.
4. Class Amphibia
4.1 General Features and Classification
Amphibians (from Greek: amphi = double, bios = life) are the first vertebrates to transition to land, but they are not fully terrestrial. They lead a "double life," typically breeding in water and living on land.
General Features
- Habitat: Require moist environments; found in freshwater and on damp land.
- Skin: Smooth, moist, glandular, and scaleless. It serves as a major respiratory organ (cutaneous respiration).
- Heart: Three-chambered (two atria, one ventricle), leading to mixed blood.
- Reproduction: Fertilization is usually external. Eggs are anamniotic (lack a shell) and must be laid in water.
- Development: Indirect, with a free-swimming aquatic larval stage (e.g., tadpole) that undergoes metamorphosis.
- Thermoregulation: Poikilothermic (cold-blooded).
Classification (Up to Order)
| Order |
Common Name |
Key Characteristics |
Examples |
| Apoda (Gymnophiona) |
Caecilians |
Limbless, worm-like, burrowing amphibians. |
Ichthyophis |
| Urodela (Caudata) |
Salamanders & Newts |
Body with a long tail. Two pairs of well-developed limbs. |
Salamandra |
| Anura (Salientia) |
Frogs & Toads |
Body short, tailless (in adults). Hind limbs are long and adapted for jumping. |
Rana (Frog), Bufo (Toad) |
4.2 Parental Care
Because their anamniotic eggs are vulnerable to desiccation and predation, many amphibians have evolved complex behaviors to protect their young.
- Nest Building: Frogs like Rhacophorus build foam nests hanging over water, so the tadpoles can drop in upon hatching.
- Guarding Eggs: Many frogs and salamanders will guard their egg clusters.
- Carrying Eggs on Body:
- The male Midwife Toad (Alytes) wraps the egg strings around his hind legs.
- The female Surinam Toad (Pipa pipa) carries her eggs embedded in pits on her back, where they complete their development.
- Brood Pouches: The male Darwin's Frog (Rhinoderma) broods the young inside his vocal sacs. In some Gastric-brooding frogs (now extinct), the female swallowed her eggs and brooded them in her stomach.
- Viviparity: Some caecilians and salamanders are viviparous, giving birth to live young.