Class Petromyzontida (lampreys)

• Jawless fish (agnathans)
• Lack paired fins
• Lack true vertebrae
• Living lampreys are specialized as parasites
• Early jawless fish (like "ostracoderms") were probably quite different from lampreys in both ecology and morphology
• Jawless condition constrains ecology of the group
• Lamprey larvae (called "ammocoetes") similar to cephalochordates like amphioxus

Class Chondrichthyes
(sharks and rays)

• Cartilagenous fish
• Group includes sharks, rays, skates, and ratfishes
• Jaws well-developed (gnathostomes)
• Paired pectoral and pelvic fins
• Cartilagenous skeleton is 2^o derivation, ancestors likely had bony skeletons
• Placoid (tooth-like) scales, homologous to true teeth (enamel, dentine structure)
• Oily liver (for buoyancy)

Class Osteichthyes (bony fish)

• Bony fish
• Split into two groups
  1. Actinopterygii - ray-finned fish
  2. Sarcopterygii / Actinistia - lobe-finned fish
• Bony skeleton
• Scales incorporate bone also
• Swim bladder - hydrostatic organ (buoyancy), analog of shark liver, homolog of lung
• Counter-current gas exchange in gills
• Lobe-finned fish (like lungfish and the coelecanth) are group that gave rise to tetrapods

Class Amphibia (amphibians)

• First tetrapod group
• Includes frogs, toads, salamanders, newts, and caecilians
• Many use cutaneous (skin) respiration as well as simple lungs - skin is vulnerable to dessication
• Two-part life cycle - aquatic larvae, terrestrial as adults (usually)
• Anamniotic eggs - must be layed in water

"Reptiles" (reptiles and birds)

• First amniote group
• Amniotic egg reduce dependence on H₂O for reproduction
• Relatively impermeable skin with keratin scales, prevents dessication
• Birds modify scales into feathers for insulation (homology), can still see reptilian scales on bird legs, feet
• Many modifications of basic reptilian form for flight and endothermy

Class Mammalia (mammals)

• Hair / fur
• Mammary glands
• Endothermy (independently evolved)
• Three major groups:
  1. monotremes
  2. marsupials
  3. placentals