Phylogeny of Fishes

• The notochord
• Filter feeding

• Lead to the dominance of vertebrates
• Allows a one way digestive system
• Its development induces the development of other structures
• Good in small animals but is not enough in bigger ones
• Vertebrae rings protect it and eventually turn it into the vertebrae we know today

• The notochord
• They act as a axis of support for the body
• Provide protection for the spinal cord and early blood supply to core structures

• Well developed sense organs in anterior (head) region
• It arose from epidermal placodes

• Cells that are really sensitive to stimuli
• Eventually make up new structures

Cells that migrate outward and make up different structures in the head such as skull bones and cartilage

• Pre-vertebrates were likely filter feeders such as amphixious 
• They were abundant in the past but today consist of lamprey and hagfish
• Possible because of pharyngeal gill slits

• Its not for certain whether jaws started in the placoderms or the ostracoderms 
• It allowed for animals to eat virtually anything; doesn't have to wait for food to come by
• Could become more predaceous 
• Could get better food, safer food, and live longer

The ostracoderms were a jawless, armored  fish that showed up in the Cambrian period (500 mya)

Class Agnatha

The hagfish - they lack bone but have the genetic code for the bone

• A class of jawless fish
• Include hagfishes
• Entirely marine
• Cartilaginous skeleton w/o body armor, no hint of vertebral development
• Feed by boring into dead or dying fish with raspy tongue
• Evidence suggests that hagfish are the most primitive living vertebrate

• A class of jawless fish
• Oldest documented vertebrate fossils
• Appear in late Cambrian-Devonian (500-400mya)
• Paired nasal openings (two nostrils)
• Most have bony heads (meaning cephalization was important early on

• A class of jawless fish
• Varied lifestyles and body shapes
• Single nasal opening (mono-)
• Fossil forms heavily armored (silurian - late Devonian)(425-400 mya)
• Bony plates down body suggests these guys were more active and got into more danger leading to these bony adaptions

• Notochord
• Pharyngeal gill slits
• Dorsal hollow nerve cord
• Post-anal tail

• Adequate growth
• Development
• Tissue Differentiation 
• Metabolic rate

Cannot change

Anus first, mouth second