Category: 1a: Jawless Fish (Agnatha)

Jawless fishes like Lamprey or Hagfish. Fish in the Agnatha Class evolved earlier than other the other two Classes of fish and their anatomy is simple: basically a tube with teeth and a tail but usually no jaw (their teeth can’t snap shut) and no other fins. They have neither scales nor spines and their main defense is slime. That’s right, some species produce SLIME when they are eaten–so much that it can clog the gills of their predators.

Traditional Classification vs Phylogenetic (using Fish as an Example)

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Traditional classification

Traditional spindle diagram of the evolution of the vertebrates at CLASS level

Conventional classification has living vertebrates grouped into seven CLASSES based on traditional interpretations of gross anatomical and physiological traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works. The extant vertebrates are:

  • Subphylum Vertebrata: 7 Classes (often taught as 5 Vertebrate Classes, by collapsing the 3 fish classes into one)

In addition to these, there are two classes of extinct armoured fishes, the Placodermi and the Acanthodii, both of which are considered paraphyletic.

Add Info on Phylogenetic (using Fish as an Example)