Agnathans: Hagfishes and Lampreys, Conodonts and

Last updated: 7/24/2022

Agnathans: Hagfishes and Lampreys, Conodonts and Ostracoderms A. Lacking into their mouths. B. Hagfishes and lampreys are the living descendants of ancient agnathan lineages. 1. Two lineages of jawless vertebrates, hagfishes (Myxinoidea) and lampreys (Petromyzontoidea), still live today. Both have skeletons composed entirely of cartilage. They have a well-developed notochord, but no true vertebrae or paired fins 2. The axial skeleton of the 60 living species of hagfishes includes only a cranium and a notochord. Hagfishes are marine scavengers that burrow in sediments on continental shelves. Hagfish life cycles are simple and lack a larval stage. 3. The 40 or so living species of lampreys have a more derived axial skeleton. Their notochord is surrounded by dorsally pointing cartilages that partially cover the nerve cord. Most are parasitic as adults. Sexually mature adults migrate to the headwaters of a stream, where they reproduce and then die. Their suspension-feeding larvae resemble adult cephalochordates. most of the earliest vertebrates used a pharynx to suck food