Chordates All vertebrates are chordates but not all chordates are vertebrates. –Examples of non-vertebrates are sea squirts and lancelets –Characteristics that they share with vertebrates at some point in their lives are: (and what they become in vertebrates) –A notochord (becomes spinal cord) –Pharyngeal pouches (becomes inner ear and jawbone in humans) –Dorsal hollow nerve chord (becomes vertebral column)

AGNATHA Jawless Fish Examples Lampreys are found in marine and freshwater environments. Like the hagfishes, they are also relatively eel-like in appearance

Feeds parasitically on other fish using its jawless tooth-filled sucker mouth. Has a notochord through all stages of their life cycle. Jawless fishes were the first vertebrates to evolve

Characteristics Cold-blooded Jaws are absent. Paired fins are absent. Early species had heavy bony scales and plates in their skin, but these have been lost in living species. In most cases the skeleton is cartilaginous. A light-sensitive pineal eye is present. External fertilization –separate sexes

Digestive system: –A cloaca is a common opening for the digestive, urinary, and reproductive tracts. Circulatory system: – Blood circulates in a pattern basic to most fish, with a two-chambered heart and a one-pass circulation. Respiratory system: – Between the external and internal gill slits are the gill chambers. –Gill arches are cartilage supports underlying the gills.