These larvae spawn in streams and live in mud burrows until metamorphosis into the parasitic adults. Eats by slipping a knot in its body forward to push chunks of meat off of dead and dying fish. 4 Hagfish are even more unusual than lampreys internally, and as shown by the cladogram on Fig. 27-2, are considered separate from other vertebrates. 3,4 Development of jaws .
Jaws are useful for biting and feeding.
If we follow along in vertebrate embryos, we will see that these initial cartilages get covered by secondary dermal jaws. CP Hickman Jr et al., Biology of Animals, 7th ed. FH Pough et al., Vertebrate Life, 4th ed.