The group Agnatha consists of the jawless fishes, the most primitive group of extant vertebrates. While most agnathan species are now extinct, fossil evidence indicates that the group was once highly successful and extremely varied. Two lineages of...

Agnatha (Greek, "no jaws") is a paraphyletic superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. Agnatha have existed since the Cambrian, and continue to live in modern times. There are two extant groups of jawless fish (sometimes...

Live motile spermatozoa of the hagfish Eptatretus burgeri were obtained for the first time, and their fine structure was examined. The spermatozoon is characterized by an extremely long midpiece. Two of the four midpiece mitochondria are extensively elongated and extend through almost the entire...