ABSTRACT
Lampreys are one of two extant members of the ancient group of jawless vertebrates, the Agnatha, whose ancestry can be traced back to at least 550 million years ago to the armoured ostracoderms. The other extant agnathan is the strictly marine group, the hagfi shes, and there is recent fossil evidence from southern China that lamprey-and hagfi sh-like agnathans coexisted (Shu et al., 1999) in the lower Cambrian period. Identifi able lamprey fossils appear in mostly marine deposits from the upper Carboniferous period and morphological evidence suggests that lamprey evolution has been relatively conservative for the past 300 million years (Potter and Gill, 2003). The taxanomic relationship between the agnathans, the lampreys and hagfi shes, has been an on-going controversy. Although extensive morphological and physiological, and some molecular, data suggest that lampreys are more similar to the jawed fi shes than they are to hagfi shes, other molecular data imply a close relationship, in fact monophyly, of the two extant agnathan groups (see Potter and Gill, 2003; Hardisty, 2006). One way in which hagfi shes and lampreys differ is in their development.
