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By M. W. Hardisty

The proliferation of medical texts and their swiftly escalating expenses calls for of an writer a few justification for the creation of one more specialized quantity; fairly person who treats of a comparatively imprecise staff of animal- the Cyclostomes-whose importance is little preferred outdoors the circle biologists. but, in the zoological literature this crew of vertebrates has regularly commanded a level of recognition, relatively dispropor tionate to the relatively small numbers of species concerned or their monetary value. This precise curiosity stems frequently from their exact phylogenetic prestige. Asjawless vertebrates the hagfish and the lamprey are considered as the only survivors of a as soon as flourishing crew of Palaeozoic vertebrates-the Agnathans-amongst that are numbered the 1st verte brates to seem within the fossil checklist. due to this courting to the fossil agnathans it was once inevitable that earlier dialogue of the phylogenetic signifi cance of the cyclostomes must have been ruled by means of comparative anatomists and palaeontologists, even though in recent times their distinct evolutionary place has more and more attracted the curiosity of comparative physiologists and scholars of molecular evolution. in the final fifteen years either the hagfish and the lamprey were the topic of separate courses describing intimately many elements in their morphology, body structure and lifestyles cycles (Brodal, A. and Fiinge, R., The Biology ofMyxine, 1963; Hardisty, M. W. and Potter, I. C., The Biology of Lampreys, 1971-72.

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Octatrema of Good Hopet E. cirrhatus M. garmani E. coast of Japan E. hexatrema M. paucidens E. coast of Japan E. burgeri S. Chile to S. M. a/finis E. profundum Argentine M. circifrons Gulf of Panama E. atami E. yangi Notomyxine N. tridentiger Argentine to E. springeri Patagonia E. /ongipinnis Neomyxine N. bip/inicata New Zealand Nemamyxine N. elongata New Zealand Distribution Chile Alaska - British Columbia California Chile. Japan S. Africa S. Pacific S. Africa E. ) at depths of 15542012 m. tM.

3. 7 Interpretations of the impressions on the dorsal plate of a heterostracan according to Stensio (left) and Halstead (right). Redrawn from Halstead, 1973. Perspectives and relationships 35 that a third horizontal member may have been present cannot be entirely excluded (Halstead, 1973b). The well-defined lateral line system is said to be of a generalized and primitive pattern, from which both the gnathostome and cyclostome condition could be derived. Stensio's reconstructions of the heterostracan brain differ radically from that proposed by Whiting and Tarlo (1965) and Halstead (1973a, b).

The resulting scheme differs in only minor respects from that adopted by Vladykov and Kott (l976a). Whereas the latter authors recognize Lampetra, Lethenteron and Entosphenus as distinct genera, these were listed by Hubbs and Potter as subgenera of Lampetra; a view that is consistent with the identical amino acid composition of the haemoglobins of L. ( Lethenteron) japonica and L. 1 ). In addition, Hubbs and Potter accorded generic status to Okkelbergia, but others have preferred to allocate this isolated North American brook lamprey to the genus Lampetra.

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