By M. W. Hardisty
The proliferation of clinical texts and their speedily escalating charges calls for of an writer a few justification for the construction of one more specialized quantity; quite person who treats of a comparatively vague workforce of animal- the Cyclostomes-whose value is little favored outdoor the circle biologists. but, in the zoological literature this team of vertebrates has continuously commanded a level of recognition, particularly dispropor tionate to the relatively small numbers of species concerned or their fiscal significance. This distinct curiosity stems primarily from their designated 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 prior dialogue of the phylogenetic signifi cance of the cyclostomes must have been ruled by way of comparative anatomists and palaeontologists, even though in recent times their designated 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 guides describing intimately many features 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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Additional resources for Biology of the Cyclostomes
Seven pairs of gill sacs are indicated whose position is of especial interest, although there is no reason to assume, as suggested by MoyThomas and Miles ( 1971) that the first in the series represents the spiracu1ar or hyoid pouch. The first two gill sacs lie immediately below the otic capsule, in contrast to the position in living lampreys, where the first pouch is situated some distance behind the auditory region. As Damas (1944) and Strahan (1958) have shown, the branchial region of the lamprey is shifted posteriorly during ontogenesis.
Amongst the lampreys of the Southern hemisphere, it is only in the genus Mordacia that a non-parasitic form has so far been recognized and this species M. praecox has been found in only two river systems in New South Wales (Potter, 1970). Looked at in a wider perspective, the persistent tendency in most lamprey genera for the evolution of non-parasitic derivatives may be seen as only one of the many viable alternative survival strategies that have been adopted by these highly adaptable animals.
In the details of the extrinsic eye muscles and the shape of the eyeball, the cephalaspids show further close parallels with the petromyzonids (Janvier, 1975). Thus the disposition and insertion of these muscles appears to have (a) (b) nasohypophysial depression (c) . e~tn primary nasohypophysial opening Fig. 10 Comparisons of the nasohypophysial tracts of an adult (a) and larval lamprey (b) and a cephalaspid (c). Redrawn from Janvier, 1974. -~ of primary nasohypophysial opening nasal part of opening part nasohypophysial depression : "-'·-~hypophysial nasohypophysial duct secondary nasohypophysial pore pineal opening 44 Biology of the Cyclostomes been identical in both groups.
Biology of the Cyclostomes by M. W. Hardisty