By M. W. Hardisty
The proliferation of medical texts and their speedily escalating expenses calls for of an writer a few justification for the creation of yet one more specialized quantity; fairly one who treats of a comparatively imprecise staff of animal- the Cyclostomes-whose value is little favored open air the circle biologists. but, in the zoological literature this crew of vertebrates has continuously commanded a level of cognizance, fairly dispropor tionate to the relatively small numbers of species concerned or their monetary value. This exact curiosity stems typically from their distinct phylogenetic prestige. Asjawless vertebrates the hagfish and the lamprey are considered as the only survivors of a as soon as flourishing workforce of Palaeozoic vertebrates-the Agnathans-amongst that are numbered the 1st verte brates to seem within the fossil list. due to this dating to the fossil agnathans it used to be inevitable that prior dialogue of the phylogenetic signifi cance of the cyclostomes must have been ruled via comparative anatomists and palaeontologists, even if in recent times their targeted 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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Additional resources for Biology of the Cyclostomes
Views on their probable feeding habits have varied from ciliary mechanisms (Heintz, 1958) to suctorial detritus feeding or even planktonic feeding (Whiting, 1972). The latter author has related the restricted size range of anaspids to their retention of a microphagous feeding mechanism similar to that of the ammocoete, suggesting that the enlarged chin may have contained a velar pumping mechanism. Other authors have suggested that this enlarged pre branchial region might have housed a rasping tongue (Parrington, 1958), foreshadowing the predaceous feeding mechanism of the lamprey (Stensio, 1958).
Cartilage Smm Fig. 8 A reconstruction of the head skeleton and associated structures of the fossil lamprey, Mayomyzon. , 1971. This may be compared with the cranial skeleton of a modem lamprey, Fig. 2. Perspectives and relationships 37 animals possessed a larval stage), this anterior position of the gills must be regarded as a primitive feature, resembling embryonic or early larval stages of present-day lampreys. In other respects also, M ayomyzon may be regarded as being less specialized than modem lampreys.
O). o). (b) The arrangement of the efferent gill ducts, with the pharynx (ph) to the left. I Condition in Myxine. 2 Posterior crowding of the gill pores in Eptatretus ( Paramyxine) atami. 3 Separate spaced gill pores as in Eptatretus burgeri. 26 Biology of the Cyclostomes that these animals are most likely to be recovered in areas where commercial fisheries are more active. This may account for the impression that certain species, such as Myxine glutinosa or E. stouti are more numerous (and also for their dominance in the scientific literature).