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TELEOSTOMES
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TELEOSTOMES , members of the third sub-class of the class Fishes, being all the fishes in which the See also:skull is invested with membrane bones, viz., the Crossopterygians, the Dipnoans, the Ganoids and the Teleosteans. They may be further defined as fishes with an ossified or cartilaginous See also:skeleton, a See also:lower See also:jaw, gills inserted on the gill-See also:arches, a single gill-opening on each See also:side (exceptionally fused with its See also:fellow on the ventral side), an opercle formed of one or several bones, the See also:body usually covered with scales or bony plates, an See also:air-See also:bladder or See also:lung, at least in the See also:primitive forms, and without copulatory paired See also:organs or " claspers." The See also:term which designates this sub-class has been adopted by See also:Sir R. See also:Owen, E. D. See also:Cope, and A. S. See also:Woodward in a less comprehensive sense, the Dipneusti being regarded by them as constituting a See also:separate stfb-class, and its inventor, C. L. See also:Bonaparte (1838) had proposed it in a more restricted sense, the sturgeons, lophobranchs and plectognaths being excluded. T. Gill (1872) was the first to use it in the acceptation taken in the See also:present See also:article. Whether the Ostracophores should be included among the Teleostomes, as recently proposed by C. T. Regan, is still open to doubt. The sub-class is here divided into four orders, but it is difficult to decide whether, in an ascending See also:series, the Crossopterygians or the Ganoids should be placed first. From the point of view of the See also:evolution of the paired fins, accepting the lateral fin-See also:fold theory as the better supported by the See also:evidence at See also:hand, there is much to say in favour of regarding the Chondrostean Ganoids as the more primitive type. From another point of view the See also:condition of the air-bladder in the existing Crossopterygians appears to represent the earliest See also:form assumed by this important See also:organ, which it seems rational to conclude was originally evolved asan See also:accessory breathing organ and later became transformed into a hydrostatic apparatus (Ganoids and Teleosteans) on the one hand, into a true lung (Dipnoans and Batrachians) on the other. Guided by the second See also:consideration, assuming that the air-bladder of the fossil Crossopterygians conformed to the type known in their See also:recent representatives, and also in deference to palaeontological See also:chronology, whatever it be See also:worth in the present See also:state of our knowledge, we shall begin the series with the Crossopterygians, which pass into the Dipnoans, and then take up the Ganoids, which See also:lead up very gradually to the Teleosteans, the dominant See also:group at the present See also:day. But we do not deny the force of the arguments adduced by Regan in attempting to show that the paired fins of the Chondrostean Ganoids are a nearer approach to the primitive condition than are those of the Crossopterygians. No doubt some day we shall become acquainted with still older Teleostomes, ,which we may expect to establish the connexion between the two types which in Palaeozoic times have evolved on parallel lines. Additional information and CommentsThere are no comments yet for this article.
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