ICHTHYOLOGY, Tit/the-61'0T (Gr. I"' fish; avyk, a discourse), is the science of fishes. It is that branch of human knowledge which treats of the aquatic gill-bearing vertebrates, popularly known as fishes to English-speaking people.
Classification of Fishes.— In different treat. ises on fishes there appear very great differ ences in the classification proposed or adopted. Often in two works of parallel scope scarcely a group will appear in both with the same boun daries or under the same name. For this con dition there are several causes. First, the tendency in some minds toward the extreme of subdivision, and in others toward the extreme of aggregation; second, the various values as signed by different authors to different sorts of characters, the actual value of each only to be determined by the final judgment of paleontol ogy; third, the tendency of many writers to give new names to old groups. On this ac count a single class order may have half a dozen virtually synon) names. Thus the terms Chondropterygis, Elacmobranckii, Ptagio stomi, Selachii, Placodii, Ataacea, and other less known names have been applied to the group of sharks and skates. Again various authors, recognizing the validity of a given group, may find it necessary at times to change its boundaries. In such a case a new name may be proposed, or a new definition be given to an old one. Either arrangement may lead to con fusion. Thus with some writers, the groups of sharks, under various names may include the order of Chimaroids, or under the same names, the Chinsteroids may be excluded from it.
The Chordata.—The great branch of chor date animals finds its origin perhaps in extinct wormlike types or possibly in marine creatures remotely allied to the horse-shoe crab and to spiders. Common opinion favors the first hypothesis. It differs essentially from the in vertebrate branches in the presence of a more or less developed notochord (which in the higher forms gives place to a backbone), and in the presence of gill-slits, connected with respiration. These gill-slits and accompanying
gill-structures are persistent in fishes, while in higher vertebrates they are mostly relegated to the embryonic stages.
The Chordate include several classes of ma rine animals leading up to the true fishes, as follows : (q.v.). Tunicala.— Ascidians (q.v.).
Lancelets (q.v.).
Hag-fishes and Lampreys (qq.v.).
Cyclis.—An extinct (Palsospoudylus) fish like form.
Fishes, properly so-called; the various primary divisions of which Elastno branchii, Ostracophori (Ostracodermi), Arth rodira and Teleostomi are usually called sub classes.
But in view of the uncertainty attached to the mutual relations and origin of these groups, we may follow recent American custom in re garding the elasmobranchs, ostracophores, ar throdires and teleostomes as 'distinct classes, the last named group containing the typical or true fishes. On anatomical grounds we must regard the Elasmobranchii (sharks) as the most primitive of these classes. As to this, palaeontol ogy gives no certain answer but the tendency of evidence is rather toward regarding the ostracophores as still more primitive. There is no doubt that fishes existed and that some of the classes were well differentiated at a period long antecedent to the deposition of the oldest known remains. The earliest remains of fossil fishes now known occur in the Ordovician or Lower Silurian deposits at Canon City, Colo rado. Among the broken fragments are ap parently parts of shields of ostracophores, scales of crocsopterygians and vertebra of a pos sible chimaroid. It is probable that primitive sharks existed still earlier than this, but no definable remains precede the Devonian.
The class or sub-class of Elasmobranchii (also called Chondropterygii, Antacea, etc.) agrees with the higher fishes in the presence of lower law, shoulder-girdle, pelvic girdle, paired fins, well developed skull, brain and viscera. The gills are well developed, and the general structure and anatomy may be described as fish like.