Zöology; a textbook for colleges and universities . re; but there isa typical shark existing today in the fresh water of LakeNicaragua, Central America. 2. The classification of fishes has given rise to manytionoffishes Differences of opinion, and is still subject to modifica-tion. The object sought is to arrange all the fishes inaccordance with their natural relationships, assumingthem to have evolved from a common ancestor. Whilethere must be a true or ideal classification, accuratelyrepresenting the historical facts, it is hardly to be ex-pected that we shall ever completely attain it, thou
Zöology; a textbook for colleges and universities . re; but there isa typical shark existing today in the fresh water of LakeNicaragua, Central America. 2. The classification of fishes has given rise to manytionoffishes Differences of opinion, and is still subject to modifica-tion. The object sought is to arrange all the fishes inaccordance with their natural relationships, assumingthem to have evolved from a common ancestor. Whilethere must be a true or ideal classification, accuratelyrepresenting the historical facts, it is hardly to be ex-pected that we shall ever completely attain it, thoughwe continually move toward it. Regarding the fishes(Pisces) as a class, we have the following principal divi-sions : (a) Subclass Elasmobranchii, or the sharks and theirrelatives. Some would separate these as a classdistinct from the Pisces. In the true sharksand rays (or skates) - - the latter being broad, 342 Classifica- Sharks andrays FISHES 343 flattened-out sharks - - the slitlike gill openingsare five to seven on each side, the skeleton is. From Animate Creation FIG. 126. A dogfish, a small species of shark, with two of its youngand two of its egg cases. cartilaginous, and the skin is beset with thorn- Piacoidlike (placoid) scales, or granules, but in Mus-telus, the dog shark, with pointed, overlappingscales. The eggs are large and comparativelyfew ; they are deposited in leatherlike cases orhatched within the body. The teeth of sharks Teeth ofare characteristic - - usually pointed or more orless serrated, often triangular, and sometimesvery large. They are extremely hard, and con-sequently are often preserved in the rocks asfossils. Associated with the elasmobranchs,but very peculiar, are the Holocephali or Chi-mseras, which are comparatively rare today, but 344 ZOOLOGY were once more numerous, and are known to beof immense antiquity. The body is long andtapering, and the thick head, with its bluntsnout and large eyes, has a most grotesque ap-pearance. The skull is ve
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectzoology, bookyear1920
