SUB;KINGD0M I.— VERTEBRATA. 



CLASS v.— PISCES. 



By R. Lydekker, B.A., F.R.S., V.P.G.S., &c. 



In the preceding section it has been shown that the tadpoles of the Amphibia 

 make a very close approximation in structure — and to a certain extent also in 

 form — to fishes. The resemblance, however, stops suddenly short in regard 

 to the structure of the limbs. In all amphibians the limbs resemble those 

 of the higher vertebrates in being composed of a definite number of segments, 

 and ternrinating in distinct digits, which are normally five in number. On 

 the other hand, although in fishes there are distinct segments in the upper 

 portion of the limbs, these do not correspond exactly with those of the higher 

 animals, and there are no structures corresponding with fingers and toes. 

 The limbs are, in fact, fins, and terminate either in a fringe of rays, or, more 

 rarely, in a jointed cartilaginous rod, bordered on one or both sides with a 

 smaller number of somewhat similar rays. In some fishes, it is true, limbs 

 are wanting ; but then this is also the case among the higher .vertebrates, 

 and does not impair the classificatory value of these appendages. Another 

 important distinction from amphibians is to be found in the circumstance that 

 the median fins' of fishes — which are always present in some form or other — have 

 rays similar to those of the paired fins. Accordingly, in a popular way, fishes 

 may be defined as aquatic, cold-blooded, back-boned animals, breathing the 

 air in water by gills (sometimes supplemented by lungs), and furnished with 

 paired nostrils, paired and median fins, the former of which (when present) 

 differ essentially from the limbs of higher animals. This definition will 

 suffice to distinguish the class Pisces from all the preceding classes, but as it 

 fails to differentiate the group from the lampreys (now regarded as a class 

 apart), it must be added that all Pisces have a functional lower jaw. 



Whether the earliest ancestors of fishes were as purely aquatic in their 

 habits as their descendants is a question somewhat difficult to answer, although 

 the reply should probably be in the aflirmative. Anyway, it is certain that 

 we have at present no indications of a direct transition between fishes and 

 the classes immediately above and immediately below tlxem, so that they 

 stand out as one of the best defined of all groups ; and there is very consider- 

 able uncertainty whether the paired fins of fishes can be regarded as the 

 ancestral type from which the limbs of higher animals were evolved. In 

 spite of certain minor variations — as exemplified by the difference in shape 

 between an eel and a flat fish — all the members of the class are characterised 

 by the adaptation of their bodily form to an aquatic life, a feature in which 

 they resemble whales and porpoises among mammals. As is well observed 

 by Dr. Bashford Dean, this adaptation to aquatic existence "has stamped 

 them in a common mould, and has prescribed the laws which direct and limit 



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