CHAPTER XXII 
Ciass PISCES—FISHES 
Sub-Class I. ELASMOBRANCHII :-— . 
Order Plagiostomi (skates and sharks). 
Order Holocéphali (Chimera and Callorhynchus). 
Several extinct orders, e.g. Acanthodei. 
Sub-Class II. TELEOSTOMI :— 
Order Crossopterygii (Polypterus), 
Order Chondrostei, ¢.g. sturgeon. 
Order Holostei, ¢.g. bony pike. 
Order Teleostei, the great majority of living fishes, 
Sub-Class III. DipNor :— 
Ceratodus, Protopterus, and Lepidosiren, and many extinct forms.. 
Fisues form the first markedly successful-class of Verte- 
brates. For though the Tunicates are numerous, most of 
them are degenerate; the level attained by the lancelets. 
is represented by, at most, two or three closely related. 
genera; and the Cyclostomes are also few in number. 
In the possession of a vertebrate axis and central nervous. 
system, in the general integration of their structure, and in. 
their great fecundity, Fishes have an easy pre-eminence. 
over their Invertebrate inferiors. With their typically 
wedge-like bodies, supple muscular tails, fin-like limbs, and. 
the like—they are well adapted to the medium in which 
they live. 
Their success may be read in the immense number of 
individuals, species, and genera, not only now, but in the: 
past; in the geological record which shows how the 
cartilaginous Elasmobranchs have persisted strongly from. 
Silurian ages, or how the mysterious decadence of the. 
“Ganoids” has been followed by a yet richer predomin- 
ance of the modern Bony Fishes; and, furthermore, im 
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