X FISHES 349 



tail-region has been converted into a feeble electric organ. In the 

 Electric Ray on the other hand — to which the Romans gave the name 

 Torpedo, still used as the generic name — ^much larger masses of muscle^ 

 connected with the branchial arches, have become converted into an 

 electric organ capable of giving powerful shocks. 



Usually included within the limits of the Elasmobranchii are a number 

 of strange, mostly deep-sea, fish grouped together under the name Holo- 

 cephali — the last survivors of an ancient group of fishes dating from 

 Palaeozoic times. They are comparatively rare fish except one species, 

 belonging to the genus Chimaera, which is common off the west coast of 

 North America. 



The Holocephali agree in their general -structure with the other 

 Elasmobranchs but they show decided peculiarities of their own. The 

 notochord persists throughout life and there are no vertebral centra 

 developed. The tail is protocercal. The upper jaw undergoes complete 

 fusion throughout its length with the cranium so that the lower jaw 

 articulates directly with the cranium (autostylie skull). And, as in the 

 more highly evolved fishes about to be described (Teleostei), the spiracle 

 is obliterated, the outer ends of the gill-clefts have become confluent into 

 a single (opercular) opening, and the cloaca has become flattened out 

 so that the nephridial and genital ducts open on the external surface 

 independently of the intestine. 



Of the Teleostomatous fishes it will be convenient to consider first 

 the Teleostei, the group which includes the great majority of existing 

 fishes and nearly all of those familiar in ordinary life. Among all verte- 

 brates the teleostean fishes stand out pre-eminent in their adaptation to, 

 and speciaUzation for, a swimming existence. 



The general form of the body may depart from the ordinary " fish " 

 shape (Fig. 145) by being " depressed," i.e. flattened in a dorso-ventral 

 direction as is the case with the Skates amongst Elasmobranchs, or it 

 may be " compressed," flattened from side to side, as in a Bream or as 

 — to an extreme extent — in the flat-fishes or Pleuronectidae, or it may 

 be much drawn out in length as in the Eel. 



There is an equipment of fins, similar in general arrangement to that 

 of Elasmobranchs. The caudal fin, while fundamentally similar to that 

 of the elasmobranch, has developed a superficial difference by assuming 

 a secondarily symmetrical (homocercal) form (Fig. 146, E and F). That 

 this symmetry is merely superficial is shown by the internal structure 

 — the tip of the vertebral column not being continued out through the 

 central line of the tail but being distinctly tilted upwards. In cases 

 where the tail loses its importance as a propelling organ it is apt to 



