564 PISCES FISHES. 



depths from 80 to 200 fathoms, the eyes tend to be larger than usual, 

 as if to make the most of the scanty light ; beyond the 2OO-fathom line 

 small-eyed forms occur with highly developed organs of touch, and 

 large- eyed forms which have no such organs, but perhaps follow the 

 gleams of "phosphorescent" organs; finally, in the greatest depths 

 some forms occur with rudimentary eyes. Many of these abyssal fishes 

 are phosphorescent; the colouring is usually simple, mostly blackish 

 or silvery ; the skin exudes much mucus ; the skeleton tends to be light 

 and brittle ; the forms are often very quaint ; the diet is necessarily 

 carnivorous. 



GENERAL NOTES ON THE STRUCTURE OF FISHES 



Fins. Along the dorsal and ventral median line of some fishes, 

 e.g. flounder, there is a continuous fin a fold of skin with dermal 

 fin-rays (dermotrichia) and deeper skeletal supports (somactids). 



In the embryos of many fishes the same continuous fringe is seen, 

 while the adults have only isolated median fins. There is no doubt 

 that these isolated median fins of which there may be two dorsals, 

 a caudal, and an anal or ventral arise, or have arisen, from a modifica- 

 tion of a once continuous fin. 



Now, the paired fins, which correspond to limbs, often resemble 

 unpaired fins in their general structure, and in their mode of origin. 

 It is possible that the paired fins may have arisen by a localisation of 

 two once continuous lateral folds. According to another theory, the 

 origin of paired fins is to be found in the visceral arches. 



The paired fins are supported by dermic fin-rays (dermotrichia} 

 and by endoskeletal pieces (somactids or radials], some of which are 

 articulated to the girdles and are then called basalia. Two main types 

 of fish fin are distinguishable (a) that best illustrated among living 

 fishes by Ceratodus^ in which a median jointed axis bears on each side 

 a series of radial rays a form often called an archipterygium ; and (&) 

 the commoner type, in which the radials arise on one side of the basal 

 pieces (an ichthyopterygium). In the bony fishes the support of the 

 fin beyond the base seems mainly due to dermal rays. 



Tail. In Dipnoi and a few Teleosteans, e.g. the eels, the vertebral 

 column runs straight to the tip of the tail, dividing it into two equal 

 parts. This perfectly symmetrical condition is called diphycercal or 

 protocercal. 



In Elasmobranchs, Holocephali, cartilaginous and many extinct 

 " Ganoids," the vertebral column is bent dorsally at the end of the tail, 

 and the ventral part of the caudal fin is smaller than, and at some little 

 distance from, the upper part. This asymmetrical condition is called 

 heterocercal. 



In most Teleostei, and in extant bony "Ganoids," the end of the 

 vertebra 1 ! column is also bent upwards, but the apex atrophies, and, by 

 the disproportionate development of rays on the ventral side, an 

 apparent symmetry is produced. The vertebral column usually ends 

 in a urostyle, the undivided ossified sheath of the notochord. Most 

 of the fin really lies to the ventral side of this. The condition is 

 termed homocercal. 



