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 200-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 (dermmotrichia) 
and by endoskeletal pieces (somactéds or radials), some of which are 
articulated to the girdles and are then called dasa/za. 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 (4) 
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 
vertebral 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. 
