vI EXTERNAL CHARACTERS ones 
The eyes of Fishes are usually very large. They are generally 
situated on the sides of the head, but in the “Star-gazers ” 
(Uranoscopus) they are on the upper surface and close together. 
In the gogele-eyed Periophthalmus the eyes seem to protrude 
from their orbits, and in a variety of a species of Carp, the Gold- 
Fish (Cyprinus auratus), the protrusion is so marked that the eyes 
seem as if on stalks. In a few species, which live either in caves 
or at very great oceanic depths, the eyes become vestigial, and are 
hidden beneath the skin, or are even covered by scales (Fig. 430). 
In the Elasmobranchs and Dipnoi the olfactory organs retain 
their primitive position as pit-like sacs on the ventral surface of 
the snout, just in front of the mouth. In the Dipnoi (e.g. Proto- 
wterus) each olfactory sac has two apertures, of which one, the 
external nostril, is placed on the under surface of the snout, while 
the other, the internal nostril, opens within the upper lip into 
the oral cavity—a feature which is unique among Fishes. In 
nearly all Teleostomi, also, each sac has two nostrils, which, how- 
ever, are situated either on the upper surface or on the sides of 
the fore-part of the head, and have no communication with the 
mouth. 
Directly behind the head in Elasmobranchs, or beneath its 
hinder part in all other Fishes, are placed the external apertures 
of the branchial clefts. In the former group these apertures 
are visible externally in the form of a series of narrow vertical 
slits, but in the latter they communicate with the exterior by 
opening on each side into a common branchial cavity, the outer 
wall of which is formed by a movable flap-like fold with a free 
hinder margin and a special internal skeleton of cartilaginous 
rays or of bony plates and rods, the gill-cover or operculum 
(Fig. 161, B). Behind the free margin of the operculum there 
is a slit-like orifice, the gill-opening or external branchial aper- 
ture, which curves from above downward and forward toward 
the chin, and places the branchial cavity in communication with 
the exterior. Through this aperture the water, which has 
entered through the mouth, traversed the gill-clefts, and bathed 
the gills, finds its exit from the body. The space on the ventral 
side of the head between the two halves of the lower jaw, and 
between the two external branchial apertures, is termed the 
“isthmus.” The size of the external branchial aperture differs 
greatly in different Fishes, according to the extent to which the 
