vi EXTERNAL CHARACTERS I 5 5 



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 goggle-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 011 the ventral surface of 

 the snout, just in front of the mouth. In the Dipnoi (e.g. Proto- 

 vterus) 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 



