392 



jaws are quickly opened. This fish reaches a 

 length of over three feet and has a mouth 

 more than a foot wide. 



Deep-sea fishes 



Many families of fishes contain deep-sea 

 species (Fig. 265) which are often curiously 

 modified. Some have very large eyes, which 

 enable them to catch as many rays of light 

 as possible; these eyes probably serve in con- 

 nection with luminescent organs. Others 

 have small or rudimentary eyes and are 

 blind; they depend upon organs of touch 

 instead of eyes. Many have large mouths 

 with long sharp teeth and enormous stom- 

 achs. The luminescent organs are variously 

 distributed over the body. One type consists 

 of a cup of secretory cells covered by a cellu- 

 lar lens. The secretion is luminous, and in 

 certain cases acts as a lure; in others, it prob- 

 ably enables the fish to see in the dark 

 abyss of the ocean. 



Lungfishes 



The ability of the lungfishes to breathe 

 air is suggestive of an intermediate stage be- 

 tween fishes and amphibians. Furthermore, 

 the Australian lungfish is able to "walk" 

 along the bottom of the rivers in which it 

 lives by using its paired fins as legs. Yet in 

 spite of such specializations in the lungfishes 

 which might lead one to conclude that they 

 were a connecting link between water and 

 land animals, the over-all evidence points 

 clearly to the fact that these vertebrates 

 have never been in the direct line of evolu- 

 tion leading from fishes to the first land- 

 living vertebrates. The lungfishes are now 

 regarded as an ancient group that has 

 changed little through the recent geologic 

 ages. 



The lungfish has an opening between the 

 nasal sac and the mouth cavity, a persistent 

 unconstricted notochord, and an air bladder 

 which opens into the pharynx and functions 

 as a lung. The Australian lungfish Neocera- 



COLLEGE ZOOLOGY 



todus (Fig. 252) lies on the bottom of 

 stagnant pools and feeds on small animals; 

 occasionally it comes to the surface in order 

 to change the air in its single lung. Because 

 of this lung it can exist in water unfit for 

 fishes that breathe entirely with gills. 



The African lungfishes, Protopterus, live 

 in the marshes of central Africa. During the 

 dry summer season they burrow about 18 

 inches into the mud, where a cocoon of 

 slime is secreted; here they remain inactive, 

 breathing with lungs and living on fat 

 stored in the kidneys and gonads until the 

 rainy season comes again. The South Amer- 

 ican lungfish Lepidosiren also hibernates 

 in the mud during the dry season. 



Fossil fishes 



A large number of species of fish are 

 known only from their fossil remains. The 

 earliest fish remains consist of spines and 

 scales from the lower Silurian or Ordovician 

 strata of the earth's crust, which were laid 

 down probably over 300 million years ago. 

 The Devonian age is called the "Age of 

 Fishes" because of the predominance of 

 fishes over the other animals that lived at 

 that time. A considerable portion of the 

 Osteichthyes are fossils: 4 of the 7 families 

 of the Neoceratodida (Dipnoi); 6 of the 7 

 families of the Crossopter^'giida; 41 of the 

 43 families of Chondrosteica; 9 of the 11 

 families of Holosteica; and but 28 of the 402 

 families of Teleosteica. The study of fossil 

 fishes is very important because of the light 

 these prehistoric forms shed upon the af- 

 finities of modern species. 



RELATIONS OF BONY 

 FISHES TO MAN 



Although a few fishes are injurious be- 

 cause they destroy valuable food fishes and 

 other useful aquatic animals, many are of 

 use to man, serving either as food or as a 

 means of recreation. Among the fresh-water 



