346 



METAZOAN PHYLA 



like that of a lobe-finned ganoid or an amphibian than like that of a 

 bony fish. 



371. Teleostei. — The teleosts are a large and relatively modern 

 group, presenting now a variety and a number of species greater than 

 at any previous time. In this division come the most of our food fishes 

 (Fig. 235). 



372. Dipnoi. — The lungfishes have for a long time been noteworthy 

 among fishes because of the belief that they represented the forms 

 from which amphibians and higher vertebrates arose. More recent 

 investigations, however, seem to throw doubt upon this belief and to 

 indicate that the lungfishes are degenerate descendants of the lobe- 

 finned ganoids, which they resemble. When the bodies of water in 



Dorsal fins 



Operculum 



Laferal 

 line 



Nostril 



Caudal 

 fjn 



Branchiosfegal 

 rays 



Fig. 235. — Common perch, Perva flavescens (Mitchill). X j^/.i- Labelled to show features 

 of a typical fish. {From Forbes and Richardson, "Fishes of Illinois.") 



which they live dry up and become foul, lungfishes possess the ability 

 to come to the surface and take air into their air bladder, which serves 

 functionally as a lung. Others live in marshes and when these dry 

 up the fish becomes dormant in the mud at the bottom, coiled in a burrow 

 lined by a capsule of hardened mucus secreted by the glands of the 

 skin. Within this capsule the lungfish remains, surrounded by a slimy 

 mucus and breathing through an air hole the margin of which is turned 

 inward to form a tube which is inserted in the mouth of the fish. During 

 this time the air bladder is used constantly as a lung. 



The lungfishes are represented by several types, one in Australia, 

 another in Africa (Fig. 236), and a third in Paraguay. Not only do 

 they have a tadpole-like larva similar to that of the Crossopterygii on 

 the one hand and the Amphibia on the other, but they also have a more 

 primitive type of embryogeny than that of any other living fish, the egg 

 being holoblastic and gastrulation taking place by invagination. 



