344 STRUCTURE OF THE VERTEBRATES 



The shark-like structure of the sturgeon (Chondrostei) makes 

 it and its relatives a good starting point for the development of 

 the Holostei and the Teleostei. The two former have always 

 been rather limited in their distribution. The teleosts (most of 

 the recent fish) have evolved in every conceivable direction. 

 Some forms are eel-like; others have become flattened dorso- 

 ventrally into wide, slow moving fish; others have flattened 

 laterally until they are almost transparent; a few are so short- 

 ened that they consist of hardly more than a head and a very 

 short body; the soles and flounders undergo a torsion during 

 development; and many of the deep-sea fish are strangely dis- 

 proportioned. The teleosts represent the extreme specializations 

 toward ''ichthyization", and represent fish-like characters as 

 we know them today. 



Early in the Silurian the lobe-finned fish (Crossopterygii) 

 developed from the primitive group. These and the Dipnoi 

 parallelled each other, and became ''de-ichthyized" in character 

 as the swim-bladder evolved into a functional lung. In their 

 early history the crossopterygians were more generalized, and 

 one small group developed the rudiments of a humerus, radius 

 and ulna. The dipnoans, on the other hand, were specialized 

 from earliest known time. The teeth had lost the generalized 

 character necessary for further evolution, and the fins had a 

 median line of bones with rays on either side. The latter could 

 not have evolved into a vertebrate hand and arm, and special- 

 ization apparently could proceed only in the direction w^hich 

 it has taken — toward degeneration. 



C. Evolution of the Amphibia 



The evidence for the evolution of the amphibia from the 

 crossopterygian fish has been summarized under separate chap- 

 ter headings. This wuU be recalled as (1) the retention of the 

 spiracle, which became the middle ear; (2) the structure of the 

 pectoral fin; (3) the swim-bladder with its ventral "trachea" 

 and its bi-lobed, vascular lungs; (4) the blood supply of the 

 lungs coming from the sixth aortic arch; (5) external gills in 

 the larva, and the general similarity between the tadpoles of 



