HOW THE FISHES LEARNED TO SWIM — HEINTZ 225 



the fossil remains, but have also endeavored to analyze them and 

 reconstruct them not only in regard to their skeleton but also in 

 regard to their apparent adaptations and manner of life, in a word, 

 their biology. The efforts have then been particularly directed to- 

 ward the elucidation of the relationship between form and function 

 in the various fossil animals, and to solve this problem wide use has 

 been made of comparisons between fossil and recent forms. During 

 later years Osborn in America and Abel in Germany, among others, 

 have worked very intensively in this direction. Abel has even intro- 

 duced a new name for this branch of paleontology which he has 

 designated as paleobiology, i. e., the investigation of the manner of 

 life of the extinct animals. 



In this article I will attempt to deal with some aspects of the bi- 

 ology of the oldest known vertebrates, the fishes and fishlike forms. 

 Through analysis of their structures it is possible to show the grad- 

 ual change in their adaptations and to draw various general con- 

 clusions from this. Investigations of the biology of different 

 groups of Paleozoic fishes have been undertaken by many others 

 (e. g., Kiaer, Jaekel, Abel), but so far as I know there has been no 

 previous attempt to give a comprehensive picture of the gradual 

 development of the different adaptations of these peculiar forms, I 

 shall therefore attempt to say a few words about the biology of the 

 oldest known fishes and fishlike forms, that is, the biology of the 

 Placoderms and Ostracoderms. They have all lived in the water, 

 and it is therefore of great interest to attempt to elucidate their 

 adaptations to life and to movement in the water, that is to 

 swimming. 



But before we proceed to a discussion of these forms in greater 

 detail, it will be better to review briefly the adaptations to swimming 

 which we find among other vertebrates.^ 



It is possible to distinguish three separate groups of organs in all 

 swimming animals: (1) the organs which cause the forward motion 

 itself, that is the organs of locomotion proper, or propulsion, (2) the 

 organs of balance serving to maintain the equilibrium of the ani- 

 mal, and finally, (3) the steering organ by which the direction of the 

 motion is determined, up or down, right or left. These different 

 groups of organs may be very differently developed in different 

 kinds of animals, but all the various modifications, in which adapta- 

 tions to movement through water are found, may nevertheless be 

 classified into four main groups or types. 



The first type, which also represents the most perfect one, is best 

 shown by the free-swimming fishes (fig. 2, A, B, C). We might for 



' More detailed descriptions of the different forms of adaptation among swimming 

 vertebrates can be found in Abel's " Palaeobiologie ", 1912. 



