HOW THE FISHES LEARNED TO SWIM HEINTZ 241 



We have here again a group of typical bottom forms, with a 

 flat ventral shield and a strongly arched dorsal armature. The most 

 peculiar feature is the development of the pectoral fins, which are 

 modified into strongly reinforced spines (fig. 10). By means of 

 a very complicated joint these spines are movably connected with 

 the body shield. It is very tempting to regard these organs as a kind 

 of oars by means of which the animals could swim through the water. 

 This interpretation is rather improbable, however, as the connecting 

 joint between the body and the spines is much too complex to permit 

 any rapid motion, and the oars themselves are furthermore too thick 

 and narrow and cannot be said to be in any manner an efficient type 

 of swimming organ. On the other hand, they may very well have 

 served as steering organs. The posterior part of the trunk and the 

 tail were very short in the oldest form, and covered by scales. In the 

 younger ones they were considerably longer and more slender and the 

 scales were completely reduced. No Antiarchi show any perfect 

 adaptation to free swimming. They all remained benthonic and only 

 varied in their ability to lift themselves from the bottom. 



I shall not attempt any detailed discussion of the two largest groups 

 of fishes, the Elasmobranchs and the Teleostomes, which are the 

 dominant forms in recent times. The Elasmobranchs are known as 

 far back as from the upper Silurian, the first Crossopterygii were 

 certainly from the Lower Devonian. 



Even the oldest Teleostomes could in all probability swim relatively 

 efficiently (fig. 11, A, D). The body was more or less oblong, and 

 they had a large caudal fin as well as median and paired fins. I shall, 

 however, not discuss their structure any further but shall proceed to 

 another problem. 



It is commonly acknowledged that the original or so-called primary 

 form in fishes was more or less torpedolike, a form which is very well 

 adapted for free-swimming in the water. The fishes have subse- 

 quently also adapted themselves, secondarily however, to other modes 

 of life and have correspondingly modified their shape. Thus some 

 of them have become flying fishes, others have changed to a bottom 

 mode of life and some have become deep-sea fishes. Conditions are 

 fairly clear insofar as we consider only the recent and the higher 

 fossil Teleostomes. In these cases we can as a rule show that for 

 instance the bottom forms have always been derived from free- 

 swimming ancestors. 



On the other hand, we have in the preceding considered a series 

 of the old Paleozoic fishes and fishlike forms and know that they 

 began from a bottom existence and only gradually learned to swim. 

 We have seen that of a great number of Ostracoderms and Placo- 

 derms perhaps onlj' a few, and then as a rule the youngest ones, can 

 really be regarded as having been nectonic forms, that is, forms 



