THE ANIMAL CLIMAX-THE CHORDATES 



289 



lei ventral folds, a dorsal fold, and a tail 

 fold which is perhaps extensive enough to 

 be called a fin. In order to be more maneu- 

 verable in their search for food, animals 

 gradually developed more elaborate ap- 

 pendages. Shark-like fossil remains of forms 

 possessing many paired fins (Fig. 13-12) 

 seem to indicate that they "had not quite 

 decided" how many pairs of appendages 

 were of the greatest utility, and, according 

 to Romer, only later did they settle down 

 to the orthodox two pairs, the pectoral 



Fig. 13-11. The oldest vertebrates are the ostracoderms 

 shown in the upper figure. They had many features 

 of modern cyclostomes shown in the lower figure. 



(chest region) and pelvic (hip region). 

 From these two pairs of fins, which became 

 so prominent in the early sharks and all 

 later fishes, evolved the appendages of land 

 forms (Fig. 13-13). 



The prehensile jaws of the early primi- 

 tive fish were another important acquisi- 

 tion, making it possible for them to become 

 free swimmers and predators, searching out 

 and capturing their prey. This, of course, 

 went hand in hand with the evolution of 

 better appendages to aid in swimming; 

 both were essential if the animals were ever 

 to become very important, and, what is 

 more, be able to get out of the water and 

 onto the land. A clue to the development 

 of the jaws can be found from a study of 

 the shark's gill arches. These differ but little 

 from the jaw itself, and in fact, they are 

 so much alike in this animal, as well as in 

 many fossil forms, that it is generally 

 agreed that the jaws have developed from 



the first gill arch. As will be shown later, 

 other important organs also develop from 

 these same primitive gill arches. 



The teeth found in the shark's jaw occur 

 in never ending rows and show a remark- 

 able similarity to its scales. It is thus clear 

 that the scales in the region of the mouth 

 opening merely enlarged and became the 

 teeth of the shark. These teeth simply grow 



Fig. 13-12. Primitive sharks had many paired append- 

 ages extending throughout the length of the body, as 

 shown in the above figure. The two conventional pairs 

 (pelvic and pectoral) appeared in later forms and in 

 all present-day vertebrates. 



over the edge of the mouth and are con- 

 tinuously shed as they wear out. Later it 

 will be shown that all teeth are modified 

 scales, including those of man (Fig. 14-4). 

 The sharks and their close relatives, the 

 rays, cannot match the success of the bony 

 fish when it comes to number and variety 

 of forais. There is, however, some diversi- 

 fication in body form among the group, 

 which becomes obvious when the ray is 

 studied. It is greatly flattened, with enor- 

 mously developed pectoral fins that look 

 more like wings (hence the name, sea bat) 

 as they undulate in the sea (Fig. 13-14). 

 The tail is drawn out to a long whip-like 

 structure, which, in the sting rays, bears a 

 spine at the tip. When annoyed, the ray 

 can inflict a painful wound. Some of the 

 rays have gone so far as to produce another 

 form of energy in considerable quantity 

 and employ it as a mechanism of defense, 

 namely, electricity, found in the electric 



