THE ANIMAL CLIMAX-THE CHORDATES 



281 



Fig. 13-3. Acorn worms (Dolichoglossus kowalevskyi) from the sand flats of Cape Cod. Note the long proboscis for 



burrowing in the sand and mud. 



pharyngeal gill slits. It is obvious that adult 

 land animals have no gill slits, but during 

 embryological development gill slits do ap- 

 pear at some stage. The structures which 

 originally produced functional gill arches 

 in fish, produced other structures in higher 

 forms, such as the sound-making apparatus 

 (larynx) and the sound-receiving appara- 

 tus (middle ear bones) (Fig. 25-11). These 

 fitted the animal better for a terrestrial ex- 

 istence and gave it a greater chance of suc- 

 cess. As already noted, the arthropods have 

 also been able to divorce themselves from 

 water and have likewise developed a new 

 means of communication by employing old 

 structures to perform new duties. Legs and 

 wings are employed in making sound; an- 

 tennae and legs in receiving sound. 



However, the chordates are set off from 

 all other animals by the possession of these 

 three characteristics noted and these must 

 have been important in contributing to the 

 success of this most important of all groups 

 of animals. Let us consider some members 

 of this phylum. 



CHORDATE BEGINNINGS 



Scientists have been perplexed about the 

 origin of the chordates and have been un- 

 able to determine which lower forms gave 

 rise to this last and perhaps most special- 

 ized group. Fossil remains have provided 

 us with a great deal of information about 

 other animals, but man's digging into the 

 earth has failed thus far to reveal any sub- 

 stantial remnants of the early chordates. 

 The reason for this is that these soft-bodied 

 animals did not remain intact sufficiently 

 long to become fossilized. In spite of the 

 lack of evidence concerning the early pro- 

 genitors of the chordates, there has been a 

 great deal of speculation as to their origin. 



The acorn or tongue worms ( Fig. 13-3 ) , 

 which are considered by many zoologists 

 to be very low chordates, were at first clas- 

 sified among the worms. Although they 

 have the three cardinal characteristics that 

 identify them as chordates, they resemble 

 the annelids more closely than any of the 

 great variety of chordate forms. At first 



