CHAPTER II 



THE EARLY CHORDATES 



The cephalochordates and the vertebrates form a closely 

 linked, progressive series; but the hemichordates and urochor- 

 dates differ so widely from the others that they are included 

 here merely to show the trial and error method on the part of 

 nature in developing the fundamental characteristics of the 

 vertebrate form. 



Hemichordata. Considered as a whole the Hemichordates 

 form a rather artificial group, the lower forms being included 

 with the chordates because of certain affinities with Balanoglos- 

 sus. The latter has unmistakable chordate form. The student 

 should not consider this inclusion as a feeling of insecurity on 

 the part of naturalists, but as the natural result of adaptive 

 radiations. The very existence of these intermediate forms with 

 doubtful affinities indicates the development of the higher 

 chordates from worm-like animals whose descendants evolved 

 in many directions. 



Balanoglossiis is a burrowing animal, varying in size from a 

 few centimeters to a meter or more in length. These chordates 

 are often called the Acorn Worms, though their worm-like char- 

 acteristics are all external. The burrowing proboscis (or ''acorn") 

 is a tough, distensible organ with muscular and connective tissue 

 bands running in both circular and longitudinal directions. At 

 the posterior end of the proboscis is the collar, with the mouth 

 within its ventral lip. The mouth empties almost immediately 

 into the pharynx, in which are located the gill slits and gills. 

 The intestine is a straight tube leading to the posterior anus. 

 The nerve cord with its small anterior ganglion, or brain, is 

 located on the dorsal side. The notochord is poorly developed. 



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