CHAPTER II 

 THE PHYLUM CHORDATA 



The characteristics of the vertebrates have been sufficiently stated 

 in the previous chapter. A vertebrate proper may be readily recog- 

 nized by applying to it the definition on page 1, but there is a 

 considerable assemblage of creatures, which, though falling short of 

 being vertebrates in some respects, are obviously vertebrate-like in 

 others, and are therefore classed with the Vertebrata or Craniata in 

 the phyliun Chordata. 



These creatures range all the way from worm-like, burrowing forms 

 such as Balanoglossus and the sessile, tubicolous Cephalodiscus, 

 through the degenerate, sedentary, flask-like tunicates, to the pro- 

 fish Amphioxus. When we include the vertebrates, the range of 

 structural diversity and degree of specialization, with Rhabdopleura 

 at one extreme and Man or the whales at the other, is so great 

 that one doubts the advisabiUty of making a single phylum so all- 

 inclusive. 



If, however, we adopt as the specifications of a chordate the pos- 

 session of a notochord, pharyngeal defts, and a medullary plate, we 

 arbitrarily throw together all animals that possess these characters. 

 It therefore becomes essential that we have exact criteria for 

 recognizing these characters in whatever guise they may be pre- 

 sented. 



Notochord. — The notochord is recognized by its position, by its 

 histological structure, by its function, and by its embryonic deriva- 

 tion. Typically it is a stiff hyaline rod (Fig. 8) covered with a con- 

 nective tissue sheath, lying ventral to the neural tube and dorsal to 

 the alimentary tract. It is composed of vacuolated cells that histo- 

 logically resemble pith. It is derived from a median dorsal strip of 

 tissue cut off from the primitive endoderm or archenteron. 



Pharyngeal Clefts (Gill-slits). — ^The term "pharyngeal clefts" is 

 preferred to "gill-slits" because it is more accurately descriptive and 

 has no dubious functional implications. The pharyngeal cleft is 

 simply an opening through the body wall in the pharyngeal region 



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