CHAPTER XIV 



PHYLUM CHORDATA: INTRODUCTION 



The Phylum Chordata (Lat. chordatus, having a cord) in- 

 cludes the vertebrate animals (mammals, birds, reptiles, am- 

 phibians, fishes, elasmobranchs, and cyclostomes) and a number 

 of marine forms (Figs. 332 to 341) that are not generally known 

 except to zoologists. All of these animals are characterized at 

 some stage in their existence by (1) a skeletal axis, the notochord, 



(2) by paired slits connecting the pharynx with the exterior, and 



(3) by a central nerve-cord dorsal to the alimentary canal and con- 

 taining a cavity or system of cavities, the neuroccele. In many 

 respects the chordates differ widely from one another, and it is 

 customary to separate them into four subphyla : — 



(1) The Enteropneusta (Gr. enter on, intestine; pneuma, 

 breathe), containing two orders of worm-like animals of some- 

 what doubtful systematic position, 



(2) The Ttjnicata (Lat. tunica, mantle), or sea-squirts, and a 

 number, of other marine forms, 



(3) The Cephalochorda (Gr. kephale, head; chorde, cord), 

 comprising only two families of fish-like animals called lancelets, 

 and 



(4) The Vertebrata (Lat. vertebratus, jointed). 



1. Subphylum I. Enteropneusta 



This subphylum is sometimes given the names Hemichorda 

 or Adelochorda. It contains two orders: (1) the Balano- 

 glossida, and (2) the Cephalodiscida. Four families and 

 about ten genera are recognized in the order Balanoglossida, 



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