CHAPTER XIV 

 PHYLUM CHORDATA: INTRODUCTION 



THE Phylum CHORDATA (Lat. c/wrdatus, 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 (i) 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 neurocoele. 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 TUNICATA (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). 



i. SUB PHYLUM I. ENTEROPNEUSTA 



This subphylum is sometimes given the names HEMICHORDA 

 or ADELOCHORDA. It contains two orders: (i) the BALANO- 

 GLOSSIDA, and (2) the CEPHALODISCIDA. Four families and 

 about ten genera are recognized in the order BALANOGLOSSIDA, 



386 



