Branch Chordata 103 



in a firm sheath, and forms an elastic supporting structure. 

 In the Vertebrata (with the exception of Amphioxus and 

 the Lampreys and Hagfishes) it becomes in the adult 

 replaced more or less completely by a segmented bony 

 or cartilaginous axis — the spinal or vertebral column." 

 (Parker and Haswell.) 



The branch or phylum Chordata is divided into three 

 subbranches or subphyla; namely : — 



Subphylum I. Adelochorda. Balanoglossus. 



Subphylum II. Urochorda. Ascidians and Sea-squirts. 



Subphylum III. Vertebrata. Fishes, Amphibians; Rep- 

 tiles, Birds, and Mammals. 



The first of these includes the remarkable worm-like 

 animals belonging to the genus Balanoglossus. The 

 second comprises the Ascidians or Sea-squirts, which are 

 often called Tunicates and grouped together under the 

 name Tunicata. The third includes the vertebrates proper, 

 in which are found the highest animals. 



In the subphylum Vertebrata, some of the very lowest 

 fish-like forms have no skull, or development of the ante- 

 rior end of the vertebral axis. These are classed under 

 the head of Acrania. All the others, or those with a skull, 

 are called Craniata. These are divided into six great 

 classes, named below in the order of their natural develop- 

 ment. The first four are cold blooded, and the other two 

 are warm blooded. 



Class I. Cyclostomata. Lampreys. 



This class comprises comparatively few forms, which 

 have in place of jaws a suctorial mouth; they have no 

 fins or other lateral appendages, and only one nasal 

 aperture. The Lampreys and Hagfishes belong to this 

 class. 



Class II. Pisces. Fishes. 



