CHAPTER XLVIII 



CLASSIFICATION OF FISHES, AMPHIBIANS, REPTILES, 



AND MAMMALS. 



Unless this chapter is mastered, there can be no understanding of 

 the textual matter which follows, as the scientific terms there used are 

 all based on what this chapter contains. 



CHORDATA 



The Chordata possess a notochord at sometime during their life's 

 history (the notochord lying between the nervous system and the ali- 

 mentary tract), a hollow central nervous system lying entirely on one 

 side of the digestive canal, and pharyngeal slits extending from the 

 pharynx to the exterior. 



The Chordata are divided into four sub-phyla, all of which develop a 

 notochord during their embryonic period, though all do not later develop 

 a bony vertebral column. 



The sub-divisions of the Chordata (Figs. 313, 314, 315, 316) are as 

 follows : 



Sub-Phylum I. Cephalochordata (Adelochorda, Fig. 312). 



The notochord runs only up to the head proper in most chordates, 

 but in the Cephalochordata, of which Amphioxus is the classic example, 

 the notochord extends to the very anterior end of the body. Amphioxus 

 is fish-like in form and is used as an example of the most primitive form 

 of the chordates. It will be remembered that there was reference made 

 to the simplicity of the embryology of Amphioxus in Part III of this 

 book. 



Amphioxus has no skull or vertebral column. The pharyngeal slits 

 are quite numerous. The true scientific name of Amphioxus is Branchi- 

 ostoma. In popular language it is often called lancelet, on account of its 

 sharp, lance-like appearance. 



Sub-Phylum II. Urochordata (Tunicates, Figs. 312, 313). 



This group possesses a notochord only in the caudal region. The 

 young are tadpole-like, and there is a metamorphosis converting the 

 tadpole into a sac-like structure. 



Order I. Larvacea (Appendicularia), free-swimming forms with 

 permanent tail. 



Order II. Ascidiacea (Tunicates or Sea-Squirts), fixed forms with- 

 out tail in the adult. 



Order III. Thaliacea (Salpians), free-swimming forms without tail 

 in the adult. 



The neurenteric canal is permanent. 



