KEY TO CLASSES AND ORDERS OF ANIMALS 47 

 2(1) Body not wormlike. 3 



3(4. 5) Body more or less sac shaped and inclosed in a gelati- 

 nous or tough tunic; soHtary or colonial; sessile or 

 free swimming. (May be minute tadpole-like forms 

 with a long tail which contains a notochord.) Always 

 with two external openings for intake and outflow of 

 water; marine. (Pratt, 555.) 



Subphylum Urochordata (Tunicata) 



4(3, 5) Body lanceolate; marine. (Pratt, 671.) 



Subphylum Cephalochordata (Leptocardia) 



5(3, 4) Body shape various, typically with a vertebral column 



of either cartilage or bone. Marine, fresh water or 



terrestrial. 



Subphylum Vertebrata (Craniata), p. 47 



Subphylum VERTEBRATA 



I (6) Fishlike animals living in water and breathing by 

 means of internal gills; lungs and appendages other 

 than fins, absent; partially developed legs may be 

 present in tadpoles. 2 



2(5) Paired fins lacking. 3 



3(4) Jaws lacking; mouth round and closed only by the 

 end of the "tongue"; internal gills; eel-like. 



Class Cyclostomata, p. 48 



4(3) Horny jaws present except in very young tadpoles 

 when the mouth has not yet broken through in which 

 case the animal has plumelike external gills; external 

 gills may be absent and partially developed legs may 

 be present in older forms. 



Tadpole larvae of Class Amphibia 



5(2) Paired fins usually present but even if lacking the 

 mouth has true jaws. Class Pisces, p. 48 



