ADELOCHORDA 



141 



The group Vertebrata consists of two divisions, the first, 

 Acrania, including the skull-less forms, e.g., the lancelet 

 (Amphioxus), while the second, and much larger divi- 

 sion, Craniata, consists of six great classes, Cyclostomata, 

 Pisces, Amphibia, Reptilia, Aves, and Mammalia. The 

 first four are "cold-blooded," the other two are " warm- 

 blooded." Cyclostomes, fishes, and amphibians have 

 gills during the whole or a part of their lives, while the 

 rest never have gills. Fishes and amphibians in embryo 

 have neither amnion nor allantois, while the animals in 

 the last three classes are provided with both. 



The skull bearing vertebrates may be grouped into 

 three provinces. 



Cyclostomes, fishes, and amphibians agree in having 

 gills or gill pouches, in wanting amnion and allantois, 

 and in possessing nucleated red blood corpuscles 

 (Ichthyopsida ). 



Birds and reptiles agree in having 

 no gills, but both amnion and allan- 

 tois, in the articulation of the skull 

 with the spine by a single condyle, 

 in the development from the skin of 

 feathers or scales, and in having 

 oval, nucleated, red corpuscles (Sau- 

 ropsi'dd). 



Mammals differ from birds and 

 reptiles in having two occipital con- 

 dyles, and their red blood corpuscles 

 are not nucleated 33 (Mammalia). 



SUBBRANCH AND CLASS i . Adelochorda 



FIG. 113. Balanoglossus, 



The principal representative of this A proboscis; c," collar"; 



i D / / r.-UJ-j **, gill slits. Enlarged. 



class is Balanoglossus, a soft-bodied, 



wormlike animal, one inch to six inches long, which lives 



in the sand and mud, along the Mediterranean coast, 



