THE SKIN AND SKELETON. 
139 
of ribs and tail, but no breast-bone; the Frog has a breast¬ 
bone. but neither tail nor ribs. As the skeleton of a Fish 
is too complicated for the primary student, we will select 
for illustration the skeleton of a Lion—the type of quad¬ 
rupeds. It should be remembered, however, that all Ver¬ 
tebrates are formed on one plan. 
In the lowest Vertebrate, Arnphioxus, the only skeleton 
is a cartilaginous rod running from head to tail. There is 
no skull, nor ribs, nor limbs. In the cartilaginous Fishes, 
