VERTEBRATE ANIMALS 151 
pectoral fins for instance can be employed as a brake, or 
used in such a way as to drive the fish backwards. Again, if 
held forwards and downwards whilst swimming, the pressure 
of the water on their upper surfaces causes the animal to 
descend ; whilst by extending them backwards and down- 
wards the pressure on the under surtaces causes the fish to 
ascend. ‘The unpaired fins act in the same way as the keel 
of a boat, serving the purpose of a cut-water, and helping to 
Cup Shapea 
Depression 
A. B. 
Fig. 99.— Vertebrae from tail region.— A, showing double row of spines and 
cup-shaped depression in centre. B, arches in spines. 
steady the fish as it moves along. ‘The extreme freedom of 
movement possessed by the paired fins, as also the great 
flexibility of their joints, should be specially noted. 
The back-bone and teeth. Amongst the most 
characteristic bones of Vertebrates are the backbone and 
the teeth. In the fish each vertebra consists of a rounded 
centrum, (solid central part) on either side of which is a 
