PROGRESSION ON AND IN THE WATER. 



65 



happens in the feathering of an oar. A very similar plan is 

 adopted by the Pteropoda, found in countless multitudes in 

 the northern seas, which, according to Eschricht, use the 

 wing-like structures situated near the head after the manner 

 of a double paddle, resembling in its general features that at 

 present in use among the Greenlanders. The characteristic 

 movement, however, and that adopted in by far the greater 

 number of instances, is that commonly seen in the fish (figs. 

 29 and 30). 



G. 29.— Skeleton of ths Perch {PercafluviatUis). Shows the jointed nature of 

 the vertebral column, and the facilities afforded for lateral motion, particu- 

 larly in the tail id), dorsal {e,f), ventral (b, c), and j^ectoral («), fins, which 

 are principally engaged in swimming. The extent of the travelling sur- 

 faces required for water greatly exceed those required for land. Compare the 

 tail and fins of the present figure with the feet of the ox, fig. 18, p. 37. — 

 (After Dallas.) 



Fig. 30.— The Salmon {Salmo salar) swimming leisurely. The body, it will be 

 observed, is bent in two curves, one occurring towards the head, the other 

 towards the tail. The shape of the salmon is admirably adapted for cleav- 

 ing the water.— Ori{/i?mZ. 



This, my readers are aware, consists of a lashing, curvi- 

 linear, or flail-like movement of the broadly expanded tail, which 

 oscillates from side to side of the body, in some instances with 

 immense speed and power. The muscles in the fish, as has 



