46 ANATOMY AND PHYSIOLOGY 



MOTION AND USE OF FINS. 



By these appendages, equivalent to arms and 

 feet, the fish performs a variety of motions. The 

 propelling instrument, the exclusive organ of pow- 

 erful movements, is the candal fluke or tail-fin, 

 which operates on the water precisely like a single 

 oar, in sculling a boat. Those on the sides are 

 only serviceable in balancing, wheeling round, and 

 in making a sudden stop. That upon the back 

 sustains a very interesting office, being, in fact, a 

 keel, which, on boats, is placed on the under side. 

 Had the keel of a fish been on the abdomen, in- 

 stead of the back, it would have destroyed its abil- 

 ity for feeding on the bottom ; moreover, there 

 would have been great danger of grounding in 

 shoal water. Nature, contemplating these objec- 

 tions, wisely placed this important appendage, by 

 which the body is kept upright, on the back, out 

 of the way, as well as out of danger.* 



* It would be worth the while to ascertain whether Dr 

 Bushnell, the inventor and only successful navigator of the 

 sub-marine boat, which was constructed for attaching kegs of 

 powder to the bottom of British ships, during the Revolution, 

 had the keel on the upper side. As his first object was to 

 sink under water, there would be circumstances in which the 

 under-side keel would fasten the boat in the mud, particularly 

 if he were scudding before a whale, who threatened to swal- 

 low the whole concern, if he did not succeed in reaching a 

 place too shoal for the pursuing enejny. 



