194 How to obtain it. 



stage and are kept in constant motion. They are rather 

 suggestive of a couple of miniature fans, and they have a some- 

 | what similar and a very important part to play, in causing fresh 

 j currents, and so rendering assistance to the gills. As a lady's fan 

 is used in a close and heated room for producing currents of air, 

 so these little fans or "pectorals " are used for producing currents 

 of water, and they form no insignificant part of the breathing 

 apparatus of the little fish. They not only assist respiration in 

 this way, but also by driving off impurities which may exist, as for 

 instance those caused by exhalation from the fish themselves. 



The accompanying figure represents an alevin trout mag- 

 nified, (i) The cranial cavity containing the brain ; (2) the gills or 

 breathing apparatus ; (3) the heart ; (4) one of the pectoral fins, 

 used at this stage for assisting respiration by causing currents, and 

 acting like a fan to the gills ; (5) here the wood-cutter has not 

 connected the dotted line, as he should have done, with one of 

 the numerous rounded bodies seen in the umbilical sac or yolk 

 bag, and which are oil globules : he has also very cleverly made 

 this dotted line to close the chief vein, making it appear as if cut 

 off, whereas, in reality, it simply disappears from view at this 

 point by being out of focus ; (6) represents the vertebral column ; 

 (7) the anus. The rudiments of the dorsal fin are quite apparent, 

 and the rays of the caudal fin or tail may also be traced. The 

 sketch is taken very soon after hatching, and the daily develop- 



