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 
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. 
Fig. 21. 
The accompanying figure represents an alevin trout mag- 
nified. (1) 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- 
