50 LECTURE VIII. 



its confinement. During the first six or eight 

 hours from its birth, it is observed to grow 

 nearly as much in proportion as in fifteen or 

 twenty days afterwards. In the young ani- 

 mal before it has left the egg, the pulsations or 

 beatings of the heart amount to about forty in 

 a minute; but immediately after hatching, they 

 are increased to the number of sixty in a minute. 

 Young fish, in this very diminutive state, are so 

 transparent as to exhibit with great distinctness 

 the course even of their larger blood-vessels. In 

 the work of Dr. Bloch we find a good representa- 

 tion of a fish in this state, viz,, the Common 

 Barbel. 



The age of fish is, according to Linnaeus, 

 determinable from the number of concentric cir- 

 cles of the vertebrae or joints of the back-bone, in 

 the same manner as that of trees is supposed to 

 be from the concentric circles of the wood. Leew- 

 enhoeck used to imagine that the age of fish might 

 be determined from the concentric circles or 

 fibres of the scales; but perhaps it may admit of 

 much doubt whether either of these opinions be 

 true. 



After this general survey of fishes as a tribe,, 



