12 INTRODUCTION. 



the corpuscles of blood, are beautifully shown 

 with a microscope of moderate power. Their 

 external appearance is remarkable. The eyes 

 are large and quite well developed ; the pec- 

 toral fins are also in an advanced stage of de- 

 velopment, and in constant and rapid motion, 

 which I think, in the more advanced stage of 

 the fish, has something to do with its resj)ira- 

 tion, as they are placed near the opening of 

 the gill covers. The other j)ortions of the fish 

 are quite rudimentary, no other fins being per- 

 ceptible, but in their place there is an attenu- 

 ated margin, or finlike substance, as on the tail 

 of the tadj)ole, commencing where the dorsal 

 fin should be, and continuing uninterruptedly 

 around the caudal, and terminating with the 

 anal fin, or rather where it should be. 



This finlike substance undergoes a constant 

 change as the fish grows older. At fourteen 

 days the dorsal, adijDOse, caudal and anal fin-s 

 are plainly seen, but as yet none of them have 

 rays, except the caudal, in which they are very 

 distinct. The rays of the caudal fin are first 

 apparent at the center, although the general 

 form of the rudimentary tail is very unsymmet- 

 rical, the superior lobe being the larger, and 

 the outline not unlike that of the tails of many 

 heterocercal fishes. At this asre the fish has 



