204 ANIMAL PEDIGREES 



symmetrically placed one on each side of the head, 

 in the position they normally hold among verte- 

 brates. We must therefore suppose that flat fish 

 are descended from other fish in which the eyes are 

 normally situated. 



The Recapitulation Theory supplies a ready test ; 

 and on employing it i.e., on studying the develop- 

 ment of the flat fish we obtain a conclusive answer. 

 A young flounder or other fish, on leaving the egg, 

 is shaped just as any ordinary fish, and has the 

 two eyes placed symmetrically on the right and left 

 sides of the head. As the young fish increases in 

 size, the shape gradually approaches that of the 

 adult ; the body increases in height and becomes 

 flattened laterally, the median, dorsal, and ventral 

 fins becoming greatly developed at the same time ; 

 and the fish now begins to adopt the habit of the 

 adult of lying on one side on the sea bottom. 

 Another change occurs : the eye of the side on 

 which the fish lies, usually the left side in a flounder, 

 becomes shifted slightly forwards, then rotated on 

 to the top of the head, and finally twisted completely 

 over to the opposite or right side. 



Crabs differ markedly from their allies, the lob- 

 sters, in the small size and rudimentary condition 

 of their abdomen or "tail." Development how- 

 ever affords abundant evidence of the descent of 

 crabs from macrurous ancestors. A crab leaves 

 the egg in what is termed the zoea condition, 

 possessing a long and clearly jointed abdomen; 

 and throughout all the earlier stages of existence 

 the abdomen remains at least as long as the body. 



