DO INSTITUTIONS OR ORGANS REAPPEAR ? 225 



ently it is very doubtful if any of them ought 

 to be regarded as the inheritance of ancestral 

 traits. Teratology contains no undoubtful case 

 of the reversion of evolution. 



On the other hand, it seems certain that in the 

 individual development of some species there is a 

 real reappearance of lost organs. The larval his- 

 tory of certain Crustacea Malacostraca l (cray-fish, 

 shrimps, etc.), seems to provide instances. 



In Stomatopoda, the youngest erychtheus larvae 

 (fig. 71, A) are formed of three parts : the head, 

 five anterior thoracic segments, each bearing a pair 

 of biramous swimming limbs (i.-v.), the three last 

 decreasing in size from before backwards (these five 

 pairs of appendages represent the five pairs of buccal 

 appendages of the adult) ; three terminal posterior 

 segments (vi.-viii.), and a caudal fin, all without 

 appendages. Tn older larvae the first and second 

 pairs are profoundly modified, losing a joint and 

 acquiring gills (fig. 71, B, i.-ii.) ; the third, fourth, 

 and fifth pairs disappear completely, or at most 

 occur as minute saccules (iii., iv., v.). New thoracic 

 segments are formed, and later on in the third 

 larval stage the thoracic appendages reappear in 

 their final form. 



Similar facts occur in the development of the 

 Decapoda Macroura, such as Palinuerus and 

 Scyllarus. While within the egg the creature 

 passes successively through nauplius and phyllo- 



1 Lang, Anatomic compardc, vol. i., p. 458. 

 P 



