EMBRYOLOGY 253 



than the caterpillar. In some cases, however, the mature 

 animal must be considered as lower in the scale than the 

 larva, as with certain parasitic crustaceans. To refer 

 once again to cirripeds: the larv^ in the first stage 

 have three pairs of locomotive organs, a simple single 

 eye, and a probosciformed mouth, with which they feed 

 largely, for they increase much in size. In the second 

 stage, answering to the chrysalis stage of butterflies, they 

 have six pairs of beautifully constructed natatory legs, a 

 pair of magnificent compound eyes, and extremely com- 

 plex antennae; but they have a closed and imperfect 

 mouth, and cannot feed: their function at this stage is to 

 search out by their well-developed organs of sense, and 

 to reach by their active powers of swimming, a proper 

 place on which to become attached and to undergo their 

 final metamorphosis. When this is completed they are 

 fixed for life: their legs are now converted into prehen- 

 sile organs; they again obtain a well-constructed mouth; 

 but they have no antennse, and their two eyes are now 

 reconverted into a minute, single, simple eye-spot. In 

 this last and complete state, cirripeds may be considered 

 as either more highly or more lowly organized than they 

 were in the larval condition. But in some genera the 

 larvae become developed into hermaphrodites having the 

 ordinary structure, and into what I have called comple- 

 meutal males; and in the latter the development has as- 

 suredly been retrograde, for the male is a mere sack, 

 which lives for a short time and is destitute of mouth, 

 stomach, and every other organ of importance, excepting 

 those for reproduction. 



We are so much accustomed to see a difference in 

 structure between the embryo and the adult that we are 



