Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 243 



when the same larva has during one stage to search for 

 food, and during another stage has to search for a place 

 of attachment. Cases can even be given of the larvae of 

 allied species, or groups of species, differing more from 

 each other than do the adults. In most cases, however, 

 the larvae, though active, still obey, more or less closely, 

 the law of common embryonic resemblance. Cirripedes 

 afford a good instance of this; even the illustrious 

 Cuvier did not perceive that a barnacle was a crustacean : 

 but a glance at the larva shows this in an unmistakable 

 manner. So again the two main divisions of cirripedes, 

 the pedunculated and sessile, though differing widely in 

 external appearance, have larvae in all their stages 

 barely distinguishable. 



The embryo in the course of development generally 

 rises in organisation ; I use this expression, though I am 

 aware that it is hardly possible to define clearly what is 

 meant by the organisation being higher or lower. But 

 no one probably will dispute that the butterfly is 

 higher 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 cirripedes : the larvae 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 complex 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 



