440 
EMBKYOLOGY. 
Chap. XIII. 
are of any use to these animals, or are related to the 
conditions to which they are exposed. 
The case, however, is different when an animal during 
any part of its embryonic career is active, and has to 
provide for itself. The period of activity may come on 
earlier or later in life ; but whenever it comes on, the 
adaptation of the larva to its conditions of life is just as 
perfect and as beautiful as in the adult animal. From 
such special adaptations, the similarity of the larvae or 
active embryos of allied animals is sometimes much 
obscured ; and cases could be given of the larvae of two 
species, or of two groups of species, differing quite as 
much, or even more, from each other than do their adult 
parents. In most cases, however, the larvae, though 
active, still obey, more or less closely, the law of com¬ 
mon embryonic resemblance. Cirripedes afford a good 
instance of this: even the illustrious Cuvier did not per¬ 
ceive that a barnacle was, as it certainly is, a crustacean; 
but a glance at the larva shows this to be the case in an 
unmistakeable manner. So again the two main divi¬ 
sions of cirripedes, the pedunculated and sessile, which 
differ 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 but¬ 
terfly is higher than the caterpillar. In some cases, 
however, the mature animal is generally considered as 
lower in the scale than the larva, as with certain para¬ 
sitic crustaceans. To refer once again to cirripedes: 
the larv8B in the first stage have three pairs of legs, a 
very simple single eye, and a prpbosciformed mouth, 
with which they feed largely, for they increase much in 
