METAMORPHOSES OF ANIMALS. 6d1 
by the geological succession of the representatives of the 
different classes, as without much doubt lancelets (or at least 
acraniate, boneless forms) were the first Vertebrates to ap- 
pear, and we know that fishes appeared before Amphibians, 
that their type culminated before the reptiles held full 
sway in Mesozoic times, and that birds, after them mam- 
mals, and, last of all, man appeared, who crowns the series 
of vertebrate forms. 
Metamorphosis.—While many animals are hatched like 
the chick with the form of the parent, others pass through 
a series of changes of form called metamorphoses ; these 
changes of form adapt the animal to changes in its sur- 
roundings, involving alterations in its mode of life—slight if 
the change of body-form is slight, thorough-going and radi- 
cal if its body becomes profoundly modified. As an exam- 
ple of a complete metamorphosis may be cited the life-his- 
tories of the jelly-fishes, the star-fish, sea-urchins, sea-cu- 
cumbers, the marine-worms, the mollusks, the crustaceans, 
insects, and the salamanders and toads and frogs, already de- 
scribed in the foregoing pages. If the student will read and 
compare these different accounts, and then consider the 
striking differences between the complicated histories of cer- 
tain species, compared with the direct mode of growth of 
other species of the same order or family, or even of the 
same genus, the inquiry will arise, What is the purpose or 
use of such a series of changes? If he look carefully into 
the embryological changes of those species which are born 
or hatched with the form of the adult, he will see that their 
embryological history is, in point of fact, a condensed suin- 
mary of the changes undergone after hatching by their co- 
species, which, to gain the same adult form, have been sub- 
jected by nature to a series of complicated, and, at first 
sight, superfluous changes of form and environment. 
Most shrimps and crabs undergo a complicated metamor- 
phosis ; in the different changes of forms they lead different 
lives, and are subjected to different surroundings, the larve, 
for the most part, being free-swimming and living near the 
surface of the water, while the parents are stationary. The 
barnacle, when very young, swims near the surface of the 
