METAM0BPH08Ea OF ANIMALS. 051 



by the goological succession of the representatives of the 

 difEerent 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 moUusks, 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 difEerent 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 suia- 

 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 difEerent changes of forms they lead different 

 lives, and are subjected to different surroundings, the larvae, 

 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 



