422 



COMPARATIVE ZOOLOGY 



sorbed atom by atom, and a new set is substituted. 

 Molting, or the periodical renewal of epidermal parts, 

 as the shell of the lobster, the skin of the toad, the 

 scales of snakes, the feathers of birds, and the hair of 

 mammals, may be termed a metamorphosis. The change 

 from milk teeth to a permanent set is another example. 

 An animal rises in organization as development ad- 

 vances. Thus, a caterpillar's life has nothing nobler 

 about it than the ability to eat, while the butterfly ex- 

 pends the power garnered up by the larva in a gay and 

 busy life. But there are seeming reversals of this law. 

 Some mature animals appear lower in the scale than 

 their young. The larval cirripede has a pair of mag- 

 nificent compound eyes and complex antennae ; when 

 adult, the antennae are gone, and the eyes are reduced 

 to a single, simple, minute eye spot. The germs of the 

 sedentary sponge and oyster are free and active. The 

 adult animal, however, is superior in alone possessing 

 the power of reproduction. Such a change from an 

 active to a fixed condition is known as retrograde meta- 

 morphosis. 



There are certain larval forms so characteristic of 

 the great groups of the animal kingdom as to demand 

 notice. Most worms leave the egg as a larva, called 

 the trochosphere (Fig. 371), an 

 oval larva, having mouth and 

 anus, and a circle of cilia an- 

 terior to the mouth. This 



f FiG. 3 72.-Veliger 



sphere of Worm, larval Stage IS COmmon tO O f Snail, magni- 

 magnified : nt, i i i fied i> velum 



mouth; <z,anus; worms with the most diverse ' 



c, circle of cilia. ^^ forms and hab j ts> ft 



is also found in many of the mollusks. The mollusks 

 usually pass through a later stage called the veliger 

 (Fig. 372), in which a circle of cil-ia homologous to 

 that of the trochosphere is borne by a lobed expansion 



