METAMOlirilOSES OF ANIMALS. 



179 



877. Analogous transformations take place in t^e Cc.nat- 



ula. In early life 

 g'^.fl^^)^ ^ (Fig. 150) it is 



fixed to the ground 



by a stem, but be- % 



comes detached at 



a certain epoch, 



and then floats 



freely in the sea, 



(Fig. 151.) On 



the other hand, 



the Polypi seem to follow a leverso 



course, many of them becoming fixed to 



the ground after having been previously 

 Fig. 150. free. 



378. The metamorphoses of mollusks, though less 

 striking, are not less worthy of notice. Thus, the oyster, 

 with which we are familiar in its adhering shell, is free 

 when young, like the clam (Mya) and most other shell- 

 fishes. Others, which are at first attached or suspended to 

 the gills of the mother, afterwards become free, as the Unio. 

 Some naked Gasteropods, the Acteon or the Eolis, for ex- 

 ample, are born with a shell, which they part with shortly 

 after leaving the egg. 



379. The study of metamorphoses is, therefore, of the 

 utmost importance for understanding the real affinities of 

 animals very difl^erent in appearance, as is readily shown by 

 the following instances. The butterfly and the earth-worm 

 seem, at the first glance, to have no relation whatever. 

 They differ in their organization, no less than in their out- 

 ward appearance. But, on comparing the caterpillar and 

 the worm, these two animals closely resemble each other. 

 The analogy, however, is only transient; it lasts only 

 during the larva state of the caterpillar, and is effaced as it 



