METAMORPHOSES OF ANIMALS. 



147 



377. Analogous transformations take place in the Comat- 

 ula. In early life 

 (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, Fi =- 151 



the Polypi seem to follow a reverse course, 

 many of them becoming fixed to the earth 

 after having been previously free. 



Fig. iso. 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 sus- 

 pended to the gills of the mother, afterwards become free, 

 as the Unio. Some naked Gasteropods, the Acteon or the 

 Eolis, for example, are born with a shell, which they 

 part with, shortly after leaving the egg. 



379. The study of metamorphosis is therefore of the ut- 

 most importance for understanding the real affinities of ani- 

 mals very different 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 if we compare 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 

 passes to the chrysalis and butterfly states. The latter be- 



