METAMOBrilOSES OF AXIXA.LS. 



357 



Fig. 372. 



seem to follow a reverse course, many of them becoming per- 

 manently fixed after having been previously free. 



560. The metamorphoses of the 

 mollusca, though less striking, are 

 not less worthy of notice. Thus, 

 the oyster, with which we are fami- 

 liar 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 Ac- 

 teon and the Eolis, for example, are 

 born with a shell, which they part 

 with, shortly after leaving the egg. 



561. The study of metamor- 

 phosis is therefore of the utmost 

 importance for understanding the 

 real affinities of animals very dif- 

 ferent in appearance, as is readily 

 shown by the f oh 1 owing 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 outward appearance. But on 

 comparing the caterpillar and the 

 worm, these two animals are seen 

 closely to 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 butter- 

 fly conditions. The latter becoming a more and more perfect ani- 

 mal, whilst the worm remains in its inferior state. 



562. Similar instances are furnished by animals belong- 

 ing to all the types of the animal kingdom. Who woufd 

 suppose, at the first glance, that a barnacle, or an anatifa, were 

 more nearly allied to the crab than to the oyster ? And, 

 nevertheless, we have seen ( 553), in tracing back the anatifa 



Fig. 373. 



rff 



