METAMORPHOSES OF ANIMALS. 143 



when we speak of metamorphoses, are those of insects. 

 Not merely is there a change of physiognomy and form, or 

 the possession of an organ more or less, but their whole or- 

 ganization is modified. The animal enters into new rela- 

 tions with the external world ; and at the same time, new 

 instincts are imparted to it. It has lived in water, and re- 

 spired by gills ; it is now furnished with a windpipe, and 

 breathes air. It passes by, with indifference, objects which 

 before were attractive, and its new instincts prompt it to seek 

 conditions which would have been most pernicious during 

 its former period of life. All these changes are brought 

 about without destroying the individuality of the animal. 

 The mosquito, which to-day haunts us with its shrill trum- 

 pet, and pierces us for our blood, is the same animal that 

 a few days ago lived obscure and unregarded in stagnant 

 water, under the guise of a little worm. 



369. Every one is familiar with the metamorphoses of the 

 silk- worm. On escaping from the egg, the little worm 

 or caterpillar grows with great rapidity for twenty days, 

 when it ceases to feed, spins its silken cocoon, casts its 

 skin, and remains enclosed in its chrysalis state.* During 

 this period of its existence most extraordinary changes 

 take place. The jaws with which it masticated mulberry 

 leaves are exchanged for a coiled tongue ; the spinning 

 organs disappear ; the gullet is lengthened and more 

 slender; the stomach, which was nearly as long as the 

 body, is now contracted into a circular bag ; the intes- 

 tine, on the contrary, becomes elongated and tortuous, 

 having also one portion much smaller than the other. 

 The dorsal vessel is also shortened. The ganglions near 

 the head approach each other, and unite into a single 



* In the raising of silk-worms this period is not waited for, but the ani- 

 mal is killed as soon as it has spun its cocoon. 



