48 GLIMPSES OF NATURE. 



of other cuttlefishes. The extinct belemnites are 

 cuttlefishes in which the shell had become internal, 

 while still retaining many of the characters of an out- 

 side shell. 



In the little living spirula (fig. 15) the shell, we 

 observe, is becoming internal. So that we have all 

 the stages at command showing us how an outside 

 shell in these animals has become internal ; how it 

 has become further degraded ; how it has attained to 

 a yet more elementary structure ; and, finally, how, 

 being no longer needed by the animal, it makes its 

 appearance in the guise of a mere plate of lime, 

 gathered from the sea-beach to feed our birds with 

 their mineral food under the name of the " cuttle- 

 bone." 



