248 EAMBLES OF A NATURALIST. 



clrian reptiles.* At the present day, however, it 

 has been found to occur in a great number of the 

 Articulata and in most Molluscs and Radiate animals. 

 In proportion as this remarkable phenomenon is ob- 

 served in more numerous and more varied types, it 

 exhibits itself under an entirely new light. For ex- 

 ample, it was formerly believed that the object of 

 metamorphosis was always to raise the organism to a 

 more perfect condition. Such is indeed the case in 

 respect to the tadpole that becomes a frog, and to 

 the caterpillar which is changed into a butterfly, but 

 the result is often precisely the reverse. By the 

 very fact of metamorphosis, the organism becomes 

 degraded, and the adult animal only retains faculties 

 which are inferior to those of its larva, as if the 

 butterfly were to become a chrysalis, f 



We will now consider what takes place in the case 

 of the Teredo. 



The larva, which is at first almost spherical and 

 entirely covered with vibratile cilia, may be com- 

 pared to a very minute hedgehog, in which every 

 spine acts as a natatory organ. It swims in all 

 directions with extreme agility, and this first state 

 continues about a day and a half. Towards the end 



* Frogs, Salamanders, &c. These animals are the only ones 

 belonging to the type of the Vertebrata which present true metamor- 

 phoses. 



f Milne Edwards has proposed to apply the term recurrent types 

 to those animals in which the very progress of development seemed 

 to result in the organic degradation of the individual. One of the 

 best examples that we can give, independently of the Teredo referred 

 to in the text, is that of the Lernea, to which I have already 

 alluded in a preceding note. 



