GENERAL STRUCTURE OF ARTICULATED ANIMALS. 75 



mode of growth, with the bones of Yertebrated animals. It con- 

 sists, in fact, of the skin, consolidated by the deposition of horny 

 or calcareous matter in its substance ; and instead of being able 

 to grow in all directions in the manner of bone, it undergoes 

 very little change when it is once fully formed, and it cannot be 

 made to increase in size except by addition to its edges. 



594. The different rings, or segments, of the body of an Arti- 

 culated animal, always bear a strong resemblance to each other ; 

 and sometimes, as in the lulus (Fig. 287), and the Scolopendra, 

 or Centipede, they seem like actual repetitions of each other. 



Dorsal arch Dorsal oar 



Dorsal member 



Ventral member Ventral arch Ventral our 



FIG. 288. VERTICAL SECTION OF A SEGMENT OF AN ANNELIDE, BELONGING 

 TO THE GENUS AlMPHINOME. 



Each ring may bear two pairs of appendages, or members ; the 

 one belonging to its dorsal arch, or the upper part of the seg- 

 ment ; whilst the other belongs to the ventral arch, or the under 

 part of the ring. In those Articulata in which the principle of 

 "division of labour" does not yet manifest itself, that is, in 

 whose bodies we find a number of similar parts adapted to 

 perform the same functions, instead of an assemblage of dif- 

 ferent parts, constructed each for its own special purpose, all the 

 segments are provided with these appendages, and their number 

 is sometimes extremely great. But in all the higher tribes, we 

 find the appendages of certain segments attaining a high de- 

 gree of development ; whilst, by a sort of compensating principle, 

 the others present themselves as mere rudiments, or are not even 

 at all discoverable. In general the appendages of the ventral 

 arch are alone developed ; and the variety of forms which they 



