70 THE STORY OF ANIMAL LIFE 



either an earthworm or a lobster, we can but rec- 

 ognise that the body consists of a number of 

 successive parts very similar to each other; and 

 since the body of each is, in section, more or less 

 round, these successive parts may very aptly be 

 termed rings. Modern writers, however, prefer 

 to call these parts not rings, but Metameres, i.e. 

 successive parts. The symmetrical arrangement 

 of the body in a series of such parts is called 

 " Metamerism " ; and the animals which possess it 

 are said to be " Metameric " in structure. Some- 

 times also the successive parts are spoken of 

 as "segments." Compare Fig. 12 ; A and C show 

 the successive body-rings of worms. 



The earthworm, with its many rings, is one of 

 the higher forms among the worms. Among the 

 lowest forms there are worms in which the ring 

 structure cannot be detected. Between the limits 

 thus marked out, there lies, so to speak, the battle- 

 ground of modern zoology. For the origin of 

 metamerism, and the pedigree of vertebrates, are 

 among the questions that are being discussed in 

 connection with various groups of the worms. 

 1 Among the lowest forms of worms are the 

 Planarian worms, already alluded to as examples 

 of the third grade of animal existence. These 

 'belong to the class Turbellaria, which is repre- 

 sented by plenty of both fresh water and marine 

 forms in our own country and on its coast. The 

 Turbellaria are divided into groups called Accela, 

 Dendroccela,andRhabdoccela. These namesallude 

 to the intestine, which in the first group is want- 

 ing, in the second branched like a tree, and in the 

 third straight. The Cestoda or tape-worms, which 

 absorb nourishment through the skin, and there- 

 fore need no alimentary canal, and possess none; 



