120 THE BIOLOGY OF AN ANIMAL. 



an animal has a back and belly differing from each other in struc- 

 ture or function, or both, as in the earthworm, is expressed by 

 saying that the body exhibits dorso-ventral differentiation. This, 

 like antero-posterior differentiation, is very feebly expressed in 

 the external features, though clearly marked in the arrangement 

 of the internal parts of the earthworm. In higher animals it 

 becomes one of the most conspicuous features of the body. 



Bilateral Symmetry. When the body is placed in the natural 

 position, with the ventral aspect downwards, a vertical plane 

 passing longitudinally through the middle will divide it into ex- 

 actly similar right and left halves. This similarity is called two- 

 sidedness or bilateral symmetry. Though not very obvious ex- 

 ternally, this symmetry characterizes the arrangement of all the 

 internal parts ; and it may be gradually traced upwards in higher 

 animals, until it becomes as striking and perfect as in the human 

 body. 



Thus a very superficial examination reveals in the earthworm 

 two fundamental laws of organization, viz., differentiation or 

 the law of difference, and symmetry or the law of likeness. And 

 these laws are of interest for the reason among many others that 

 earthworms, like other organisms, have had a history as a race, 

 have come to l>e by a gradual process (cf. p. KM). And biology 

 must answer the questions how and why certain parts have be- 

 come symmetrical and others differentiated. Without entering 

 into a full discussion of the question at this point, it may be said 

 that the main cause of symmetry or differentiation has probably 

 been likeness or unlikeness of function or of relation to the en- 

 vironment. Earthworms show antero-posterior and dorso-ventral 

 differentiation, because the anterior and posterior extremities, or 

 the dorsal and ventral aspects, have been differently used and ex- 

 posed to different conditions of environment. And on the other 

 hand, the organism is bilaterally symmetrical, because the two 

 sides have been similarly used and have been exposed to like con- 

 ditions of environment. 



Metamerism. Another general feature of the earthworm is 

 of great importance in other animals, including the higher forms. 

 The body is marked off by transverse grooves into a series of 

 similar parts like the joints of a bamboo fishing-rod, or like the 

 joints of fingers (Fig. 72). These parts are called metameres, or 

 more often somites, and the body is consequently said to have a 



