44 THE BIOLOGY OF AN .ANIMAL. 



Dorso-ventral Differentiation. In living or well-preserved spe- 

 cimens, the body is not perfectly cylindrical, but is somewhat 

 flattened, particularly near the posterior end, and has a slightly 

 prismatic four-sided form. One of the flattened sides, slightly 

 darker in color than the other, is habitually turned upwards, and 

 is therefore called the back, the opposite or lower side, commonly 

 turned downwards, being the belly. For the sake of accuracy, 

 however, biologists are wont to speak of the dorsal aspect (back) 

 and ventral aspect (belly) of the body ; and the fact that an animal 

 has a back and belly differing from each other in structure or 

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

 that the body exhibits dorse-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 

 exactly similar right and left halves. This similarity is called 

 two-sided likeness, or bilateral symmetry. Though not very 

 obvious externally, this symmetry characterizes the arrangement 

 of all the internal parts; and it may be gradually traced up- 

 wards in higher animals, until it becomes as striking and perfect 

 as in the human body. 



Thus a very superficial examination reveals in the earth- 

 worm two fundamental laws of organization, viz., differentia- 

 tion or the law of difference, and symmetry or the law of like- 

 ness. And these laws are of interest for the reason among 

 many others that earthworms, like other organisms, have as a 

 race had a history, have come to be by a gradual process (cf. 

 p. 99). And biology must strive to answer the questions how and 

 why certain parts have become symmetrical and others differ- 

 entiated. Without entering into a full discussion of the ques- 

 tion at this point, it may be said that the main cause of sym- 

 metry or differentiation has probably been likeness or unlikeness 

 of function, or of relation to the environment. Earthworms 

 show antero-posterior and dorso-ventral differentiation, because 

 the anterior and posterior extremities, or the dorsal and ventral 



