CHAPTER VI 



THE WORMS 



52. General Characteristics. The bodies of the animals 

 comprising the two preceding groups are exposed on all 

 sides equally to the water in which they live and are radi- 

 ally symmetrical ; but in the worms, one side of the body 

 is fitted for creeping, and for the first time we note a well- 

 marked dorsal (back) and ventral (under) surface. In the 

 former, the body, like a cylinder, may be divided into simi- 

 lar halves by any number of planes passing lengthwise 

 through the middle ; but in the worms, the right and left 

 halves only are exposed equally to their surroundings, and 

 there is, accordingly, only one plane which divides the body 

 into corresponding halves, so that these animals, like all 

 higher forms, are bilaterally symmetrical. In creeping, also, 

 one end of the body is directed forward and it thus be- 

 comes correspondingly modified. It usually bears the 

 mouth, and may be provided with eyes, feelers, or organs 

 of touch, and various other structures which enable the 

 worm to recognize the nature of its surroundings. The 

 nervous and muscular systems are better developed than in 

 the foregoing groups, and we note a greater vigor and defi- 

 niteness in the animal's movements, and in various ways the 

 worms appear better able to avoid or ward off their enemies, 

 recognize and select their food, and in general adapt them- 

 selves to the conditions of life. 



The division of the worms is a very large one, and in 

 some respects difficult to define, owing to the close resem- 



59 



