CHAPTER VI 
THE WORMS 
42, 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- 
44 
