52 



NATURAL HISTORY. 



WORMS. 



The term worm is rather indefinite. To the ordinary mind the word 

 is associated with something loathsome ; yet among the forms enumerated 

 among the worms are some of the most beautiful objects of nature, while 

 the history of others furnishes some of the most interesting facts in the 

 whole realm of science. From the scientific standpoint the term worms, 

 or its Latin equivalent, vermes, is also indefinite. The group bearing this 

 name is a most heterogeneous assemblage of animals, which doubtless will 

 be broken up into many distinct branches. For our purposes, however, it 

 is not necessary to descend into details, and we will accept the group of 

 worms in its widest sense, but without attempting to define it. 



Worms exist everywhere, — on the land, in fresh water, and in the 

 sea, — while a large number spend a part, or even the whole of their 

 existence as parasites in other animals. Though this subject of parasitism 

 is not a pleasing one at first sight, yet some phases of it are really very 

 interesting and very important, as will be seen from the following pages. 



In the first of the groups of worms, — the flat worms, or Plathelminthes, 

 — we meet all the conditions of life enumerated in the preceding para- 

 graph, and this "environment" has its modifying influence on the mem- 

 bers of the group. A form living in moist earth must needs be differently 

 constituted from one living in the water, and one of these latter in turn 

 from one infesting the organs or tissues of another animal. A free life calls 

 for means of protection and locomotion which are entirely 

 unnecessary in a form living in the flesh or digestive tract of 

 another animal. These latter, of course, can have no use for 

 eyes ; all they need is means for eating and for reproducing 

 their kind, and insuring the young finding the proper place 

 for their further life. 



Most common of the free forms are those known as Plana- 

 rians. These abound in both fresh and salt water. In the 

 ooze at the bottom of ponds one can readily find a number 

 of oval, flat worms, some dark, others nearly white, the 

 body of all being very thin. In the lighter specimens, which 

 rarely measure over three-quarters of an inch in length, we can see more 

 or less distinctly all the features of structure. With the naked eye we 



Fig. 50. — A fresh- 

 water planarian 

 t Polya lis). 



