THE WORMS 



45 



blance which many of them show to animals in other 

 groups. All the invertebrates, therefore, except the crabs 

 and insects, were placed in one group until subsequent 

 study made it possible to classify them more exactly. Ac- 

 cording to the general shape of the body, and the arrange- 

 ment of internal organs, worms are divided into a number 

 of groups, chief among which are the flatworms, the thread 

 or roundworms, and the ringed worms or annelids. 



THE FLATWORMS 



43. Form and habitat. The flatworms, as their -me 

 indicates, are much flattened, leaf-like forms, some species 

 living in damp places on land, 

 in fresh - water streams or 

 ponds, or along the seacoast, 

 while a variety of other spe- 

 cies are parasitic. The free 

 forms (Fig. 27) are usually 

 small, barely reaching a length 

 greater than five or seven cen- 

 timeters (2 to 3 inches), but 

 some of the parasitic species 

 (Fig. 31) attain the great 

 length of six to thirteen me- 

 ters (20 to 40 feet). 



The free-living forms usu- 

 ally occur on the under side 

 of stones, and frequently are 

 so delicate that a touch is 

 sufficient to destroy them. A 

 few species are almost trans- FIG. ST.-A, fre 8 h-water natworm (Pto- 



. naria) ; B, marine flatworm (Lepto- 



parent, While many are COl- piana}. Enlarged, from Nature. 



ored to harmonize completely 



with their surroundings, so that, even though fragile and 

 defenseless, they escape the attacks of enemies by being 

 overlooked. The night-time or dark days are their hunting 



