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 name 
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- Fis. a7.—A, fresh-water flatworm (Pla- 
& naria); B, marine flatworm (Lepto- 
parent, while many are col- plana). 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 
