52 ANIMAL FORMS 
may be cut into a number of pieces, each of which will 
develop into a complete individual; and this power of recov- 
ery from the injuries produced by enemies is of the great- 
est service in the perpetuation of the species. This ability 
is also present in certain flatworms, and some species are 
known which voluntarily separate the body into two por- 
tions, each of which becomes an adult. In other species a 
similar process results in the formation of a chain of six 
individuals, placed end to end, the chain finally breaking 
up into as many complete worms. It is possible that the 
tapeworm may also be looked upon as a great chain of 
united individuals produced by the division of a single 
original parent, which becomes adapted for attaching the 
others until they separate. These latter are capable only of 
a very sluggish movement, and, devoid of mouth and ali- 
mentary canal, are not able to digest their food, but their 
life work is to so lay their eggs that they may develop into 
other individuals, and for this they are well adapted. 
NEMATODES (THREADWORMS) 
52. General characters.—This class of worms is com- 
posed of an enormous number of different species, some para- 
sitic, others free all or a portion of their lives, and in view of 
the fact that they inhabit the most diverse situations it is 
remarkable that they are so uniform in their structure. In 
all the body is slender, and the general features of its organ- 
ization may be readily understood from an examination of 
the “vinegar eel” (Fig. 32, A). This small worm (not an 
eel), a millimeter or two in length, lives on the various forms 
of mold that grow in fermenting fruit juices, especially 
after a little sugar or paste has been added. A tough cuti- 
cle surrounds the body, preserving its shape and at the 
same time protecting the delicate organs against the action 
of the acids in which it lives. Through this may be seen 
great bands of muscles extending the entire length of the 
body and producing the wriggling movements of swimming 
