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 



