THE ORIGIN OF INSECTS 31 



known collectively as Appendiculata. They are divided 

 into three sections: (1) the marine worms, earthworms 

 and leeches, called Chaetopoda, or "bristle-footed"; (2) 

 the wheel animalcules, or Rotifera; (3) the " foot-jawed" 

 animals, or Gnathopoda, including insects. 



According to Sir Kay Lankester, " the Gnathopoda 

 have been derived from ancestors of the lower grade 

 represented by the Chajtopoda. They have the surface of 

 the body covered by a hard, horny skin (except in 

 Peripatus and immature grubs), and the legs or appen- 

 dages divided into hard, movable joints, whence they are 

 often called Arthropoda (i.e. with jointed feet). They 

 are definitely characterised by having one or several of 

 the pairs of limbs, which belong to the rings behind 

 the head, turned towards the middle line, so that their 

 hard, horny projections can be made by the muscles 

 to meet one another, and nip any foreign body. These 

 modified legs are called 'jaws,' or 'foot-jaws,' and are 

 never found in the Chastopoda. They are easily seen 

 in the stag-beetle and the wasp. Another leading 

 feature in which the Gnathopoda (also called the 

 Arthropoda) have developed away from and to a higher 

 level than the Chaatopoda is in having from one to three 

 of the series of rings which immediately follow the simple 

 head-lobe of Cheetopods, incorporated and completely 

 fused with it to form the ' head,' by means of the back- 

 ward shifting of the mouth. These new head segments 

 or rings, which have, as it were, ' slipped ' in front of the 

 mouth, are distinguishable in very early growth, but 

 not in the adult. They have their limbs modified as eyes 

 or as long tactile organs (antennas), or rarely as nippers." 



Gnathopoda, as above distinguished, are divided into 

 six great groups, called classes. The first class includes 

 the strange caterpillar-like animal Peripatus and its con- 



