128 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



surround the orifice, and aid the work of the 

 mouth. 



In the lower Crustacea (Entomostraca) we never 

 find more than three pairs of appendages converted 

 into gnathites, and these are, in a general way, com- 

 parable to the mandibles and maxillse of the crayfish. 



The true Insects or Arthropoda Hexapoda have, 

 likewise, three pairs of mouth appendages, and these, 

 again, are known as mandibles and maxillse ; but the 

 mandible is never provided with the three-jointed 

 " palp," which is found in the crayfish. 



No series of structural changes in relation to the 

 different modes of taking in food is more interesting 

 than the really remarkable variations which are found 

 in the size and shape of the gnathites of insects, and 

 nowhere, perhaps, do we see more distinctly the in- 

 fluences of those two prime factors in organic evolution, 

 heredity and adaptation. 



As Meynert has pointed out, we find in winged 

 insects two chief types of mouth organs ; in some the 

 mandibles are hinged on to the sides of the head, and 

 the first pair of maxillse have a less perfect articu- 

 lation ; sometimes, indeed, the latter merely slide on 

 the sides of the hard parts which bound the mouth ; 

 in others the mandibles and maxillse are not arti- 

 culated, but can be withdrawn inwards, or protruded 

 outwards. 



In the former the articulation is such that the 

 jmathites work from side to side and are fit to act as 



O 



cutting 1 or biting: organs ; in the latter they can be 

 pushed into an object or laid side to side, so that they 

 form stabbing or sucking 1 parts. 



It is of supreme interest to observe that among 

 the members of the lower grade of insects, or that in 

 which wings are never developed (Aptera), the 

 mouth organs sometimes (Campodea) remain in an 

 undifferentiated or generalised condition ; though not 



