Chap, iv.] MOUTH ORGANS OF TRACHEATA. 127 



covering of a crayfish's body, we are compelled to 

 turn to the history of development for an explanation. 

 When we do this, we find that the epiblastic infoldings, 

 which form respectively the stomodoeum and the 

 proctodoemn, are carried very far inwards, and that 

 only a small portion of the archenteron, or region 

 primitively lined by hypoblast, remains in the adult 

 organisation. The developing lobster, as compared 

 with the developing crayfish, has a much shorter proc- 

 todeal invagination. 



While it is absolutely true of all the animals here 

 spoken of as Arthropoda that some of their appendages 

 are converted into mouth organs or gnathites, the 

 number is by no means always so large or the arrange- 

 ments so complicated as those which we have just 

 found to obtain in the crayfish. In Peripatus, for 

 example, one pair only of appendages are modified to 

 serve as jaws, which have the special function of 

 cutting blades. In the Scorpion, where there are no 

 appendages in front of the mouth, there are only two 

 pairs specially adapted to the service of the mouth, 

 and these have their free ends pincer-shaped, and not 

 converted into cutting or biting organs ; this arrange- 

 ment will be the more clearly understood when one 

 remembers that these animals suck the juices rather 

 than eat the tissues of their prey. 



The differences between the Chilopodous and 

 Chilognathous Myriopoda allow us to say little 

 that is true of them both ; in both, however, we see 

 here, as in other parts of their organisation, characteris- 

 tics of a less high degree of differentiation than those 

 that obtain in the crayfish, on the one hand, or the 

 cockroach on the other ; there are two or three pairs 

 of gnathites, and these are always jointed. One is 

 often converted into u poison gland in the Chilopoda, 

 and in them also, as in the scorpion, the basal por- 

 tion of some of the succeeding pairs of appendages 



