328 J. D. Dana — Observations on the Derivation, etc. 



The fact that in low-grade Insects, there is no proper meta- 

 morphosis, while in the higher, as they rise in grade, the larval 

 stage is lower and lower in embryonic level, suggests that the 

 larval stage results from an attendant retrograde embryonic 

 change to a line parallel with the Myriapod, and beyond to the 

 memberless condition of a worm. The principle appears to be 

 a general one among animals, and thence, the higher the 

 species the longer the stage of youth. 



The relations in body-segments and limbs between the classes 

 of Crustaceans, Limuloids, Arachnids, Myriapods and Insects 

 are shown in the following table. The segments of the body 

 are numbered along the left margin ; the zero opposite signifies 

 that the segment, though present has no appendage. 



CRDSTACEANS. 



LIMDLOIDS. 



ARACHNIDS. 



MYRIAPODS. 



INSECTS. 



Tetradecapods. 



lurypterus. 



Pterygotus. 



Limulus. 



Scorpion. 



Phrynus. 



Lithobius. 





1. 1st Ant. " 





1 



Ant. 1 



Ant. "> 



Falces 





Falces 



i 



Ant/ 



■O 



Ant. 





2. 2d Ant. 





M-P. 





M-P. ; 



M-P. ! 



M. 



w 



M. Jffi 



M. 



- 



o 



M. 



a 



3. M. 



-d 



M-P. 



d 



M-P. -o 



M-P. 



<6 



P. ' 





P. " 



i 



Mx. j 



Mx. & L. , 



U 





o 





m 





03 





o 









y, 









4. Mx. 



ill 



M-P. 



W 



M-P. 



l±l 



M-P. 



W 



P. 





P. 



r- 



P. 1 



p - 



* 



5. Mx. 





M-P. 





M-P. 





M-P. 



P. 





P. 



— 





' 



d 





















H 



P. 



?■ 



o 



6. Mx. J 



M-P. J 



M-P. J 



M-P. J 



P. . 



M 



OS 

 Si 



o 



P. , 





P. 





P. 



Eh 





















1. P. 1 



Fol. P. 1 



Fol. P.] 



Fol. P. 1 





0. ' 



EH 



0. 





P. 





0. 





2. P. 





Foi. P. 





Fol. P. 





Fol. P. 





Comb. 





0. 





P. 





0. 





3. P. 



X 







- 









Fol. P. 



=3 



0. 





0. 





P. 





0. 





4. P. 



O 







s* 







- 



Fol. P. 



O 



0. 





0. 



c 



P. 



03 



0. 



C 



5. P. 



EH 







- 

 EH 



S 



Fol. P. 



•G 

 H 



0. 





0. 



S 

 o 

 -a 



P. 



o 



- 



0.- 



S 







6. P. 















Fol. P. 





0. 





0. 



< 



P. 



< 



0. 



<! 



7. P. _ J 



J 



J 



J 





0. ] 





0. 

 0. 





P. 

 P. 





0. 

 0. 

















1. App. 1 



1 



1 



1 





0. 1 





0. J 





P. 





0. 





2. App. 



a 









. 





c* 



0. 



p 







P. 









3. App. 



s 



o 







a 



1 g 





o 



0. 



o 







P. 









4. App. 



JC 



o 



■o 



h§ 





•a 



0. 









P. J 









5. App. 



<3 







«! 



< 





< 



0. 



< 

































15 pairs 







6. App. J 



J 



J 



J 





0. J 









of ieet in 

 Lithobius, 























21inScolo- 























pendra,200 























in some 























Myria- 





















pods. 







