V. DIl'LOPODA. SUMMARY OF IMPORTANT FACTS. 497 



young escape from the egs with three pairs of legs, a point once thouglit to 

 show resemblances to the Hexapoda, but wliich does not, for these le<i's are 



Fig. 539.— Hexapod young of Strongylosmna. (After Metschnikoff.) 

 on the fourth, sixth, and seventh somites of tlie body. Tlie Iulidve have 

 elongate cylindrical bodies ; Spirobolus.^ Glomerid.e short, capable of 



Fig. .540. — lulus uiazimus. (After Schmarda 



rolling into a ball ; Polydesmid^, with wing-like processes to the 

 segments, giving thema flattened appearance. Paur(JPida; minute ; body 

 with twelve segments, tending to fuse to six. Pauropus* Eurypauropus* 

 More uncertain in position are the Symphvl.v [Scolopendrella *) , but from 

 the position of the genital opening tlioy are placed here. 



Summary of Important Facts. 



1. The ARTIIEOPODA are animals with evident internal and 

 external segmentation (metamerism). 



2. The metamerism is expressed internallj' in the ladder-like 

 nervous system, in the strncture of the heart, and in the arrange- 

 ment of segmental organs and tracheae so far as these are present. 



3. The onter segmentation is expressed in the rings of the clii- 

 tinous coat of the body as well as in the metameric arrangement 

 of the appendages. 



4. From the similarly metameric Annelida the Arthropoda are 

 distinguished by the presence of jointed appendages, at most a 

 pair to a somite. The appendages may be divided according to 

 function into antennae, jaws, accessory jaws, feet, and swimmerets. 



5. A further distinction is the grouping of tlie somites into 

 regions of which usually head, thorax, and abdomen are recognized. 



6. The head bears the tactile and eating apjiendages; the 



