F. DIPLOPODA. SUMMAET OF IMPORTANT FACTS. 497 



young escape from the egg with three pairs of legs, a point once thought to 

 show resemblances to the Hexapoda, but which does not, for these legs are 



Fio. 539. Hexapod young of Strongylosoma. (After Metschnikoff.) 



on the fourth, sixth, and seventh somites of the body. The IULID.E have 

 elongate cylindrical bodies ; Spirobolus* GLOMERID^: short, capable of 



FIG. 540. lulus maximus. (After Schmarda.) 



rolling into a ball ; POLYDESMID.E, with wing-like processes to the 

 segments, giving them a flattened appearance. PAUROPODA : minute ; body 

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

 More uncertain in position are the SYMPHYLA (Scolopendrella *), but from 

 the position of the genital opening they are placed here. 



Summary of Important Facts. 



1. The ARTHROPODA are animals with evident internal and 

 external segmentation (metamerism). 



2. The metamerism is expressed internally in the ladder-like 

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

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



3. The outer segmentation is expressed in the rings of the chi- 

 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 the somites into 

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



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



