NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA — SNODGRASS I25 



of jaws. The protarthropods thus developed into Protomandibulata. 

 At this point arose the crustacean branch, in which the simple proto- 

 cephalon is still the definitive head in a large number of forms, though 



Md iMx -^1 C 



Fig. 53. — Head and mouth parts of Chilopoda. 



A, head of Scutigera forceps Raf. B, poison claws (first legs) and second 

 body segment of Lithobius sp., ventral view. C, first and second maxillae of 

 Lithohius, ventral view. D, head of Lithobius with maxillae removed, ventral 

 view. E, right mandible and associated head structures of Lithobius, ventral 

 view. F, right mandible of Lithobius, dorsal view. G, hypopharynx, hypo- 

 pharyngeal suspensoria, and preoral mouth cavity of Lithobius, ventral view 

 (labrum and clypeus removed). 



a', primary posterior articulation of mandible; Ant, antenna; c, secondary 

 anterior articulation of mandible ; Clp, clypeus ; Cx, coxa ; E, eye ; For, foramen 

 magnum; h, ventral inflection of cranium; HAp, hypopharyngeal apodeme; 

 Hphy, hypopharynx (metastoma) ; HS, hypopharyngeal suspensorium (ful- 

 tura) ; L, leg; Lm, labrum; Md, mandible; mr, mandibular rod; iMx, 2Mx, first 

 and second maxillae ; ProC, preoral mouth cavity ; Sex, subcoxa ; Stn, sternum ; 

 T, tergum ; /, 2, frontal and clypeal muscles of labrum ; 3, 4, frontal and clypeal 

 muscles of hypopharyngeal suspensorium; 5, 6, 7, cranial muscles of same; 8, 

 ventral dilator muscles of pharynx; 9, cranial muscle of mandibular stipes; 10, 

 cranial flexor of mandibular lacinia (origin lateral on cranium) ; //, adductor 

 muscle of mandible; 12, protractor muscle of mandible; 13, stipital flexor of 

 mandibular lacinia. 



in several groups a more extensive head has been evolved by adding 

 to the protocephalon the following three, four, or five somites. Cepha- 

 lization, however, continued also in the main protomandibulate line, 

 and produced here a composite head of standardized structure in 



