48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8 1 



hypopharynx (fig. i8 D, HS). The plates appear to be chitinous 

 remnants of the mandibular sternum. They are best developed in the 

 myriapods. In Lifliobiiis (fig. 21 A). Scolopcndra (B). and Scutigcra 

 (C), each plate is a large, irregular sclerite ( HS) attached laterally to 

 the lower margin of the head wall at a point (d) before the base of the 

 mandible, and ending mesally in the side of the hypopharynx. In some 

 chilopods a process on the anterior free part of the mandible articu- 

 lates against the hypopharyngeal plate of the same side. 



Attems (1926) describes the suspensorial plates of the hypo- 

 pharynx in the chilopods as a mandibular support (" kommandibular 

 Geriist "), but the homologous sclerites and their apodemal processes 

 in the diplopods he calls the " tentorium." The writer has not ob- 

 served corresponding structures in the Crustacea. In insects the 

 hypopharyngeal supports are variously developed, but are usually 

 reduced, and often rudimentary. In Machilis (fig. 21 E, HS) their 

 outer ends are broadly fused with the basal angles of the clypeus 

 iCIp) ; in Japyx ( D) the plates are reduced and united in a W-shaped 

 sclerite in the base of the hypopharynx ; in Dissostcira (fig. 42 B, C, 

 HS) they are slender bars extending outward to the bases of the 

 adductor apodemes of the mandibles ; in Microcentrum (fig. 20 D, 

 HS) they are rudimentary prongs diverging from the base of the 

 hypopharynx. In many cases a process extends from each hypo- 

 pharyngeal bar into the lateral walls of the mouth, where it supports 

 the insertion of the retractor muscle of the mouth angle (figs. 42 B, 

 44,^8), and may give rise to an extensive pharyngeal skeleton. In 

 the bees these processes form the long rods bearing the protractor 

 muscles of the pharynx, though the hypopharyngeal bars themselves 

 are lacking. 



In the chilopods and in the apterygote insects, an apodemal process 

 arises from the inner end of each suspensorial plate of the hypo- 

 pharynx, and extends posteriorly below the sides of the pharynx (fig. 

 21 A, C, D, HA). Upon these apodemes arise the retractor muscles 

 of the hypopharynx, the ventral dilators of the pharynx, and ventral 

 adductors of the mandibles, the first maxillae, and the second maxillae. 

 These muscles are all properly sternal muscles, and their origin in the 

 Chilopoda and Apterygota on the hypopharyngeal apodemes, which 

 are sternal apophyses of the head, attests a primitive relation in these 

 groups between the muscles of the gnathal appendages and the sternal 

 parts of their segments. In some of the Crustacea, the corresponding 

 muscles have their origins on a central endoskeletal structure that 

 arises on the sternal region of the gnathal segments behind the mouth 

 In many Crustacea, however, and in the Diplopoda, the ventral 



