/ 



68 SMITHSONIAN MISCELLANEOUS COLLKCTIOXS VOL. 8l 



meat is in no sense to be homologized with the tentorium as developed 

 in some of the higher Crustacea and in the pterygote insects. Each 

 mandible of Spirontocaris is provided with two dorsal productor 

 muscles (/), but a reductor was not observed. Spironiocavis preserves 

 the primitive single dorsal point of articulation of the mandible (a) 

 with the wall of its segment. In higher decapods, the am])hipods. and 

 the isopods, where the mandible may have a double hinge with the 

 wall of the head, the musculature of the organ is modified in a manner 

 to be described later. 



The simple mechanism of the mandible of the higher pterygote in- 

 sects is well understood ; the complicated musculature of the mandible 

 in Apterygota has l)een given scant attention, and the derivation of 

 the pterygote jaw mechanism from that of the Apterygota has been 

 almost ignored. Borner ( 1909) has given the first comparative account 

 of the mandibular musculature in the more generalized insects, and 

 has pointed out certain points of similarity with the musculature of 

 higher crustaceans. He did not, however, carry his comparisons to 

 the myriapods, and thereby missed some fundamental relations. 



The mandibles of the Machilidae will serve best as an example of 

 the more generalized apterygote jaw. The mandible of MacJiilis or 

 of NcsoinachUis (fig. 27 D, Md) is surprisingly similar in form to 

 that of the crustacean Apus {A), except that it has a long incisor 

 point in addition to a broad molar lobe. In this latter character the 

 machilid jaw resembles the mandibles of some of the decapod crus- 

 taceans, such as Spi'irotocar'is and Virhhis, as has l)een pointed out by 

 Crampton (1921b). The mandible of Machilis is suspended by a 

 single dorsal point of articulation (a) against the lateral wall of the 

 head. The cavity of the elongate base of each organ is filled by a mass 

 of muscle fibers (KLk). and these fibers from the two mandibles con- 

 verge upon the ends of a common transverse tendon (k) that passes 

 through the base of the hypopharynx. Here, in an insect, therefore. 

 we find the same type of dumb-bell adductor uniting the two mandibles 

 as occurs in the Diplopoda and in lower Crustacea. In Madiilis. 

 however, there is a second and larger set of adductor fil)ers (KlJ) 

 which has its origin on the hypopharyngeal apodemes (HA). Machi- 

 lis, therefore, in the possession of two dififerentiated sets of mandibular 

 adductor fibers, combines the primitive condition of the chilopods 

 with the specialized condition of the diplopods and lower crustaceans. 

 The tergal musculature of the mandible in Machilis is simple, con- 

 sisting of an anterior promotor (/) and a posterior remotor (/). The 

 two muscles are disposed exactly as in Apus (A), and are in entire 



