NO. 3 INSECT HEAD SNODGRASS 85 



of the annelid, Nereis virens (fig. 33 D). Here a dorsal proniotor 

 and a remoter (/, /) arise on the tergal wall of the segment, and a 

 ventral promoter and a remotor {K, L) on the midline of the sternal 

 wall. The ventral muscles are repeated regularly in all the segments 

 of the worm (C, E, K, L), but in the more posterior segments the 

 dorsal muscles, though present (E, I, J), are less symmetrical in ar- 

 rangement, and the primary remotor (/) is subordinated to a large 

 oblique remotor (/) that arises on the anterior margin of the seg- 

 ment. This last muscle is described by Borner (1921) as being the 

 typical dorsal remotor of the parapodium, but by comparison with the 

 simpler anterior appendages (D) it appears to be a secondary acquisi- 

 tion, for the muscle (E, /) dipping beneath it has the same insertion 

 on the parapodial base as that of the tergal promoter of tlic anterior 

 parapodium (D, /). Borner's claim, however, that this simple type of 

 limb musculature presented by Nereis must represent the primitive 

 motor mechanism of an appendage turning forward and backward on 

 a vertical axis through its base is scarcely to be questioned. 



The basal muscles of an appendage do not necessarily retain their 

 original functions, nor their primitive simplicity, for an alteration in 

 the basal articulation of the appendage may change the fundamental 

 movements of the limb, and thereby give quite a different action to 

 the muscles, which, in turn, may shift in position, or become split up 

 into segregated groups of fibers, thus multiplying the number of in- 

 dividual muscles actually present. 



Returning now to a further consideration of the muscles of the 

 gnathal appendages of the arthropods, it is not difficult to draw a 

 parallel between the musculature of a mandible, or of an insect 

 maxilla, and that of the annelid parapodium (fig. 33D, E), or with 

 the hypopthetically primitive musculature of an arthropod limb base 

 as expressed in figure 34 A. In the mandible of Scutigera (fig. 26 B, 

 C), Apus (fig. 27 A), Hetcrojapyx (fig. 27 C), Ncsomachilis (fig. 

 27 D), a tergal promoter (/) and a remoter (/) have the typical re- 

 lation to the appendage. In some forms the tergal remotor appears 

 to be lacking (Diplepeda, fig. 26 A; Spiroiitocaris, fig. 27 B). The 

 cranial flexor of the mandibular lacinia m diplopods and chilopods 

 (fig. 26 A, B, C, iJcc) is probably derived from the tergal promoter, 

 since it arises en the dorsal wall of the head and goes dorsal (an- 

 terior) to the ventral muscles. The sternal promoter and remotor, 

 which are distinct muscles in the annelid parapodium (fig. 33 C, D, E, 

 K L), are united in the gnathal appendages of the arthropods, where 

 they become ventral adductors (A'L) as a result of the free movement 



