8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IIO 



tennules of mandibulate arthropods (fig. 2 E, lAnt), as they were 

 formerly thought to be, nor are they the mandibles, as some arach- 

 nologists still persist in calling them. Functionally, the chelicerae 

 might be said to be the "jaws" of the arachnid, but their action is 

 remote from the mouth and consists of grasping, holding, tearing, 

 crushing, or piercing. 



Some students of arachnid embryogeny, as Laurie (1890) and 

 McClendon (1904), say the definitive preoral position of the che- 

 licerae results from a posterior displacement of the mouth, while 

 Renter (1909) says that as the mouth moves caudad there is a simul- 

 taneous forward movement of the chelicerae. Since the mouth and 

 the labrum retain their primitive positions at the anterior pole of 

 the animal, the result, however produced, is the same as if the 

 chelicerae had migrated anteriorly and dorsally around the mouth. 

 In most cases the chelicerae come to lie entirely above the level of 

 the epistome (fig. 2 A), and so close together that they reduce the 

 area of the primary embryonic head lobe between them to a narrow 

 vertical strip. 



The chelicerae have the same essential structure and musculature 

 in both the Xiphosurida (fig. 3 A) and the Arachnida (C). They 

 are composed of three segments in Limulus (A), Palpigradi, Scor- 

 pionida (B), Phalangida (C), and many Acarina ; they are two- 

 segmented in Solpugida (F), Pedipalpida, Chelonethida, Araneida 

 (G), and some Acarina. The uniformly simple structure of the che- 

 licerae precludes the possibility of determining the homology of the 

 cheliceral segments with the segments of a leg. The terminal segment 

 is the "movable finger," which, except in Araneida (G), is usually 

 opposed by an immovable process. The movable finger may be dorsal 

 on the supporting segment, or it may be ventral, and in some forms 

 lateral. The cheliceral pincer resembles the chela of a chelate pedipalp, 

 but the movable finger of the chelicera has always both an opening 

 and a closing muscle (fig. 3 A, D, E, F, G), while the movable finger 

 of a pedipalp chela has only a closing muscle (fig. 5 E). When the 

 chelicera is three-segmented (fig. 3 A, C), the middle segment is 

 strongly musculated from the basal segment. The extrinsic muscles 

 of the chelicerae arise on the dorsum of the prosoma (fig. 3 C), there 

 being no cheliceral muscles corresponding with the ventral muscles 

 of the other appendages. 



The pedipalps and the legs. — The pedipalps are the second postoral 

 appendages of the Arachnida; they are thus the homologues of the 

 mandibles of mandibulate arthropods ; but arachnologists commonly 

 call them the "maxillae," or at least they give this term to the coxae, 



