6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 10 



various names as "epipharynx," "camerostome," "rostrum," "lingula," 

 "tonguelike process," and "styletlike process." The arachnid labrum 

 is variable in size and shape in different groups, but it is always present 

 as a lobe of some form projecting above and beyond the mouth at 

 its base (fig. 2 A, Lm). 



Proximal to the labrum, and supporting the latter, there is in most 

 arachnids a distinct median plate (fig. 2 A, Epst) below the chelicerae. 

 This plate has been regarded as a basal part of the labrum, or rec- 

 ognized as an individual structure under a variety of names, such 

 as "clypeus," "epipharynx," "intermaxillary jugum," "subcheliceral 

 plate." Since the plate in question is usually well separated from the 

 labrum, and is postoral in position, it is evidently a part of the head 

 wall ; it corresponds with the epistome (clypeus) of mandibulate 

 arthropods, and hence may be so named in the arachnids. 



The arachnid epistome is not always distinctly separated from the 

 labrum, and it may be more or less invaginated into the anterior body 

 wall beneath the chelicerae, but it is to be identified by one or both 

 of two characteristic relations to other structures. First, the plate 

 is usually united with the pedipalp coxae, forming a bridge between 

 their dorsal surfaces (fig. 2 A, Epst) ; and second, it always gives 

 origin, either directly or by means of a basal apodeme, to the dorsal 

 dilator muscles (D, did) of the pharynx (Phy). In the xiphosurid 

 Lmmhis also an epistomal plate (fig. 2 C, Epst) may be distinguished 

 from the labrum {Lm) ; it here supports the chelicerae {Chi), and, 

 while it is not united with the pedipalp coxae {IICx), it sends out a 

 long arm on each side close to the coxal margin. 



The chelicerae. — The chelicerae, being the first postoral appendages 

 of the arachnid, must represent the corresponding appendages of the 

 mandibulate arthropods, and these appendages are the second antennae 

 of Crustacea, or their homologues, the vestigial premandibular ap- 

 pendages transiently present in some insect embryos. The homology 

 of the arachnid chelicerae with the crustacean second antennae is 

 accepted by Stormer (1944) as obvious from the facts of compara- 

 tive anatomy, and is fully confirmed by the origin of the cheliceral 

 nerves from the tritocerebral lobes of the brain, as shown by Holm- 

 gren (1920) and by Hanstrom (1928). The similar position of the 

 arachnid chelicerae and the crustacean second antennae on the head 

 is at once evident on comparison of a facial view of an arachnid 

 (fig. 2 A) with that of an amphipod (E), and in the phalangid 

 Leiobunum (fig. 16 A), as in the amphipod Talorchestia (fig. 2E), 

 a median bar (/) connects the epistome with the dorsal wall of the 

 head. The chelicerae of the Chelicerata, therefore, are not the an- 



