14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IIO 



In the scorpion (fig. 5 E) the basal segment of the chela (Tb) 

 contains a great mass of depressor muscles (dptar) of the movable 

 finger, divided into three distinct bundles of fibers, two of which are 

 lateral and one ventral. Above the ventral muscle runs a strong 

 median tendon (F), attached distally on the base of the finger, 

 and giving insertion proximally to a large, dense, fusiform muscle 

 (dpptarf) in the patella (Pat). Functionally this muscle is an ef- 

 fective depressor of the finger, but its origin in the patella suggests 

 that it represents the patellar branch of the depressor of the pretarsus 

 in the scorpion leg (fig. 4 C, dpptar). A similar muscle is shown 

 by Chamberlin (1931) to be present in the pedipalp of the pseudo- 

 scorpion (fig. 5 G, dpptar F). 



The mouth and the preoral cavity. — The mouth of an arthropod 

 embryo lies between the base of the labrum and the venter of the 

 first postoral somite. Though a labrum is almost always present and 

 overhangs the mouth, the sternal region of the postoral somite is 

 seldom to be identified in the adult structure. 



In the arachnid order Palpigradi the mouth is on the end of a 

 snoutlike cone projecting from between the bases of the chelicerae 

 (fig. 6 B, D), and the pedipalps are entirely postoral in position (D, 

 Pdp). The mouth cone is formed by a dorsal plate, clearly the labrum 

 (Lm), and a similar ventral plate. The ventral plate cannot possibly 

 be referred to the segment of the postoral pedipalps, and is therefore 

 most reasonably regarded by Borner (1902) as the sternum of the 

 cheliceral segment (IS), retained in the Palpigradi, but lost, or not 

 recognizable as such, in any other arachnid. 



Among the other arachnid orders there is nearly always present 

 an under-lip structure, but it never corresponds with the suboral 

 plate of the Palpigradi. In the Araneida the functional under lip is 

 the projecting sternum of the pedipalp segment; in the Phalangida 

 it is the sternum of the first leg segment ; in the Scorpionida it is 

 formed by coxal lobes of the first and second legs ; in the Thely- 

 phonidae and Ricinulei it is the united coxae of the pedipalps ; in 

 most of the Acarina it is a long lobe, known as the hypostome, pro- 

 duced from the united pedipalp coxae. In these orders the pedipalp 

 coxae are turned forward at the sides of the mouth so that there is 

 enclosed a preoral cavity between the labrum above, an under lip 

 below, and the pedipalp coxae on the sides. 



That the external feeding organs of the Arachnida constitute a 

 "beak," or are derived from such a structure, seems to be an idea 

 prevalent with many writers. Bernard (1895, p. 391), for example, 

 in discussing what he calls the "beak" of the Solpugida, says : "The 



