NO. lO FKEDING ORGANS OF AKACHMOA — SNOLH.RASS 5 



therefore includes also the narrow median strip of the anterior body 

 wall connecting the epistome with the carapace between the cheliceral 

 bases. The somite of the chelicerae is postoral in the early embryo 

 (A, B, /), but the chelicerae in their preoral transposition invade the 

 anterior part of the primary cephalic region (E, Chi), while the 

 sternal part of the cheliceral somite, except possibly in Palpigradi, 

 i)ecomes so reduced that it is not recognizable in the adult structure. 

 The appendages of the second postoral somite, the so-called pedipalps 

 (A, B, Pdp), retain the primitive position in the I'alpigradi, but in 

 the other arachnid orders they move forward to the sides of the mouth 

 (E, Pdp), and generally their dorsal walls unite with the epistome 

 {Epst). Either the sternal plate of the pedipalp somite {IIS) or 

 some other structure forms a lower lip projecting in front of the 

 mouth, and there is thus enclosed, between the pedipalp coxae on the 

 sides and the labrum {Lm) above, a preoral cavity (PrC) for the 

 reception of food. 



The simple basic structure of the anterior part of an arachnid is 

 well shown in a medium sagittal section, such as is represented dia- 

 grammatically at D of figure 2. The eye region of the dorsum, the 

 intercheliceral space, the epistome (Epsl), and the labrum (/-'") 

 represent the cephalic lobe of the embryo. Erom the anterior margin 

 of the back or carapace (Cp), the membranous front wall of the 

 body (a-c) is retlectcd downward or obliquely backward, and bears 

 the chelicerae {Chi) in their secondarily acquired supraoral position. 

 Below the chelicerae the epistome {Epst) extends forward, and sup- 

 ports the labrum {Lm). The labrum is a free lobe, but the epistome 

 lies between the pedipalp coxae and is generally united with their 

 dorsal surfaces (A). Beneath the base of the labrum is the mouth 

 (D, Mtli), Projecting below the mouth is a lower lip, which may be 

 the deutosternum {IIS), as shown in the diagram, or some other 

 structure replacing the latter. Between the labrum and the lower lip, 

 however the latter may be formed, is the preoral food cavity {PrC) 

 enclosed laterally by the pedipalp coxae. The mouth leads directly 

 into the sucking organ known as the pharynx (D, Phy), the dorsal 

 dilator muscles of which {did) are seen in the section to take their 

 origins on the epistome {Epst). Modifications of these features occur 

 in each of the arachnid orders, and are carried farthest in the Acarina, 

 but they are clearly all derived from a simple basic structure. 



The labrum and the epistome. — A labrum is a part of the standard 

 equipment of all arthropods from trilobites to insects, and there is 

 no apparent reason for calling the preoral lolx! of an arachnid any- 

 thing else than labrum. Yet we find the organ dcscrilKd under such 



