Jj . E. Robinson and J. Davidson 
223 
the anterior portion of the basis capituli, the buccal cavity is seen to 
occupy a central position (see Pt. I, PI. V, fig. 17). It is supported in 
front by its continuity with the buccal appendages, and laterally by the 
stout chitinous struts which are formed by the internal basal portions of 
the hypostome and palps. The I'oof of the buccal cavity is continued 
backwards wiihin the cavity of the basis capituli in the form of a broad 
chitinous plate—the subcheliceral plate (sub ch. p .)—the dorsal surface 
of which is applied throughout its extent to the ventral surfaces of the 
dilated basal portions of the cheliceral shafts. The subcheliceral plate 
is very thin in its median portion, but acquires rigidity from its stoutly 
thickened lateral margins, which thickening continues directly into the 
chitinous struts alluded to above. The subcheliceral plate serves a 
dual purpose. It forms a smooth surface on which the bases of the 
chelicerae slide in the movements of protrusion and retraction, the raised 
thickened margins helping to maintain the chelicerae in position. Its 
ventral surface affords a means of attachment for the dorsal dilator 
muscles of the pharynx. The posterior margin of the subchelicei’al 
plate like the lateral margins is thickened, and is also indented by a 
broad shallow notch. 
The pharyngeal orifice is situated in the middle of the Hoor of the 
buccal cavity at the posterior extremity of the hypostomal gutter (h. g.) 
(see Part I, PI. VI, fig. 21, o. ph.). The opening is guarded above by a 
tongue-like cuticular flap (tg.) formed from a forward continuation of 
the floor of the buccal cavity, an arrangement which appears to prevent 
the direct access of the saliva to the pharynx during the operation of 
sucking. Samsonk in her description of the buccal cavity of 1. ricinus, 
refers to the portion of the cavity lying above the tongue-like process— 
the cul-de-sac into the posterior angles of which the salivary ducts 
open—as the “salivary cavity” (Speichelhbhle), and includes, as part of 
the buccal cavity, the space below the tongue-like process, which is 
actually the posterior end of the hypostomal gutter, where it forms 
the upper part of the pharyngeal orifice. In Fig. 16 (Part I, PI. V), 
the space which appears between the buccal cavity and the roof of the 
pharynx, is the continuation of the body cavity into the base of the 
tongue-like proce.ss, and has no communication with the alimentary 
canal. 
1 Samson, K. (1909 a), p. 192. 
