L. E. Robinson and J. Davidson 
35 
The position and general relations of the capitulum have already 
been alluded to (p. 26). The ventral situation of the organ, as seen 
in the Argasidae, in contradistinction to the terminal situation ex¬ 
hibited in the Ixodidae, is due, in all probability, not so much to a 
shifting backwards of the cai^itulum, as to a forward extension of the 
body. It has been shown that in the larval stage the capitulum is 
almost terminal, and that the apparent backward movement is pro¬ 
gressive through the nymphal stages. 
The Basis Capituli. 
The basis capituli (b.c.) comprises the whole proximal portion of 
the capitulum. Seen in situ, from the ventral aspect (see PI. IV, fig. 8), 
the outline is roughly rectangular ; the lateral margins are slightly 
INVAGINATED PORTION 
OF CHELICERAL SHEATH 
Text-fig. 2. 
The asterisk indicates the constricted pharyngeal orifice. 
convex and the structure becomes narrower anteriorly. The postero- 
ventral margin is concealed by a transverse fold of the ventral 
integument. From the lateral aspect (see Text-fig. 2) the basis 
capituli is seen to taper down in its distal portion, and thus presents a 
somewhat conical outline. The entire base is open and communicates 
with the body cavity through a large foramen in the ventral body wall. 
The posterior rim of the basis is attached all round to the margins of 
3—2 
