7 ^^ STING STRUCTURE, 
exterior and interior walls are united at their edges, and 
in the pouch-like portion of the sheath are nearly in 
juxtaposition, forming the oblong valve chamber (a). A 
cross-section is given 
in Fig. 35, c, showing 
the space {n) between 
the walls. At the 
point (Fig. 35, b, c) 
where the contraction 
is seen, the space be- 
gins to alter its form 
(Fig. 35> d), and con- 
tinues to diminish (e), 
until at the apex the 
two walls unite and 
form a solid cutting 
edge (h). 
It will be seen that 
the space n (Fig. 35, 
c) has no connexion 
with so that the 
poison injected into 
the base of the sheath follows the channel a (Fig. 35, 
D and E^) between the lancets. Near the upper end 
of the sheath are two flexible arms, which curve in 
arcs of a circle, and carry the guides on which the 
lancets slide. A cleft commences at the lower part 
of the valve chamber, and taking the form of a shallow 
groove, continues to the end of the sheath. The 
lancets lie in contact with this groove, with their 
barbed edges (k Fig. 34), extending outwards over 
