ANDREWS: ANNULUS VENTRALIS. 
42R 
The annulus is thus separated from the openings of the oviducts 
by the entire length of the sternum of the region bearing the fourth 
legs. 
The exoskeleton of the annulus is so thick and rigid that it cannot 
readily be bent, while the exoskeleton just next to it posteriorly and on 
each side is thin and pliable. Anteriorly the annulus is rather firmly 
fastened to the thick sternal plate between the fourth pair of legs by 
somewhat thinner exoskeleton that acts like a leather hinge so that 
when pressure is exerted upon the annulus it may be raised from a 
horizontal position into a much inclined one, somewhat like a trap 
door. This up and down motion is rendered possible by the pliable, 
thin, exoskeleton posterior to the annulus and anterior to the stiff 
part of the sternum between the fifth legs. 
By cutting through the thin parts of the exoskeleton posterior to 
the annulus and on each side of it and by cutting the stiff hinge-like 
connection of annulus and sternal plate anterior to it, the entire exo¬ 
skeleton of the annulus may be removed with some little injury to the 
underlying tissues of the annulus. 
A more detailed view of such a severed annulus exoskeleton as 
seen from the ventral and somewhat from the posterior points of view 
is generalized in figure 1 (pi. 43), since each specimen presents slight 
differences in ratio of transverse to longitudinal diameter, in size and 
shape of protuberances and depression, and in amounts of curvature 
of different regions. 
Each annulus presents two large subcentral, tuberosities anterior 
to a deep transverse depression which is bounded posteriorly by an 
elevated rim. On the median plane this rim juts out anteriorly into 
the depressed area as a bent ridge which we will speak of later as the 
promontory. Along this ridge is a sharp groove, or suture, which 
leaves the median plane and passing down to the right along the 
promontory suddenly turns to the left, on a low level, and crossing the 
median plane, ends in a narrow opening more or less concealed by the 
overhanging tuberosities. This opening, or orifice, is on the left of 
the median plane and is somewhat overhung both bv the large tuberos¬ 
ity on the left and by a transverse spur from the large tuberosity on the 
right. This transverse spur is a prolongation of the right tuberosity 
crossing the median plane and ending more or less under the edge of 
the left tuberosity. Anteriorly the annulus shows a wide median 
groove between the anterior faces of the large tuberosities. The 
