ANDREWS: ANNULUS VENTRALIS. 
431 
disappears posteriorly in the deep transverse groove that represents 
the elevated transverse rim of figure 1. The bent ridge thus follows 
in general the course of the zigzag suture seen on the ventral side as 
passing from the anterior orifice to the posterior ending of the suture 
upon the posterior rim. The amount of protuberance of this- ridge is 
variable, but in general a posterior view of the annulus exoskeleton, 
as in figure 3' (pi. 43), shows the ridge as a transverse mass projecting 
above the rim of the annulus. In this view the high, sheer, posterior 
face of the annulus with its slight median groove, ends below in a sharp 
edge where the exoskeleton was cut off from the thin membrane poste¬ 
rior to the annulus. The mass below this is the above curved ridge 
while the two dark masses above are the two tuberosities in the back¬ 
ground, the ventral side being uppermost. 
When the annulus was removed and dried with filter paper and the 
ridge broken open it was found to be a hollow structure having a thick, 
brittle wall and a cavity lined by a thin, smooth, yellow, ehitin-like 
layer. 
When the exoskeleton of the annulus was made translucent, the 
true form of this curved ridge, that projects dorsally from the rest of 
the exoskeleton, was seen to be that of a tube or trumpet, with a wide 
mouth anteriorly and a small closed tip posteriorly. Such a trans¬ 
lucent annulus (pi. 43, fig. 5) seen from the ventral side shows the 
posterior rim and promontory and the posterior borders and the spur 
of the tuberosities of figure 1 as more opaque regions in a generally 
clear field and in addition the dorsal ridge showing through the trans¬ 
lucent ventral exoskeleton as a bent, hollow tube with very thick walls. 
The central cavity of the bent tube opens anteriorly on the left by a 
curved orifice partly overhung by the left tuberosity and the transverse 
spur of the right tuberosity. From the orifice the cavity may be seen 
passing to the right, then backward and to the left, finally to end in a 
small median enlargement lying above the middle of the promontory. 
As shown in the figure the curve of the tube is like that of the zigzag 
suture seen on the surface, but the suture does not exactly overlie the 
cavity of the tube; it is short and angular while the cavity of the tube 
is long and sinuous. 
The dorsal ridge of the annulus is thus like a trumpet. We will 
speak of its transverse anterior part as the vestibule opening to the 
left by a somewhat crescentic orifice, at the bottom of the transverse 
depression of the annulus. The cavity following the vestibule is the 
