ANDREWS: ANNULUS VENTRALIS. 
433 
The annulus is all the protuberant mass of connective tissue and 
epidermis covered by exoskeleton that lies just below the sternal artery 
where the above nerve ring exists, that is, where the descending artery 
passes forward and backward as the sternal artery. It will be seen 
that the exoskeleton runs in and out over the protuberances and 
depressions formed by the connective tissue and epidermis. The larg- 
Fig. B. 
est protuberance here shown is the annulus; the other large protuber¬ 
ance, to the left in the figure, is the ventral transverse ridge shown in 
text-figure A posterior to the annulus and between the fifth legs. On 
the right of text-figure B is the very thick exoskeleton that forms the 
bottom of the sternal groove all along the second, third, and fourth 
somites as seen in text-figure A. Where this median exoskeleton 
suddenly becomes thin in section, text-figure B, is the hinge-like ante¬ 
rior border of the annulus. In like manner where the very thick exo¬ 
skeleton on the anterior face of the sternal ridge of the fifth somite 
becomes thin, it passes over into the posterior edge of the annulus. 
In removing the annulus to gain such views as figures 1 to 5, in plate 
43, the exoskeleton is cut across the two thin regions shown in text- 
figure B. A horizontal cut through these thin places removes the 
annulus as a truncated cone of connective tissue covered by epidermis 
and exoskeleton. In this exoskeleton is a cavity so sinuous as to be 
cut in three places in the above figure. Such a cut leaves behind on 
the animal a bleeding surface of connective tissue close under the 
sternal artery, and the edges of this wounded surface are formed by 
exoskeleton overlying epidermis. 
The three cavities in the thick exoskeleton of the annulus as repre- 
