464 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
shown in figure 39 (pi. 48), that is, that the tube cavity ends bluntly 
some distance anterior to the ending of the suture on the surface while 
the plane or slit connecting suture and cavity of tube ends in a slant¬ 
ing wrinkled line leading from the posterior end of the surface suture 
inward and forward some distance to meet the cavity of the tube as 
shown in figure 39 (pi. 48). A very thin section, as in text-figure D, 
may thus cut across the tip of the trumpet tube surrounded by solid 
exoskeleton and also cut the outer part of the posterior ending of the 
suture slit. Sperm, if it were forced along the tube, would readily 
pass out of the suture, when opened, and find no recess or side pockets 
to lodge in. 
Thus this annulus is simple both in having no specialized vestibule 
and in having no recess. 
Owing to the great simplicity of this annulus it is possible to pull 
away the exoskeleton from the underlying soft tissues and to obtain 
(pi. 48, fig. 40), as it were, the die upon which the exoskeleton was 
made. Seen from the ventral side the living part of the annulus thus 
laid bare is a projecting mass of connective tissue covered by epidermis 
and with a curved longitudinal groove along it. The elevated mass 
projects anteriorly on each side and posteriorly it somewhat overhangs 
the epidermis of the sternal surface posterior to the annulus. The 
bluntly ending posterior part of the groove is on the median line and 
more anteriorly the groove sweeps in a semicircle to one side, finally 
to pass over a ridge down into the deep depression upon the anterior 
face of the annulus. The external suture of the exoskeleton (pi. 47, 
fig. 34) thus indicates the axis of the groove of epidermis shown in 
figure 40 (pi. 48). The posterior boundary of the groove is a narrow 
transverse elevated rim which is the tissue that fits into the space 
shown in figure 39 (pi. 48) between the exoskeleton of the posterior 
face of the annulus and the exoskeleton of the trumpet. 
While the essentially living part of the exoskeleton is thus an eleva¬ 
tion bearing a bent groove, the exoskeleton that fits over this tissue 
annulus is so thick that it nearly fills the groove solidly and leaves but a 
small cavity connecting with the exterior by a fine slit. Thus the 
wide flaring epidermis groove (pi. 48, fig. 40) is occupied by a thick- 
walled pocket (pi. 46, fig. 24) and the wide external opening in the 
epidermis is reduced to a suture open for a short distance only, ante¬ 
riorly, as the orifice into the exoskeletal tube. 
The microscopic structure of the tissue annulus (pi. 48, fig. 40) is 
