ANDREWS: ANNULUS YENTRALIS. 
4(33 
the groove on the anterior face of the annulus, as might be supposed 
(pi. 47, fig. 34), but the sharp cleft in the anterior rim of the plateau 
puts the anterior groove into direct communication with the suture 
(pi. 48, fig. 38), which thus seems to run forward into that groove 
(pi. 47, fig. 34). 
A general view of the exceedingly simple cavity of this annulus is 
given in figure 39 (pi. 48) which represents a thick longitudinal section 
cleared in oil of cloves and seen from the left side, anterior being to 
the right and ventral above. On the right is seen the steep anterior 
face, on the left the steeper posterior face, and above, the top of the 
plateau whence projects downward the trumpet and its cavity opening 
to the surface by widely opened clefts. On the right in the background 
is the far, or right, side of the groove of the anterior face (pi. 47, fig. 34; 
pi. 48, fig. 38); the heavy line represents the bottom of the groove. 
This bottom line rises over a ‘divide/ which is the top of the slope 
leading up the outer face, and it then falls down through the narrow 
cleft in the anterior rim of the plateau to the bottom of the suture 
plane or mouth of the orifice. Thence to the left in figure 39 (pi. 48) 
the dark line is the bottom of the cavity of the trumpet. This tube 
cavity is but a slight enlargement of the opened suture plane leading 
to the surface. Toward the right of figure 39 (pi. 48) it is seen in 
cross section as it comes somewhat toward the observer and then it 
passes far away to the right of the animal (pi. 46, fig. 24), to return 
toward the observer and be cut across again in this section. The 
terminal part of the cavity on the left of the figure is the posterior 
median part of figure 24 and, lying near the plane of the section, is 
represented as a light area, enlarged below and opening to the surface 
by an open cleft. 
The above views make self evident the interpretation of the exo¬ 
skeleton of the annulus as an elevated mass pitted in as a crooked 
longitudinal pocket whose mouth is open all along its length in 
some preparations, but in Nature closed except for a short distance 
anteriorly. 
While such views as figure 24 (pi. 46) and figure 39 (pi. 48) seem to 
show that the annulus is more simple in possessing no recess, yet some 
cross sections, text-figure I), show a small posterior cavity near the tip 
of the tube and apparently isolated from the median suture. This is, 
however, but the tip of the trumpet tube itself and not a side pocket 
or recess. The appearance of an isolated recess is due to the fact 
