SEA-FISITERIES LABORATORY. 397 
median line exactly under the anterior aorta. It now 
enters the pericardium and passes through the ventricle 
of the heart. 
The course of the alimentary canal in P. operculares 
is somewhat different, but only in the visceral mass. 
Instead of the descending limb passing down to the end 
of the reproductive region, it stops short at about the 
junction of the seminal portion with the ovarian, and 
forming a loop at this point returns to the digestive gland 
as the ascending lmb, without passing through the 
ovarian region at all. 
The descending portion of the intestine in transverse 
section (fig. 37) is almost circular, but owing to differences 
in the cells lining the cavity, there is a trace of division 
into two compartments. The larger part of the cavity is 
lined by epithelium cells, which are elongated and many 
times their width in length (fig. 42). Most conspicuous, 
however, are the cilia which they bear. These are strong 
looking, of considerable length, and present in great 
numbers. ‘These cells have finely granular contents, and 
often contain large granules of a_ green colour. 
Separating this region from another to be presently 
described are two ridges situated opposite one another 
(fig. 37), and formed of epithelial cells about twice as 
long as those previously described, but their cilia are not 
s0 numerous nor as strong, and the cells do not contain 
the round green granules. The cells lining the smaller 
cavity (fig. 37, Al.c. 5’) are much shorter than any of the 
others, being only about a quarter as long as those of the 
ridges. Like those cells, they bear much shorter and 
weaker cilia. ‘The division between these two parts of the 
intestine is but slight, and the differences are due to the 
character of the cells. Projecting into the larger 
compartment of the intestine (fig. 37, Crs.) from the 
Ge 
