65 



median line exactly inule]- tlie aiiieiidi" aoita. It no\\' 

 enters the pericardium and passes through the ventricle 

 of the heart. 



The course of the alimentary canal in P. opercularix 

 is some>^hat different, but only in ihe visceral mass. 

 Instead of the descendino- limb passing down to the end 

 of the reproductive region, it sto]is short at about the 

 junction of the seminal poition witli the ovarian, and 

 forming a loop at this point returns to the digestive gland 

 as the ascending limb, without passing through the 

 ovarian region at all. 



The descending ]iortion of the intestine in transverse 

 section ffig. 87) 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 

 (tig. 37), and formed of epithelial cells about twice as 

 long as those preA^iously described, but their cilia are not 

 so numerous nor as strong, and the cells do not contain 

 the round green granules. The cells lining the smaller 

 cavity (hg. 87, Al. e. 3') 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 ffig. 87, CHs.) from the 



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