20 bulletin: museum of compaeative zoology. 



In many of the Paleonemertea the digestive canal is a simple 

 cylindrical tube without any diverticida whatever (Text-fig. 5), but 

 in other genera of this order, and in all members of the other orders, 

 except Malacobdella, this tube exhibits closely placed, paired, lat- 

 eral lobes, or pouches, throughout the middle and posterior portions 

 of the body. These metameric pouches arise from a flattening and 

 widening of the canal, correlated with the constriction of bands of 

 tissue at regular intervals, thus giving rise to alternating wider and 

 narrower segments. These pouches ai'e deepest in the flatter and 

 wider forms of both the Hoplonemertea and Heteronemertea and 

 reach their greatest develo])ment in Dkepanopiiorus, Ceeebratu- 

 Lus, and Langia, Avhere tlie central cylindrical tube is but a small 

 fraction of the width of the intestinal pouches. An intermediate 

 condition occurs in Garinomella, w^here more or less regularly 

 arranged pouches are present when the body of the worm is con- 

 tracted (PI. 10, figs. 63-65), but largely or wholly disappear when 

 the body is well extended. 



In some species of Paraxejiertes, Cerebratulus, and other 

 genera the intestinal lobes are forked once or twice distally, and in 

 Pelagoxemertes they are profusely branched. The digestive tube 

 is further complicated by the presence of a large caecal appendage 

 in most Hoplonemertea, as described below. 



A. Paleonemertea and Heteronemertea. — In the Paleone- 

 mertea devoid of lateral intestinal lobes there are no distinct 

 morphological divisions to the alimentary canal, although the his- 

 tological features vary in its different portions. In the other 

 Paleonemertea and in the Heteronemertea the mouth, situated 

 immediately behind the brain, opens into a long esophagus, or 

 fore gut, the posterior portion of which is usiially differentiated 

 histologically, and sometimes morphologically, to form a second 

 chamber, the " stomach." This latter opens into the intestine 

 2)roper, or mid gut, with its paired metameric pouches. Near the 

 anus the pouches become gradually smaller and in a few forms, as 

 Carixoma (PI. 12, fig. 77), a considerable terminal portion is 

 devoid of diverticula, forming a distinct rectum, or hind gut. In 

 most nemerteans, however, this portion of the digestive tract is 

 very inconspicuous. 



Buccal cavity and esophagus. — The mouth is lined with highly 

 columnar, ciliated and glandular cells, and opens into a broad esoph- 



