Pilidiuyyi of Certain Nemerteans. 247 



TTie Digestive Tract. 



At the end of twenty-four hours the wall of the enteron in M. 

 caeca and C. leidyi becomes divided into two distinct regions — an 

 anterior, buccal cavity, or esophagus, composed of flattened cells, and 

 a posterior blind sack, or intestine, of columnar cells provided with 

 very long cilia (Fig. 2, PI. xxxiii ; Fig. 1, PI. xxxiv). Later cell- 

 processes grow out to form a sort of intestinal valve which separates 

 the two cavities completely. These processes (Fig. 3, PI. xxxiv) are 

 produced by the half-dozen cells which lie immediately between the 

 two cavities. They are devoid of cilia, and closely resemble the 

 blunt pseudopodia of an amoeba. Ordinarily they meet in the center, 

 so that the two cavities of the digestive tract are not in direct com- 

 munication. 



The cells of the valve do not actually fuse together, however, as 

 may be seen by placing a little finely powdered carmine in the water 

 with the embryos. The particles of the carmine are readily taken 

 into the mouth and collected at the posterior end of the esophagus. 

 Here they are churned round and round against the intestinal valve. 

 When a considerable quantity has been collected, the pseudopod- 

 like ends of the cells forming the valve separate, as in a true sphinc- 

 ter, and the carmine passes into the intestine. There it is rapidly 

 revolved for a time by the motions of the long cilia lining the cavity. 



Hubrecht (16) and Arnold (1) find in Desor's larva that at a cer- 

 tain stage in the development the esophagus is for a long time 

 completely cut off from communication with the intestine by a solid 

 growth of cells. At a later period these cells separate, and communi- 

 cation is again established. It seems probable that this mass of cells 

 is represented in the pilidium in a rudimentary way by the amceboid 

 cell-processes of the intestinal valve. 



Eventually the particles of carmine, or of undigested food-material, 

 are thrown out of the mouth, but not at the same region where they 

 entered ; for these particles always enter at the anterior end of the 

 mouth and leave at the posterior. 



The mechanism by which the in-coming is sepai'ated from the out- 

 going current consists of a thickened fold of large cells covered with 

 especially strong cilia. This fold, which we may call the buccal 

 ridge, extends forwards and downwards on each side (Fig. 3, PI. 

 xxxiv; Fig. 1, PL xxxv, a) to the border of the mouth-opening, where 

 it is continuous with the lateral lips. 



These oblique folds, which are so greatly developed in M. caeca 

 and C. leidyi, are represented in C. lacteus and rtxarginatus only by 

 slight thickenings of the posterior wall of the buccal cavity. 



