Pilidium of Certain N'emerteans. 249 



sented by the opening of the esophagus into the intestine. This 

 becomes closed for a long time, as found by Hubrecht. 



In the nemerteans with direct development, both the esophagus 

 and the rectum are formed by special invaginations of the ectoderm, 

 and are both widely separated from the blastopore. 



The development of the intestinal tract in the pilidium, however, 

 offers little positive evidence as to whether the esophagus is ectoder- 

 mic in origin, because in this form the whole digestive tract (except 

 the rectum) is formed by an almost continuous invagination. From 

 a comparison with other forms, as well as from the histological pecu- 

 liarities of the esophagus of the adult nemertean, it seems highly 

 probable, as urged by Burger in his splendid monograph, that this 

 organ is here likewise of an ectodermic origin, A strong point in 

 favor of this is the fact that the intestinal valve is indicated at a 

 very early period. The histological differences likewise manifest 

 themselves very early. As Burger also states, such a supposition is 

 necessary in order that the nephridial invaginations in different 

 groups of nemerteans may not appear to originate in different layers 

 of the body. 



We may thus conclude that the process of invagination involves 

 not only all of the entoderm cells on the lower pole of the blastula, 

 but also some of the surrounding ectoderm cells by a continuous 

 process of infolding. The blastopore would be pushed inward, and 

 would be marked by the intestinal valve, or an homologous thicken- 

 ing of the epithelium. 



The whole digestive tract is capable of movement to a consider- 

 able extent independent of that of the body-walls. In life the form 

 of its parts, especially of the esophagus, is continually changing. 

 This movement, which is sometimes vigorous, is accomplished by the 

 contraction of the fibers of the few muscular cells which lie directly 

 beneath the epithelium of the digestive canal, and of other fibers 

 which connect this canal with the other parts of the body. 



Histologically the two portions of the digestive tract show marked 

 differences in structure. Both in M. cceca and in (J. leidyi fhe 

 esophagus is very large (Fig. 3, PI. xxxiv) and is bordered by 

 extremely thin epithelium. In the earlier stages of the pilidium 

 the cells of the esophagus were as large as those of the intestine. 

 The esophagus later increases to several times its original size with- 

 out any increase whatever in the number of cells composing it, so 

 that each cell must cover a large surface and becomes correspond- 

 ingly thin. In C. marginatus (Fig. 6, PI. xxxiii) and also in C. lacteus 



