248 W. R. Coe — Development of the 



During the first two days only a few particles of powdered car- 

 mine will be ingested, but after this time it is devoured in large 

 quantities. 



The mouth of the pilidium develops directly out of the gastrula- 

 mouth, although the true blastopore is pushed inward and marks 

 the opening of the esophagus into the intestine. In the early gas- 

 trula the opening becomes oval, and this shape is retained in the 

 pilidium. 



On the sides of the mouth the epithelium of the esophagus is sepa- 

 rated from that of the under surface of the pilidium by a pair of 

 thickened folds, or lips (Fig. 2, PL xxxiv), covered with especially 

 strong cilia. These lips are united in front, but are widely separated 

 posteriorly and become continuous with the pair of buccal ridges 

 which reach inwards towards the intestine. The relation of these 

 parts is well shown in Fig. 3, PI. xxxiv. In this figure one of the lips 

 is seen between the lines converging at m, and represents the extent 

 of the mouth proper. It is only between these lips that water 

 or food-particles can enter the esophagus. The lip is continued 

 posteriorly into the buccal ridge, a, which finalh^ becomes lost in 

 the wall of the esophagus. It is between the two buccal ridges that 

 water with substances in suspension passes out from the esophagus to 

 the exterior. 



In Fig. 1, PI. xxxiv, which represents an earlier stage, no such 

 buccal folds are present, and consequently there is no definite cur- 

 rent of water passing through the esophagus. At this stage very 

 few, if any, particles of carmine are taken into the digestive canal. 



When the buccal ridges have become established, a current of 

 water carrying food-particles, etc. is constantly passing into the 

 esophagus through the mouth proper, and out again between the 

 posterior buccal ridges. In passing through the esophagus the solid 

 material is collected by the cilia and passed into the intestine, as was 

 described above. 



The esophagus of the pilidium is found by Salensky (28), Butschli 

 (6)^ and others to develop directly into the esophagus of the adult 

 nemertean. Likewise from the intestine the corresponding part 

 of the adult nemertean develops. Hubrecht (16) finds that this 

 is likewise true of Desor's larva. He also considers the esophagus 

 to be composed of entoderm, and that the blastopore becomes the 

 mouth of the nemertean. Arnold (1), on the other hand, believes 

 that the esophagus is formed by an invagination of " secondary " 

 ectoderm, by which the blastopore is pushed inward and is repre- 



