188 CHAS. B. WII-SON. 



of two kinds, distiiigiiished by the intensity with wliich they 

 are stained. 



Wall -eel Is. — These are more numerous, are prismatic in 

 shape, and contain smaller nuclei. They do not stain very 

 deeply, and are evidently the true entoderm. 



Gland-cells. — Between the wall-cells are others scattered 

 over the stomach, but most numerous at its anterior end 

 (fig. 80). They are larger than the wall-cells, and are flask- 

 shaped, with the narrow neck of the flask pointing inward, 

 while the rounded body contains a large nucleus. They stain 

 very deeply, and are evidently gland-cells as first stated by 

 Metschnikoff (33) and later by Blirger (13), and not nerve- 

 cells as claimed by Salensky (45). 



The inner surface of both stomach and oesophagus is 

 covered thickly with fine cilia, which are slightly longer in 

 the stomach than in the oesophagus. At the base of the cilia 

 is another structureless membrane, covering the whole inte- 

 rior of the intestine. 



At the junction of stomach and intestine is the lai'ge 

 sphincter muscle already described. It allows free passage 

 of food particles when relaxed, but closes the opening entirely 

 when contracted. Since the muscles are usually contracted 

 in preserved specimens, it often happens that sections are 

 obtained which appear similar to those given by Hubrecht 

 (25) ; but watching the living larva for a few moments is 

 sufficient to do away entirely with any idea that the stomach 

 ends blindly at both ends. At about the sixth day a swelling 

 is noted on the superior wall of the stomach posterior to the 

 pyloric valve. This develops into a shallow evagination 

 whose walls are made up of cells smaller and more crowded 

 than the adjacent stomach entoderm. But this evagination 

 was carried no further in the oldest pilidiums reared, and so 

 its ultimate purpose could not be determined. 



Summary. — 1. A segmentation cavity appears very early, 

 and increases until the blastula consists of a single layer of 

 cells elongated at right angles to the surface and surrounding 

 this cavity. 



