2^8 OTT. [Vol. VII. 



toward the median line ventral to the intestine and forms the 

 commissure. 



The eyes and ciliated pits are now developed. The ciliated 

 pits are formed by a simple depression of the integument. 



According to Wagner the wall of the pharynx is formed by 

 a mass of parenchyme cells which appears on the ventral side 

 of the intestine, and not from the integument, which has been 

 pushed in through this mass until it touches the intestine. The 

 depression of the integument stops at the ventral side of this 

 mass of cells and does not go through it. The integument joins 

 the new pharynx when its lumen opens to the outside. The 

 pharynx opens into the intestine just before the bud separates 

 from the mother, — not at any long time previous to the separa- 

 tion. 



In Stenostoma leucops when it is budding I found no ridge on 

 the outside of the intestine at the anterior end of the new bud, 

 but rather a circular groove on the intestinal wall. No septa 

 were seen. My observations on the development of the brain 

 and ciliated pits confirm those of Wagner on Microstoma. 



Graff ('82, p. 79) says that in those Rhabdocoels, which have the 

 simple pharynx (Microstomida and Macrostomida) the pharynx 

 is a simple depression of the integument forming a tube which 

 connects the mouth with the intestine. According to this the 

 pharynx of Stenostoma leucops is of epiblastic origin. 



As was stated on page 271, there is a distinct line of demarca- 

 tion between the pharynx and the cone-shaped depression of the 

 integument leading to it. These two parts differ in the relations 

 of the longitudinal and circular muscle fibres to the epithelial 

 cells and in the character of the epithelial cells. The two struc- 

 tures differ also in their development, as has been shown by 

 Wagner for Microstoma and by myself for Stenostoma. The 

 walls and the lumen of the pharynx are formed within the par- 

 enchyme before there is any break in the integument, so that 

 the lumen of the pharynx may open to the outside. The 

 pharynx is formed directly from the ventral mass of parenchyme 

 cells, not from a depression of the integument which reaches 

 to the anterior end of the intestine. The depression of the 

 integument reaches only to the anterior end of the new pharynx, 

 thus forming the cone-shaped depression of the integument 



