No. 3.] STUDY OF STENOSTOMA LEUCOPS O. SCHM. 28 1 



and therefore equivalent to the pharyngeal apparatus of Poly- 

 clads. 



Neither in Microstoma nor in Stc7iostoma have we any evi- 

 dence that such a migration actually takes place. In Micro- 

 stoma there is, on the other hand, no reason for denying the 

 possibility of such a migration. 



In Steiiostoma if the cells which form the new pharynx had 

 been derived from the ectoderm, we should expect the pharynx 

 resulting from them to show the same histological character of 

 its lining cells and the same arrangement of the muscle-fibre 

 layers that we see in the general ectoderm and in the cone- 

 shaped depression. 



In respect to the character of its epithelial cells the pharynx 

 of Stenostoma agrees more nearly with the intestine than it 

 does with the integument. In respect to the second point the 

 muscle-fibre layers of its walls show an arrangement the reverse 

 of that which obtains in the integument and identical with that 

 found in the intestine. It is thus much more in accordance with 

 the facts to regard the parenchyme cells from which the new 

 pharynx is formed as potentially entoderm and to homologize 

 what I have called the pharynx with the pharynx of the Triclads. 

 The parenchyme cells which form the pharynx are probably not 

 derived from the entoderm which lines the adult intestine. It 

 is more likely that they are a part of the original mesenchyma- 

 tous tissue which filled the blastocoel and from which the ento- 

 derm of the alimentary canal was differentiated. 



We thus arrive at the conclusion that the so-called pharynx 

 of the Microstomidae consists of two parts : (i) a "cone-shaped 

 depression " homologous with the Polyclad pharynx, and (2) a 

 "pharynx" homologous with the similarly named structure of 

 the Triclads. 



Water Vascular System. 



In the living worm the water vascular system may be seen to 

 consist of a light gray, almost transparent tube, with numerous 

 fine branches. This tube opens to the outside at or near the 

 posterior end of the body. Sometimes the opening may be 

 seen in optical section, in the middle of the triangular area at 

 the posterior end of the body, and sometimes it may be seen 



