PHARYNX OF MICROSTOMA CAUDATUM 315 



b) the gland cells which leave the epithelium to sink radially 

 into the mesenchyme, and, (c) a few cells at the fundus of the 

 young pharynx. These last cells appear to be modified as tran- 

 sitional cells between the epithelium lining the pharynx and the 

 enteric epithelium. We may therefore call them transitional 

 cells (figs. 11 and 12, T.C.). These transitional cells increase in 

 number and form a conspicuous plug protruding into the lumen 

 of the pharynx at the time communication between pharynx 

 and enteron is established. Both in position and histological 

 detail these cells suggest the 'Schliesszellen ' which Mattiesen 

 ('04) describes for the developing pharynx of a planarian embryo. 

 The region of the transitional cells never becomes extensive. The 

 most extensive mass of cells in the pharynx is that of the gland 

 cells. Here the differentiation is rapid and very many gland 

 cells are formed before the young pharynx communicates with 

 the lumen of the enteron. Indeed, many of the gland cells of 

 an advanced pharynx are filled with secretion granules before the 

 lumina of pharynx and enteron are continuous. No empty gland 

 cells, however, have been found in a pharynx that does not lead 

 into the enteron, though the fission of the animal had advanced 

 to where the parent enteron had completely divided. The pos- 

 terior zooid is thus provided with a pharynx whose glands are 

 charged with secretion products, ciliated pits and a 'brain' which 

 lies quite near the cleavage plane. The new zooid is completed 

 with the formation of a rounded anterior extremity. 



SUMMARY 



1. The pharynx is provided with a sucking disc which serves 

 the animal in securing food. The pharynx is much more highly 

 distensible than previously described. 



2. The pharynx is aided in its prehensile functioning by the 

 anterior caecum of the enteron. 



3. The secretions of the glands of the pharynx paralyze Hydras. 

 This paralysis is local, confined to the parts of the prey's body 

 that have been ingested and is not permanent, the prey recover- 

 ing if egested. 



