PHARYNX OF PRORHYNCHUS APPLANATUS 479 



plasm at three regions (a), (b), (c). The nucleus of the mature 

 cell is rarely found well fixed in prepared sections. 



Many rhabdocoeles, for example, Dalyella, Stenostoma and 

 Microstoma, have gland-cells projecting into the mesenchyme 

 from the wall of the isthmus which connects the pharynx with 

 the enteron. For a time, we had supposed that the cushion 

 cells of Prorhynchus applanatus might be such cells, but in 

 no case have we been able to find that they have ducts. On 

 the other hand, places occur in which the transition from general 

 mesenchymal cells to the typical cushion cells is apparent. In 

 a cell which has departed little from the mesenchymal type, 

 there is a slight vacuolization of the cytoplasm at the pole nearer 

 the mass of more advanced cells. The nucleus of such cells has 

 undergone but little change (fig. 4:-A). Later the cell enlarges 

 as shown at figure 4, B. The cytoplasm becomes more vacuo- 

 lated, and, at this phase in the development of the cushion cell, 

 the nucleus reaches its maximum size (fig. 4, N). When further 

 growth ensues the nucleus decreases in size (fig. 3, N). The 

 cushion cells of Prorhynchus applanatus therefore are not homol- 

 ogous with the gland cells of the pharynges of other rhabdocoeles, 

 but are of mesenchymal origin. 



In both their function and origin they suggest, therefore, the 

 so-called Leydig's cells, which have been described for Crustacea. 



SUMMARY 



1. The pharynx of Prorhynchus has a double function, (a) 

 food prehension; (b) offense and defense. 



2. In correlation with this duplex functional demand there is 

 a structural differentiation analogous to that of the proboscis 

 of the higher rhabdocoeles. 



* 3. There are cushion cells (Ley dig-like cells) developed at the 

 base of this highly organized pharynx, which protects the ad- 

 jacent organs from the recoil of the pharynx. 



4. These cells are of mesenchymal origin and are not homolo- 

 gous with the pharyngeal gland-cells of other rhabdocoeles. 



