89 



portion of the pyloric chamber, so as to form — as Huxley 

 suggested — a very effective filtering apparatus. 



The dorsal part of the pyloric chamber has a 

 comparatively large cavity. In transverse sections 

 through the pyloric region the lateral walls of the dorsal 

 portion are roughly at right angles to one another. 



The supra-arnpullary wall (fig. 36, s. amp.) is 

 immediately above the supra-ampullary ridges, and is 

 almost horizontal, thus forming the floor of the upper 

 region. 



The pleuro-pyloric wall (pp.) is on the outer side of 

 the supra-ampullary wall and turns upwards almost at 

 right angles to the latter. This portion of the wall may 

 be complicated by the presence of folds (up. /.). 



The dorsal wall is simple in structure. 



Thus in the posterior two-thirds of the pyloric region 

 the lumen is divided into a wide dorsal portion and a 

 narrow ventral portion, the two parts being capable of 

 complete separation by the concrescence of the inner 

 portions of the supra-ampullary ridges. 



The anterior third of the pyloric region is compara- 

 tively simple, and shows no such division into dorsal and 

 ventral portions. 



It is probable that in the anterior part of the pyloric 

 region the contents undergo a certain amount of separa- 

 tion. For instance, any hard shell-like structures 

 belonging to the creatures taken in as food will be 

 separated from the soft and nutritious parts. The hard 

 parts pass backwards along the wide dorsal chamber, and 

 by means of an elaborate system of valves they are carried 

 directly into the hind-gut without coming into contact 

 with the unprotected walls of the mid-gut (fig. 40, vol.). 

 The valves are flap-like structures projecting backwards 

 from the upper side of the posterior end of the fore-gut. 



