378 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



presents numerous thickenings to form an elaborate 

 system of plates and teeth known as the "Gastric Mill." 

 This will be described below. 



The posterior wall of the cardiac fore-gut is 

 invaginated on its ventral surface to form (he cardio- 

 jjyloric valve separating the cardiac region from the 

 pyloric region. 



The Pyloric fore-gut (PL V, tigs. 34, 36) presents a 

 very complicated arrangement. 



In the posterior two-thirds of the pyloric region the 

 chitin of the ventral wall is thickened at each side to 

 form the pyloric ampullae (amp.). These are clearly 

 seen from the outside as swellings on the floor of the 

 p}doric region. The ampullae have their chitinous lining 

 thrown into well-defined longitudinal parallel ridges. 

 From the summits of the ridges there are numerous fine 

 setae projecting into the cavity of the pyloric chamber. 

 Each seta has numerous small hook-like branches. The 

 two ampullae meet in the mid-ventral line in a well- 

 defined ridge — the inter-amjndlary fold (PL V, fig. 36, 

 i.a.f.). 



The ventro-lateral walls immediately above the 

 ampullae have the chitin enormously thickened at each 

 side to form cushion-like pads projecting into the cavity 

 of the pyloric chamber immediately above the ampullae. 

 These are the supra-ampullary ridges | kk voute ampul- 

 laire," Mocquard] (amp. c). Each cushion has a convex 

 surface which faces inwards and downwards, and the 

 upper parts almost meet in the middle line. The presence 

 of the supra-ampullary ridges and the inter- ampullary 

 fold causes the cavity and the ventral part of the pyloric 

 chamber to be reduced to a narrow two-rayed fissure. The 

 supra-ampullary ridges are covered with numerous fine 

 setae, which stretch across the narrow lumen of this 



