CRYSTALLINE STYLE OF LAMELLIBRANCHS 77 



current ciliary channels leading to the base of the incurrent si- 

 phon, whence they are expelled. The mouth is a slit-like aperture 

 situated between the palps. It is provided with muscles by 

 means of which it may be closed in case any objectionable matter 

 gets by the palps. So far as I know, no one has followed the 

 ciliary mechanism beyond this point. 



The oesopha'gus is a thin-walled tube, dorsoventrally com- 

 pressed. Its inner surface is furrowed by a number of distinct 

 longitudinal grooves, one of which is very much larger than the 

 others. Where the oesophagus joins the stomach the epithelium 

 is raised to form a distinct ridge encircling the orifice; in Lamp- 

 silis this ridge forms a sort of shelf on the ventral side, which 

 projects into the stomach, nearly to the gastric shield. 



All of the longitudinal grooves of the oesophagus end at the 

 stomach except the large one, which extends across the left side 

 of the ventral wall of the stomach to the base of the gastric 

 shield. The cilia in the oesophagus carry the streams of food 

 material up to the shelf-like projection in its ventral wall and 

 also up the large furrow to the base of the gastric shield. 



The lateral walls of the stomach in proximity to the gastric 

 shield bear tufts of very powerful cilia. When the walls are in 

 their normal position the effective stroke of these is such as to 

 set into rotary motion any material in contact with them. This 

 rotation is in a clockwise direction when viewing the animal 

 from the anterior end. 



All ridges and furrows of the stomach wall, except the region 

 beneath the gastric shield, bear strong cilia which keep up con- 

 stant currents within the stomach. In general these may be 

 resolved into two main groups: first, those currents on the 

 ridges and raised parts of the gastric epithelium, together with 

 all streams from the oesophagus, which lead to the region around 

 the gastric shield; second, those of the deeper grooves and fur- 

 rows of the stomach wall which, save for exceptions to be noted 

 later, lead into the large furrow which passes to the right of the 

 major typhlosole and forms the intestine proper. 



A detailed description of the comparative anatomy of the 

 stomach and the ciliary mechanisms in each of the lamelli- 



