XIIl] ANODONTA 305 



palps to a band of ciliated ectoderm running along the inner side of 

 the mantle lobe near its edge until it reaches the ventral siphon when 

 it is ejected. The current produced by the frontal cilia covering the 

 outer lamella of the outer gill-plate passes upwards to join a current 

 produced by cilia on the mantle : that produced by the frontal cilia 

 covering the inner lamella of the outer gill-plate passes upwards to 

 join the downward current on the outer lamella of the inner gill-plate. 

 The current on the mantle curves round to join the current bearing 

 away the rejected food, but if the palps are swollen out so as to touch 

 the mantle, the food conveyed by this current passes to them and can 

 be utilised by the mouth. 



The alimentary canal shows a considerable resemblance to that 

 of the snail. No trace, however, of radula, buccal mass, crop or 

 salivary glands, is to be seen. A short oesophagus leads at once 

 into the stomach, which is a wide sac receiving right and left the 

 ducts of the two lobes of the liver. The intestine runs vertically 

 down into the foot, makes several loops there and then turns back 

 and reaches nearly to the point from which it started, i.e., the 

 hinder end of the stomach. Thence it pursues a straight course 

 through the pericardium and over the posterior adductor muscle, to 

 end in an anal papilla which projects into the epibranchial mantle- 

 cavity. The cavity of the first part of the intestine after it leaves the 

 stomach is invaded by two opposite folds termed the major and minor 

 typhlosoles which almost divide it in two viz. a narrow groove on the 

 right and a cylindrical space on the left. The former, termed the intes- 

 tinal groove, serves to convey undigested matter to the anus the latter, 

 termed the style-sac, secretes a clear gelatinous rod called the crystal- 

 line style. This rod is kept in constant rotation by cilia and is worked 

 forward towards the stomach. Where its front end touches the stomach 

 wall this forms a cuticular structure called the gastric shield. The 

 front end is constantly melting away : and as the rod contains a 

 digestive ferment it has been called solid gastric secretion. The rod 

 is not found in specimens which have been deprived of food for some 

 time, such as those commonly used for dissection in the class-room. 

 The straight concluding portion of the intestine is called the rectum. 

 The pericardium is situated in the mid-dorsal line posterior to the 

 stomach. The fact that it surrounds the rectum is the consequence 

 of its origin in the embryo as a pair of sacs lying to the right and 

 left of the intestine, which later meet above and below this organ. 



There are, as mentioned above, two kidneys formerly called 

 "nephridia" in the mussel. These, frequently termed the organs 

 s. & M. 20 



