96 THE FRESHWATEE MUSSEL 



animal and vegetable, is swept in at the hinder 

 end of the shell by the inhalent stream of water, 

 and carried forwards to the mouth by the action 

 of the cilia covering the palps. 



b. The oesophagus is a short straight tube passing 

 almost vertically upwards behind the anterior 

 adductor. 



0. The stomach is a slightly dilated chamber whose 

 cavity is partially subdivided by folding of its 

 walls. 



d. The intestine arises by a small orifice from the ven- 



tral surface of the stomach, and runs backwards 

 and downwards into the visceral mass, where it is 

 closely surrounded by the generative gland. It 

 follows the course shown in fig. 27, and finally 

 passes into the rectum. 



e. The rectum, which is rather wider than the intes- 



tine, runs upwards through the visceral mass to 

 the anterior end of the pericardial cavity, through 

 which it runs back horizontally, surrounded by 

 the ventricle. Behind the pericardial cavity it 

 turns slightly downwards, runs over the posterior 

 adductor, and opens into the cloacal cavity by a 

 slit-like anus. 



The ventral wall of the rectum is folded so 

 as to form a prominent longitudinal ridge, the 

 typhlosole, projecting into its cavity. Thistyph- 

 losole is largest at the junction of the rectum with 

 the intestine. 



2. The liver is a large many-lobed gland surrounding the 

 stomach and adjacent parts of the oesophagus and 

 intestine, and opening into the stomach by several 

 bile-ducts. 



G. The Reproductive System. 



The sexes are separate. The reproductive organs are 

 simple, and very similar in the two sexes. 



