BRITISH ZOOPHYTES. 205 



The stomacli is a bag in which the process of diges- 

 tion is carried on, and has three layers, the inmost of 

 which consists of cells containing a brown matter, 

 variable in quantity with the supply of food, and is 

 regarded as showing a glandular structure formed for 

 the purpose of secreting bile, and, therefore, identical 

 in function with the liver of higher organisms. The 

 base of the stomach differs in structure from the upper 

 part, and is often observed to separate from the 

 remainder of the stomach by a constriction, where- 

 by it appears to seize upon the different portions 

 of the food in turn, and after a temporary pressure to 

 release them. The intestine opens out of the upper 

 end of the stomach, which is generally prolonged into 

 a wide chamber called the pyloric vestibule, at the 

 upper end of which is the pyloric valve (p), which has 

 functions similar to that of the cardia. Before passing 

 into the intestine the food accumulates in the pyloric 

 vestibule, where the whole of the nutritious matter is 

 absorbed, and the rest in the form of pellets is sent 

 through the intestine, and discharged at the anal 

 orifice. In Bowerhanhia and some other species the 

 upper part of the stomach is occupied by a gizzard 

 (fig. 2 gi), the walls of which are very muscular, and 

 provided with a crushing apparatus in the form of 

 pointed projections called gastric teeth. 



The intestine is not ciliated. 



The nervous system of the Polyzoa is limited to a 

 single ganglion which is found (Plate XVI., fig. 1 g) 

 on the side of the oesophagus nearest to the anal orifice. 

 From this ganglion nervous filaments have been traced 

 to the lophophore and tentacles, and to the oesophagus . 

 That side of a polyzoon on which the nervous ganglion 



