HIGH SCHOOL ZOOLOGY. 39 



concerned in the sense of c(iuiliV)riuni and direction, fur which purpose 

 the arraugemeut in space of the semi-circular canals would appear to lit 

 them. 



49. The Intestinal System.— To this belongs the Ali- 



montaiy canal, with its appendages, tlie liver, air-bladder, &c.; 

 ihe gills, which also come under this category, wc shall, how- 

 ever, reserve for separate treatment. 



We have already studied the osseous boundaries of the gape, 

 and seen the distribution of teeth on these. There are no soft 

 flexible lii)S, although the skin in this position is more richly 

 provided with the tactile organs described in § 9 than elsewhere. 

 In other respects except in the distribution of pigment and 

 thickness, there is little difference between the mucous mem- 

 brane which lines the mouth-cavity and the external skin. 

 There can hardly be said to be a tongue in the same sense as in 

 the hifdier vertebrates, but the mticous membrane which clothes 

 the hypo-hyal bones is certainly thicker than it is elsewhere in 

 the mouth. In the living fish the action of the superior and 

 inferior pharyngeal tooth-plates can be seen j the presence of 

 any foreign body causes them to close reflexly on it, and the 

 muscles in connection with them enable them to pass it down 

 into the oesophagus. This is the begining of the alimentary 

 tract proper, in which we recognize three chief divisions, the 

 oesophagus and stomach, the small intestine, and the large in- 

 testine. The latter terminates in the anus, in front of the anal 

 fin, and is separated from the small intestine by an ileo-CCBcal 

 valve, while the small intestine is separated from the stomach 

 by the pyloric valve. The limits of the various regions are 

 therefore distinct, but there is no such limit between the oeso- 

 phagus and the stomach, although considerable structural dif- 

 ferences exist between these two organs. 



50. The whole intestinal tract is constructed on the same gene- 

 ral plan throughout ; where it lies free in the ctelom, it is cov- 

 ered by a serous coat which is continoiis with the ccelomic 



