xii PHYLUM CHORDATA 347 



The body-cavity in which the enteric canal and other 

 organs are contained is lined with a membrane, the peri- 

 toneum. This is reflected over the surface of the contained 

 structures, and folds of it serve to suspend the various organs 

 and connect them together. The best developed of these 

 folds is the mesentery (defective in the dogfish), by means 

 of which the intestine is attached to the dorsal wall of the 

 body-cavity. 



The organs of respiration of the dogfish are gills adapted 

 for receiving oxygen from the air dissolved in sea-water; 

 those of the lizard and the rabbit are lungs adapted for 

 breathing air directly. The movements of respiration have 

 been already referred to. In the dogfish these movements 

 have the effect of causing water to be taken in by the mouth, 

 and to pass out from the pharynx to the exterior through 

 the gill-slits. In passing out, the water flows over the gills, 

 which are sets of vascular elevations on the walls of a series 

 of five pairs of chambers the branchial sacs opening 

 internally into the pharynx, and externally communicating 

 with the surrounding water through the branchial slits. In 

 this way the needed oxygen is constantly being taken up, 

 and the carbon dioxide given off. The walls of the branchial 

 sacs are supported by the hyoid and branchial arches. 



Inspiration and expiration of air in the lizard and rabbit 

 take place through the nostrils. The nasal chambers into 

 which the nostrils lead communicate internally with the 

 mouth-cavity or the pharynx through a pair of apertures 

 known as the internal or posterior nares. On the floor of 

 the pharynx behind the root of the tongue is a slit-'like 

 aperture, the glottis, opening behind into a chamber known 

 as the larynx, the wall of which is supported by cartilages. 

 From the larynx the air passes backwards along a tube, the 

 trachea, the wall of which is supported by numerous rings 



