xii PHYLUM CHORDATA 345 



vessels in the wall of the enteric canal and are thus 

 conveyed throughout the body to be distributed. The 

 fatty matters, however, pass into a system of minute vessels 

 the lacteals which ramify in the wall of the intestine. 

 The lacteals are not blood-vessels, but belong to the 

 lymphatic vascular system. The lacteals combine together 

 and in the Rabbit open into a large trunk the thoracic 

 duct by means of which the absorbed emulsion, or chyle 

 as it is termed, is conveyed to one of the great veins. 



The body-cavity in which the enteric canal and other 

 organs are contained is lined with a membrane, the 

 peritoneum. 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 

 Dog-fish), by means of which the intestine is attached to the 

 dorsal wall of the body-cavity. 



The organs of respiration of the Dog-fish 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 Dog-fish 

 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. 



