Chap, vi.] CHORDATA. 231 



the lowest members of all three divisions are 

 branchiate, the higher Yertebrata pass from an 

 amphibious or amphipnous stage to one in which 

 outgrowths of the enteric tract, or lungs, are alone 

 the respiratory organs. In the Cephalochordata 

 and Urocliordata respiration is always effected by 

 gills, and there are some very striking points of 

 agreement between the two sets of forms. 



In both, the gill slits are formed by an ingrowth 

 from without, and an outgrowth from within; in both 

 it is the anterior portion of the enteric tract which is 

 so affected, and in both the water of respiration enters 

 by the same orifice as the food. In those Tunicata 

 that retain the chordate tail throughout life (Appeiidi- 

 cularia), the water that passes in at the mouth passes 

 out by a cylindrical tube on either side ; but, on the 

 other hand, water may enter by these " spiracula " 

 and pass out by the mouth. 



In the rest, as also in Ampliioxus, an outgrowth 

 of the body wall on either side gives rise to the 

 formation of a peribranchial chamber, into which the 

 water streams from the gills; the folds which form 

 the walls of this chamber unite along the greater part 

 of their length, but leave an orifice (atriopore) 

 by which the water can escape to the exterior. This 

 atriopore may either open, as in Ascidia, close to the 

 incurrent orifice or mouth, or it may be at the aboral 

 end of the body, as in Pyrosoma ; in compound 

 ascidians there is a single common excurrent orifice. 

 In some the water is forcibly driven out, and then, 

 just as in Cephalopods, the excurrent stream aids in 

 locomotion. In Amphioxus the atriopore is on the 

 ventral surface, and not far in front of the anus. 



The ancestors of the present race of Verteforata 

 were aquatic forms that breathed the oxygen which 

 was dissolved in the water in which they dwelt ; 

 or in other words, they had gills. This mode of 



