302 



CLIMAX OF THE ANIMAL KINGDOM— PHYLUM CHORDATA 



ally a baby is born with a definite tail when this rudimentary structure 

 develops too far, but this is generally quietly and inconspicuously re- 

 moved so that there are few that know that it ever happened. There is 

 no such post-anal tail among invertebrates since the anus normally is 

 found near the posterior tip of the body. 



The Child's Heredity, Popenoe, Saunders 



Fig. 21.2. A baby with a tail. All persons have tails as embryos and quite a number 



of babies are born with tails like this Portuguese infant. However, these can easily 



be removed surgically so that few ever know of these cases. 



Pharyngeal Gill Clefts. The external pharyngeal region of all em- 

 bryonic chordates bears a series of gill clefts. In those forms that de- 

 velop gills for respiration, these clefts extend inward and perforate the 

 pharynx to produce gill slits with both internal and external openings. 

 In those that use lungs for respiration, either the clefts never completely 

 penetrate the pharynx and form no gill slits or, if the slits are formed, 

 they become closed during embryological development. 



Blood Flows in a Posterior Direction in the Dorsal Vessel. As the 

 blood is pumped over the body, it flows backward along the dorsal 

 wall just under the notochord or vertebral column in a vessel called 



