GILL CLEFTS AND HEART 



though dissociated from the respiratory function, 

 remains for life. That cleft is the external ear hole, 

 which opens internally by what is called the eustachian 

 tube into the pharynx. Thus, though the gill clefts in 

 the higher vertebrates are no longer used for breathing 

 purposes, being replaced by the lungs, they persist for a 

 time to emphasize the relationship between the higher 

 and the lower vertebrates. 



The ventrally placed heart is another marked 

 characteristic of the vertebrata. The beat of the heart 

 in man is felt through the chest walls and not through 

 the back. In all animals the heart has the same posi- 

 tion ; it lies on the opposite side of the body to that which 

 lodges the brain and spinal cord. In many invertebrate 

 animals the position of these two important organs is 

 exactly the reverse. In worms, insects and crustaceans, 

 the heart, if it exists, is on the dorsal side and the 

 larger portion at any rate of the central nervous system 

 is on the ventral side, the side upon which the animal 

 progresses, or which is turned towards the ground in 

 progression as in those animals which possess legs. 



Anatomy reveals many other characteristics of 

 vertebrate animals ; but the principal ones are those 

 which we have just briefly sketched. The large as- 

 semblage of creatures classifiable as vertebrates splits 

 up readily into the five sub-groups mentioned above, 

 all of which are considered in the following pages. 



