That is to say, the left and right sides are generally mirror images of each other. If there are two functionally similar body parts, they are usually found roughly equidistant from the center line, parallel to each other. Note the location of the woman's eyes, nostrils, and cheeks relative to the center line of her body. In the case of humans, other mammals, birds, and reptiles, lungs replace gill slits after the embryonic stage of development. Frogs replace them with lungs in the transition from tadpoles to adults.
Fish retain their gill slits all of their lives.
C hordates also have a notochord at some phase in their life cycle. This is a rudimentary internal skeleton made of stiff cartilage that runs lengthwise under the dorsal surface of the body. Generally, there is a single hollow nerve chord on top of the notochord. Among humans and the other vertebrates , the notochord is replaced by a more complex skeleton following the embryonic stage of development. M embers of the phylum Chordata also often have a head, a tail, and a digestive system with an opening at both ends of the body.