106 PRIMITIVE ANIMALS 



old opening into the throat of the spiracular gill-cleft. 

 The hyomandibular cartilage, which in the fish was 

 used to suspend the jaws to the skull, has been 

 pressed into the service of the middle ear, and in 

 the lower forms of land Vertebrates, the Amphibia, 

 Reptiles and Birds, supplies the auditory ossicle for 

 conveying the sound-waves from the drum of the 

 ear to the internal ear (Fig. 22 B). In the Mammal 

 further changes have occurred ; not only does the 

 hyomandibular cartilage serve as an auditory ossicle, 

 the stapes, but additional ossicles, the incus and 

 malleus, are present which have been segmented 

 off the jaw articulation and correspond to the 

 quadrate and articular bones of the lower land 

 Vertebrates. 



The rest of the gill apparatus, including the body 

 of the hyoid arch and all the succeeding arches, 

 though formed as separate and distinct arches in the 

 embryo land Vertebrate, are fused together in the 

 adult to form the hyoid cartilage which supports the 

 throat and vocal apparatus, so that all traces of their 

 gill nature is lost, and were it not for their embryonic 

 history which involves not only themselves but the 

 blood-vessels, nerves and muscles, associated with 

 them, we should be hardly able to prove the un- 

 doubted origin of the land Vertebrate from a fish- 

 like ancestor. That this has occurred is, however, 

 certain ; the gradual embryonic and larval changes 



